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Mole concept 5c

Chapter 5 - Mole Concept And Stoichiometry Exercise Ex. 5C

Question 1

Give three kinds of information conveyed by the formula H₂O.

Solution 1

Information conveyed by H₂O

(1)That H₂O contains 2 volumes of hydrogen and 1 volume of oxygen.

(2)That ratio by weight of hydrogen and oxygen is 1:8.

(3)That molecular weight of H₂O is 18g.

Question 2

Explain the terms empirical formula and molecular formula.

Solution 2

The empirical formula is the simplest formula, which gives the simplest ratio in whole numbers of atoms of different elements present in one molecule of the compound.

The molecular formula of a compound denotes the actual number of atoms of different elements present in one molecule of a compound.

Question 3

Give the empirical formula of:

(a) C₆H₆ (b) C₆H₁₂O₆ (c) C₂H₂ (d) CH₃COOH

Solution 3

(a) CH (b) CH₂O (c) CH (d) CH₂O

Question 4

Find the percentage of water of crystallisation in CuSO₄.5H₂O. (At. Mass Cu = 64, H = 1, O = 16, S = 32)

Solution 4

Question 5

Calculate the percentage of phosphorus in

(a) Calcium hydrogen phosphate Ca(H₂PO₄)₂

(b) Calcium phosphate Ca₃(PO₄)₂

Solution 5

(a) Molecular mass of Ca(H₂PO₄)₂ = 234

So, % of P = 2 31 100/234 = 26.5%

(b) Molecular mass of Ca₃(PO₄)₂ = 310

% of P = 2 31 100/310 = 20%

Question 6

Calculate the percent composition of Potassium chlorate KClO₃.

Solution 6

Molecular mass of KClO₃ = 122.5 g

% of K = 39 /122.5 = 31.8%

% of Cl = 35.5/122.5 = 28.98%

% of O = 3 16/122.5 = 39.18%

Question 7

Find the empirical formula of the compounds with the following percentage composition:

Pb = 62.5%, N = 8.5%, O = 29.0%

Solution 7

Element % At. mass Atomic ratio Simple ratio

Pb 62.5 207 1

N 8.5 14 2

O 29.0 16                          6

So, Pb(NO₃)₂ is the empirical formula.

Question 8

Calculate the mass of iron in 10 kg of iron ore which contains 80% of pure ferric oxide.

Solution 8

In Fe₂O₃ , Fe = 56 and O = 16

Molecular mass of Fe₂O₃ = 2 56 + 3 16 = 160 g

Iron present in 80% of Fe₂O₃ =

So, mass of iron in 100 g of ore = 56 g

mass of Fe in 10000 g of ore = 56 10000/100

= 5.6 kg

Question 9

If the empirical formula of two compounds is CH and their Vapour densities are 13 to 39 respectively, find their molecular formula.

Solution 9

For acetylene , molecular mass = 2 V.D = 2 13 = 26 g

The empirical mass = 12(C) + 1(H) = 13 g

n =

Molecular formula of acetylene= 2 Empirical formula =C₂H₂

Similarly, for benzene molecular mass= 2 V.D = 2 39 = 78

n = 78/13=6

So, the molecular formula = C₆H₆

Question 10

Find the empirical formula of a compound containing 17.7% hydrogen and 82.3% nitrogen.

Solution 10

Element % At. mass Atomic ratio Simple ratio

H 17.7 1

N 82.3 14

So, the empirical formula = NH₃

Question 11

On analysis, a substance was found to contain

C = 54.54%, H = 9.09%, O = 36.36%

The vapour density of the substance is 44,calculate;

(a) its empirical formula, and

(b) its molecular formula

Solution 11

Element % at. mass atomic ratio simple ratio

C 54.54 12 2

H 9.09 1 4

O 36.36 16 1

(a) So, its empirical formula = C₂H₄O

(b) empirical formula mass = 44

Since, vapour density = 44

So, molecular mass = 2 V.D = 88

Or n = 2

so, molecular formula = (C₂H₄O)₂ = C₄H₈O₂

Question 12

An organic compound ,whose vapour density is 45, has the following percentage composition

H=2.22%, O = 71.19% and remaining carbon. Calculate ,

(a) its empirical formula, and

(b) its molecular formula

Solution 12

Element % at. mass atomic ratio simple ratio

C 26.59 12 1

H 2.22 1 1

O 71.19 16 2

(a) its empirical formula = CHO₂

(b) empirical formula mass = 45

Vapour density = 45

So, molecular mass = V.D 2 = 90

so, molecular formula = C₂H₂O₄

Question 13

An organic compound contains H = 4.07%, Cl = 71.65% chlorine and remaining carbon. Its molar mass = 98.96. Find,

(a) Empirical formula, and

(b) Molecular formula

Solution 13

Element%at. massatomic ratiosimple ratio

Cl71.6535.51

H4.0712

C24.28121

(a) its empirical formula = CH₂Cl

(b) empirical formula mass = 49.5

Since, molecular mass = 98.96

so, molecular formula = (CH₂Cl)₂ = C₂H₄Cl₂

Question 14

A hydrocarbon contains 4.8g of carbon per gram of hydrogen. Calculate

(a) the g atom of each

(b) find the empirical formula

(c) Find molecular formula, if its vapour density is 29.

Solution 14

(a) The g atom of carbon = 4.8/12 = 0.4 and g atom of hydrogen = 1/1=1

(b) Element Given mass At. mass Gram atom Ratio

C 4.8 12 0.4 1 2

H 1 1 1 2.5 5

So, the empirical formula = C₂H₅

(c) Empirical formula mass = 29

Molecular mass = V.D 2 = 29 2 = 58

So, molecular formula = C₄H₁₀

Question 15

0.2 g atom of silicon Combine with 21.3 g of chlorine. Find the empirical formula of the compound formed.

Solution 15

Since, g atom of Si = given mass/mol. Mass

so, given mass = 0.2 28 = 5.6 g

ElementmassAt. massGram atomRatio

Si5.6280.21

Cl21.335.53

Empirical formula = SiCl₃

Question 16

A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29, find its molecular formula.

Solution 16

% of carbon = 82.76%

% of hydrogen = 100 - 82.76 = 17.24%

Empirical formula = C₂H₅

Empirical formula weight = 2 x 12 + 1 x 5 = 24 + 5 = 29

Vapour Density = 29

Relative molecular mass = 29 x 2 = 58

N =  

Molecular formula = n x empirical formula

  = 2 x C₂H₅

 = C₄H₁₀

Question 17

In a compound of magnesium (Mg = 24) and nitrogen (N = 14), 18 g of magnesium combines with 7g of nitrogen. Deduce the simplest formula by answering the following questions.

(a) How many gram- atoms of magnesium are equal to 18g?

(b) How many gram- atoms of nitrogen are equal to 7g of nitrogen?

(c) Calculate simple ratio of gram- atoms of magnesium to gram-atoms of nitrogen and hence the simplest formula of the compound formed.

Solution 17

(a) G atoms of magnesium = 18/24 = 0.75 or g- atom of Mg

(b) G atoms of nitrogen = 7/14 = 0.5 or 1/2 g- atoms of N

(c) Ratio of gram-atoms of N and Mg = 1:1.5 or 2:3

So, the formula is Mg₃ N₂

Question 18

Barium chloride crystals contain 14.8% water of crystallization. Find the number of molecules of water of crystallization per molecule.

Solution 18

Barium chloride = BaCl₂.x H₂O

Ba + 2Cl + x[H₂ + O]   

=137+ 235.5 + x [2+16]

=[208 + 18x] contains water = 14.8% water in BaCl₂.x H₂O

=[208 + 18 x] 14.8/100 = 18x

=[104 + 9x] 2148=18000x

=[104+9x] 37=250x

=3848 + 333x =2250x

1917x =3848

x = 2molecules of water

Question 19

Urea is very important nitrogenous fertilizer. Its formula is CON₂H₄.Calculate the percentage of nitrogen in urea. (C=12,O=16 ,N=14 and H=1).

Solution 19

Molar mass of urea; CON₂H₄ = 60 g

So, % of Nitrogen = 28 100/60 = 46.66%

Question 20

Determine the formula of the organic compound if its molecule contains 12 atoms of carbon. The percentage compositions of hydrogen and oxygen are 6.48 and 51.42 respectively.

Solution 20

Element % At. mass Atomic ratio Simple ratio

C 42.1 12 3.5 1

H 6.48 1 6.48 2

O 51.42 16 3.2 1

The empirical formula is CH₂O

Since the compound has 12 atoms of carbon, so the formula is

C₁₂ H₂₄ O_12.

Question 21

(a) A compound with empirical formula AB₂, has the vapour density equal to its empirical formula weight. Find its molecular formula.

(b) A compound with empirical formula AB has vapour density 3 times its empirical formula weight. Find the molecular formula.

(c)10.47 g of a compound contained 6.25 g of metal A and rest non-metal B. Calculate the empirical formula of the compound [At. wt of A = 207, B = 35.5]

Solution 21

(a) Now since the empirical formula is equal to vapour density and we know that vapour density is half of the molecular mass i.e. we have n=2 so, the molecular formula is A₂B₄.

(b) Since molecular mass is 2 times the vapour density, so Mol. Mass = 2 V.D

Empirical formula weight = V.D/3

So, n = molecular mass/ Empirical formula weight = 6

Hence, the molecular formula is A₆B₆

(c) 

Given:

Wt. of the compound: 10.47g

Wt. of metal A: 6.25g

Wt. of non-metal B: 10.47 – 6.25 = 4.22g

Hence, the empirical formula is AB₄

Question 22

A hydride of nitrogen contains 87.5% per cent by mass of nitrogen. Determine the empirical formula of this compound.

Solution 22

Atomic ratio of N = 87.5/14 =6.25

Atomic ratio of H= 12.5/1 = 12.5

This gives us the simplest ratio as 1:2

So, the molecular formula is NH₂

Question 23

A compound has O=61.32%, S= 11.15%, H=4.88% and Zn=22.65%.The relative molecular mass of the compound is 287 amu. Find the molecular formula of the compound, assuming that all the hydrogen is present as water of crystallization.

Solution 23

Element % at. mass atomic ratio simple ratio

Zn 22.65 65 0.348 1

H 4.88 1 4.88 14

S 11.15 32 0.348 1

O 61.32 16 3.83 11

Empirical formula of the given compound =ZnSH₁₄O₁₁

Empiricala formula mass = 65.37+32+141+11+16=287.37

Molecular mass = 287

n = Molecular mass/Empirical formula mass = 287/287=1

Molecular formula = ZnSO₁₁H₁₄

=ZnSO₄.7H₂O

Chapter 11 - Study of Compounds D. Sulphuric Acid Exercise Intext 1

Question 1

Comment, sulphuric acid is referred to as

(a) King of chemicals

(b) Oil of vitriol

Solution 1

(a) Sulphuric acid is called King of Chemicals because there is no other manufactured compound which is used by such a large number of key industries.

(b) Sulphuric acid is referred to as Oil of vitriol as it was obtained as an oily viscous liquid by heating crystals of green vitriol.

Question 2

Sulphuric acid is manufactured by contact process

(a) Give two balanced equations to obtain SO₂ in this process.

(b) Give the conditions for the oxidation of SO₂

(c) Name the catalyst used.

(d) Why H₂SO₄ is not obtained by directly reacting SO₃ with water.

(e) Name the chemical used to dissolve SO₃ and also name the product formed.

(f) Name a gas that can be oxidized to sulphur.

Solution 2

(a) Two balanced equations to obtain SO₂ is:

(i) 4FeS₂ + 11O₂ 2Fe₂O₃ +8SO₂

(ii) S +O₂ SO₂

(b) The conditions for the oxidation of SO₂ are:

(i) The temperature should be as low as possible. The yield has been found to be maximum at about 410⁰C-450^oC

(ii) High pressure (2 atm) is favoured because the product formed has less volume than reactant.

(iii) Excess of oxygen increases the production of sulphur trioxide.

(iv) Vanadium pentoxide or platinised asbestos is used as catalyst.

(c) Vanadium pentoxide (V₂O₅)

(d) Sulphuric acid is not obtained directly by reacting SO₃ with water because the reaction is highly exothermic which produce the fine misty droplets of sulphuric acid that is not directly absorbed by water.

(e)The chemical used to dissolve SO₃ is concentrated sulphuric acid. The product formed is oleum.

(f) Hydrogen sulphide.

Question 3

Why the impurity of arsenic oxide must be removed before passing the mixture of SO₂ and air through the catalytic chamber?

Solution 3

Impurity of ARSENIC poisons the catalyst [i.e. deactivates the catalyst]. So, it must be removed before passing the mixture of SO₂ air through the catalytic chamber.

Question 4

(a) Name the catalyst which helps in the conversion of sulphur dioxide to sulphur trioxide in step C.

(b) In the contact process for the manufacture of sulphuric acid, sulphur trioxide is not converted to sulphuric acid by reacting it with water. Instead a two -steps procedure is used. Write the equations for the two steps involved in D.

(c) What type of substance will liberate sulphur dioxide from sodium sulphite in step E?

(d) Write the equation for the reaction by which sulphur dioxide is converted to sodium sulphite in step F.

Solution 4

(a) The catalyst which helps in the conversion of sulphur dioxide to sulphur trioxide in step C is Vanadium pentoxide.

(b) The two steps for the conversion of sulphur trioxide to sulphuric acid is:

(i) SO₃ + H₂SO₄ H₂S₂O₇

(ii) H₂S₂O₇ + H₂O 2H₂SO₄

(c)The substance that will liberate sulphur dioxide in step E is dilute H₂SO_4.

(d) The equation for the reaction by which sulphur dioxide is converted to sodium sulphite in step F is:

SO₂+2NaOH Na₂SO₃+H₂O

         Or

Na₂O+SO₂ Na₂SO₃

Chapter 11 - Study of Compounds D. Sulphuric Acid Exercise Ex. 11

Question 1

Why is water not added to concentrated H₂SO₄ in order to dilute it?

Solution 1

Water is not added to concentrated acid since it is an exothermic reaction. If water is added to the acid, there is a sudden increase in temperature and the acid being in bulk tends to spurt out with serious consequences.

Question 2

Give two balanced reactions of each type to show the following properties of sulphuric acid.

(a) Acidic nature

(b) Oxidising agent

(d) Non-volatile nature

Solution 2

Balanced reactions are:

(a) Acidic nature:

(i) Dilute H₂SO₄ reacts with basic oxides to form sulphate and water.

2 NaOH+H₂SO₄ Na₂SO₄+2H₂O

(ii) CuO+H₂SO₄ CuSO₄+H₂O

(iii) It reacts with carbonate to produce CO₂.

Na₂CO₃+H₂SO₄ Na₂SO₄+H₂O+CO₂

(b) Oxidising agent:

H₂SO₄ H₂O +SO₂ +[O]

Nascent oxygen oxidizes non-metals, metals and inorganic compounds.

For example,

Carbon to carbon dioxide

C+H₂SO₄ CO₂ +H₂O +2SO₂

Sulphur to sulphur dioxide

S +H₂SO₄ 3SO₂ +2H₂O

(d) Non-volatile nature:

It has a high boiling point (356^oC) so it is considered to be non-volatile. Therefore, it is used for preparing volatile acids like hydrochloric acid, nitric acid from their salts by double decomposition reaction.

NaCl + H₂SO₄ NaHSO₄ +HCl

KCl + H₂SO₄ KHSO₄ +HCl

Question 3

Give a chemical test to distinguish between:

(a) Dilute sulphuric acid and dilute hydrochloric acid

(b) Dilute sulphuric acid and conc.sulphuric acid

Solution 3

(a)Bring a glass rod dipped in Ammonia solution near the mouth of each test tubes containing dil. Hel and dil. H₂SO₄each.

(b)

1. Dilute sulphuric acid treated with zinc gives Hydrogen gas which bums with pop sound.

Concentrated H₂SO₄ gives SO₂ gas with zinc and the gas turns Acidified potassium dichromate paper green.

2.Barium chloride solution gives white ppt. with dilute H₂SO₄, This white ppt. is insoluble in all acids.

Concentrated H₂SO₄ and NaCl mixture when heated gives dense white fumes if glass rod dipped in Ammonia solution is brought near it.

Question 4

Name the products formed when hot and concentrated sulphuric acid reacts with the following:

(a) Sulphur

(b) NaOH

(c) Sugar

(d) Carbon

(e) Copper

Solution 4

(a) When sulphuric acid reacts with sulphur the product formed is Sulphur dioxide is formed.

S +2H₂SO₄ 3SO₂ + 2H₂O

(b) When sulphuric acid reacts with sodium hydroxide it neutralizes base to form sodium sulphate.

2NaOH + H₂SO₄ Na₂SO₄ + 2H₂O

(c) When sulphuric acid reacts with sugar it forms carbon

C₁₂ H₂₂O₁₁ 12C + 11H₂O

(d) When sulphuric acid reacts with carbon it forms carbon dioxide and sulphur dioxide gas.

C +2H₂SO₄ CO₂ + 2H₂O + 2SO₂

(e) When sulphuric acid reacts with copper it forms copper sulphate and sulphur dioxide.

Cu + H₂SO₄ CuSO₄ + 2H₂O + SO₂

Question 5

Why is:

(a) Concentrated sulphuric acid kept in air tight bottles?

(b) H₂SO₄ is not a drying agent for H₂S?

(c) Sulphuric acid used in the preparation of HCl and HNO₃? Give equation in both cases.

Solution 5

(a) Concentrated sulphuric acid is hygroscopic substance that absorbs moisture when exposed to air. Hence, it is stored in air tight bottles.

(b) Sulphuric acid is not a drying agent for H₂S because it reacts with H₂S to form sulphur.

H₂SO₄+H₂S 2H₂O+SO₂+S

(c) Concentrated sulphuric acid has high boiling point (356^oC). So, it is considered to be non-volatile. Hence, it is used for preparing volatile acids like Hydrochloric acid and Nitric acids from their salts by double decomposition.

NaCl+H₂SO₄ NaHSO₄ +HCl

NaNO₃ +H₂SO₄ NaHSO₄ +HNO₃

Question 6

What property of conc. H₂SO₄ is made use of in each of the following cases? Give an equation for the reaction in each case

(a) In the production of HCl gas when it reacts with a chlorine

(b) In the preparation of CO from HCOOH

(c) As a source of hydrogen by diluting it and adding a strip of magnesium

(d) In the preparation of sulphur dioxide by warming a mixture of conc. Sulphuric acid and copper -turnings.

(e) Hydrogen sulphide gas is passed through concentrated sulphuric acid

(f) Its reaction with (i) ethanol (ii) carbon

Solution 6

(a) Due to its reducing property. i.e, it is a non-volatile acid.

NaCl+ H₂SO₄ NaHSO₄ + HCl

(Conc.)

(b) It is a dehydrating agent.

HCOOH CO + H₂O

(c) Magnesium is present above hydrogen in the reactivity series so sulphuric acid is able to liberate hydrogen gas by reacting with magnesium strip.

Mg + H₂SO₄ MgSO₄+H₂

(d) Due to its oxidizing character

Cu +H₂SO₄ CuSO₄ +2H₂O +SO₂

(e) Due to its oxidizing property Hydrogen sulphide gas is passed through concentrated sulphuric acid to liberate sulphur dioxide and sulphur is formed.

H₂S + H₂SO₄ S + 2H₂O + SO₂

(f) 

(i) Ethanol

(ii) Carbon

Question 7

What is the name given to the salts of:

(a) Sulphurous acid

(b) Sulphuric acid

Solution 7

The name of the salt of

(a) Hydrogen sulphites and Sulphites.

(b) Sulphate and bisulphate.

Question 8

Give reasons for the following.

(a) Sulphuric acid forms two types of salts with NaOH

(b) Red brown vapours are produced when concentrated sulphuric acid is added to hydrogen bromide.

(c) A piece of wood becomes black when concentrated sulphuric acid is poured on it

(d) Brisk effervescence is seen when oil of vitriol is added to sodium carbonate.

Solution 8

(a) Two types of salts are formed when sulphuric acid reacts with NaOH because sulphuric acid is dibasic.

NaOH + H₂SO₄ NaHSO₄ + H₂O

2NaOH + H₂SO₄ Na₂SO₄ + 2H₂O

(b) When hydrogen bromide reacts with sulphuric acid the bromine gas is obtained which produce red brown vapours.

2KBr+3H₂SO₄ 2KHSO₄+SO₂+Br₂ +2H₂O

(c) A piece of wood becomes black when concentrated sulphuric acid is poured on it because it gives a mass of carbon.

(d) When sulphuric acid is added to sodium carbonate it liberates carbon dioxide which produces brisk effervescence.

Na₂CO₃+H₂SO₄ Na₂SO₄ +H₂O+CO₂

Question 9

Copy and complete the following table:

Solution 9

Question 10

Give the odour of gas evolved and name the gas produced when sodium sulphide is added to solution of HCl in water.

Solution 10

When sodium sulphide is added to solution of HCl, Hydrogen sulphide gas is produced. It has rotten egg like smell.

Question 11(a)

Which property of sulphuric acid accounts for its use as a dehydrating agent?

Solution 11(a)

Sulphuric acid is powerful dehydrating agent on account of its strong affinity towards water.

Question 11(b)

Concentrated sulphuric acid is both an oxidizing agent and a non-volatile acid. Write one equation. Each to illustrate the above mentioned properties of sulphuric acid.

Solution 11(b)

Concentrated sulphuric acid as

 i. Oxidising agent:

 The oxidising property of conc. sulphuric acid its due to the fact that on thermal decomposition, it yeilds nacent oxygen [O].

 H₂SO₄→ H₂O + SO₂ + [O] 

 ii. Non-volatile acid:

 conc. sulphuric acid has high boiling point (338°C) that why it is said to be a non volitile compound, therefore it is used for preparing volatile acids like hydrochloric acids, nitric acids from there salts by double decomposition

  H₂SO₄ + NaCl → NaHSO₄ + HCl

Question 12

Some properties of sulphuric acid are listed below. Choose the property A, B, C or D which is responsible for the reactions (i) to (v). Some properties may be repeated:

A. Acid

B. Dehydrating agent

C. Non-volatile acid

D. Oxidising agent

(i) C₁₂H₂₂O₁₁ +nH₂SO₄ 12C + 11H₂O + nH₂SO₄

(ii) S + 2H₂SO₄ 3SO₂ +2H₂O

(iii) NaCl +H₂SO₄ NaHSO₄ + HCl

(iv) CuO + H₂SO₄ CuSO₄ +H₂O

(v) Na₂CO₃ + H₂SO₄ Na₂SO₄ + H₂O +CO₂

Solution 12

(i) B

(ii) D

(iii) C

(iv) A

(v) A

Question 13

(a) Name the acid formed when sulphur dioxide dissolves in water.

(b) Name the gas released when sodium carbonate is added to a solution of sulphur dioxide.

Solution 13

(a) The acid formed when sulphur dioxide dissolves in water is sulphurous acid.

(b) Carbondioxide gas is released when sodium carbonate is added to solution of sulphur dioxide.

Question 2008

1. Dilute sulphuric acid will produce a white precipitate when added to a solution of :

 i. Copper nitrate

 ii. Zinc nitrate

 iii. Lead nitrate

 iv. Sodium nitrate

2. Identify the following substance: Liquid E can be dehydrated to produce ethene.

c. Copy and complete the following table relating to an important industrial process and its final output.

4. Making use only of substances given : dil. sulphuric acid, sodium carbonate, zinc, sodium. sulphite, lead, calcium carbonate : Give equations for the reactions by which you could obtain :

 i. hydrogen

 ii. sulphur dioxide

 iii. carbon dioxide

 iv. zinc carbonate [2 steps]

5. What property of conc. H₂SO₄ :

 i. is used in the action when sugar turns black in its presence.

 ii. allows it to be used in the preparation of HCl and HNO₃ acids.

f. Write the equations for :

 i. dil. H₂SO₄ and barium chloride

 ii. dil. H₂SO₄ and sodium sulphuide

Solution 2008

a. (C) Lead nitrate

b. Liquid E is Ethanol.

c. 

d.  

 i. Zn + dil. H₂SO₄→ ZnSO₄ + H₂ 

 ii. Na₂CO₃ + dil. H₂SO₄→ Na₂SO₄ + H₂O + CO₂

 iii. Pb(NO₃) + dil. H₂SO₄→ PbSO₄  + 2HNO₃

 iv. Zn + dil. H₂SO₄→ ZnSO₄ + H₂

ZnSO₄ + Na₂CO₃→ ZnCO₃ + Na₂SO₄

e.  

 i. The property of concentrated sulphuric acid which allows it to be used in is used in the action when sugar turns black in its presence is its dehydrating property. 

 ii. The property of concentrated sulphuric acid which allows it to be used in the preparation of hydrogen chloride and nitric acid is its non-volatility.

H₂SO₄ + BaCl₂ →  BaSO₄ + 2HCl

Question 2009

a. Name the gas evolved [formula is not acceptable]. The gas that can be oxidised to sulphur.

Solution 2009

Hydrogen Chloride

Question 2010

a. Give the equation for :

 i. Heat on sulphur with conc. sulphuric acid

 ii. Reaction of - sugar with conc. sulphuric acid.

b. Give a balanced equation for the conversion of zinc oxide to zinc sulphate.

c. Select the correct answer from A, B, C.

A. Sodium hydroxide solution

B. A weak acid

C. Dilute sulphuric acid.

The solution which liberates sulphur dioxide gas, from sodium sulphite.

Solution 2010

a.  

 i. S + H₂SO₄ → 3SO₂ +2H₂O.

 ii. C₁₂H₂₂O₁₁  + Conc. H₂SO₄ → 6C + 6H₂O

b. ZnO + H₂SO₄ → ZnSO₄ + H₂O. 

c. C) Dilute sulphuric acid. 

Question 2011

a. State your observation when - Sugar crystals are added to a hard glass test tube containing conc. sulphuric acid.

b. Choose the correct answer from the choices - The gas evolved when dil. sulphuric acid reacts with iron sulphide.

 i. Hydrogen sulphide

 ii. Sulphur dioxide

 iii. Sulphur trioxide

 iv. Vapour of sulphuric acid

c. Give a balanced equation for :

 i. Dilute sulphuric acid is poured over sodium sulphite.

 ii. Manufacture of sulphuric acid by the - contact process.

d. State the property of sulphuric acid shown by the reaction of conc. sulphuric acid when heated with

 i. potassium nitrate

 ii. carbons

Solution 2011

a. Charring of sugar takes place. Sulphuric acid dehydrates sugar leaving behind carbon which is black in colour.

b. i. Hydrogen sulphide

c. i.

ii.

d. 

 i. Non-volatile nature

 ii. as an oxidising agent

Question 2012

a. Name - The gas produced on reaction of dilute sulphuric acid with a metallic sulphide.

b. Some properties of sulphuric acid are listed below. Choose the role played by sulphuric acid as A, B, C or D which is responsible for the reactions (i) to (v). Some role/s may be repeated.

 1. Dilute acid

 2 Dehydrating agent

 3. Non-volatile acid

 4. Oxidising agent

c. Give balanced equation for the reaction : Zinc sulphide and dilute sulphuric acid.

Solution 2012

a. Hydrogen Sulphide (H₂S).

b. 

 i. (B) Dehydrating agent

 ii. (D) Oxidising agent

 iii. (C) Non-volatile acid

 iv. (A) Dilute acid

 v. (D) Oxidising agent

c. ZnS + dil.H₂SO₄→ ZnSO₄ + H₂S

Question 2013

a. State one appropriate observation for : Conc. H₂SO₄ is added to a crystal of hydrated copper sulphate.

b. In the given equation - S + 2H₂SO₄→ 3SO₂ + 2H₂O :

 Identify the role played by conc. H₂SO₄.

 i. Non-volatile acid

 ii. Oxidising agent

 iii. Dehydrating agent

 iv. None of the above

c. Give a balanced equation for : Dehydration of concentrated sulphuric acid with sugar crystals.

d. Identify the substance underlined : A dilute mineral acid which forms a white precipitate when treated with barium chloride solution.

Solution 2013

a. when Conc. H₂SO₄ is added to a crystal of hydrated copper sulphate,it removes water of crystalisation from salt.

b. ii. Oxidising agent 

c. C₁₂H₂₂O₁₁ + Conc. H₂SO₄ → 6C + 6H₂O

d. Sulphuric acid (H₂SO₄)

Question 2014

a. Write balanced equations for the following : Action of concentrated sulphuric acid on carbon.

b. Distinguish between the following pairs of compounds using the test given within brackets. Dilute sulphuric acid and dilute hydrochloric acid [using barium chloride solution].

c. State the conditions required for the following reactions to take place : The conversion of sulphur dioxide to sulphur trioxde.

d. Give one equation each to show the following properties of sulphuric acid :

 i. Dehydrating property

 ii. Acidic nature

 iii. As a non-volatile acids

Solution 2014

a. C + H₂SO₄ → CO₂ + 2H₂O + 2SO₂.

b. Sulphuric acid precipitates the insoluble sulphate of barium from the solution of barium chloride.

BaCl₂ + H₂SO₄→ BaSO₄ + 2HCl

Dilute HCl does not react with barium chloride solution, and thus, no precipitate is produced in the reaction.

c. Two conditions for the conversion of sulphur dioxide to sulphur trioxide is as follows: 

Temperature: 450-500° C Catalyst: V₂O₅

d.  

 i. Dehydrating property of sulphuric acid:

H₂SO₄ has a great affinity for water, and therefore, it acts as a dehydrating agent. 

 ii. Acidic nature of sulphuric acid:

 It acts as a strong dibasic acid.

 H₂SO₄→ 2Hi⁺ + SO₄^2-

 It reacts with metals, metal oxides, metal hydroxides, carbonates etc. to form metallic sulphates and hydrogen at ordinary temperature.

 Mg + H₂SO₄→ MgSO₄ + H₂↑ 

 CuO + H₂SO₄→ CuSO₄ + H₂O

 2NaOH + H₂SO₄→ Na₂SO₄ + 2H₂O

 ZnCO₃ + H₂SO₄→ ZnSO₄ + H₂O + CO₂↑ 

 iii. As a non-volatile acid:  

It has a high boiling point, so it is used to prepare volatile acids such as HCl, HNO₃ and acetic acid from their salts.

NaCl + H₂SO₄→ NaHSO₄ + HCl

NaNO₃ + H₂SO₄→ NaHSO₄ + HNO₃

CH₃COONa + H₂SO₄→ NaHSO₄  + CH₃COOH

Question 2015

(a) In the manufacture of sulphuric acid by contact process, give the equations for the conversion of sulphur trioxide to sulphuric acid.

(b) Give equations for the action of sulphuric acid on

(i) Potassium hydrogen carbonate

(ii) Sulphur

(c) Identify the acid in each case.

(i) Acid which produces sugar charcoal from sugar

(ii) Acid on mixing with lead nitrate solution produces white ppt. which is insoluble even on heating

Solution 2015

(a) In the contact process for the manufacture of sulphuric acid, the equations for the conversion of sulphur trioxide to sulphuric acid are

 SO₃ + H₂SO₄ → H₂S₂O₇

  (oleum or pyrosulphuric acid)

 H₂S₂O₇ + H₂O → 2H₂SO₄

(b)  

(i) Action of sulphuric acid on potassium hydrogen carbonate

 2KHCO₃+ H₂SO₄→ K₂SO₄+ 2H₂O + 2CO₂↑ 

(ii) Action of sulphuric acid on sulphur

 S + 2H₂SO₄→ 3SO₂ + 2H₂O

(c)  

(i) Concentrated sulphuric acid

(ii) Concentrated sulphuric acid

Chapter 10 - Study of Compounds C. Nitric Acid Exercise Intext 1

Question 1

What is: (a) aqua fortis (b) aqua regia (c) Fixation of Nitrogen?

Solution 1

(a) Aqua fortis: Nitric acid is called aqua fortis. Aqua fortis means strong water. It is so called because it reacts with nearly all metals.

(b) Aqua Regia: Conc. Nitric acid (1part by volume) when mixed with conc. Hydrochloric acid (3 parts by volume) gives a mixture called aqua regia. It means Royal water.

HNO₃ +3HCl NOCl +2H₂O +2[Cl]

(c) Fixation of Nitrogen: The conversion of free atmospheric nitrogen into useful nitrogenous compounds in the soil is known as fixation of atmospheric nitrogen.

Question 2

During thunderstorm, rain water contains nitric acid. Explain with reactions.

Solution 2

During lightning discharge, the nitrogen present in the atmosphere reacts with oxygen to form nitric oxide.

N₂+ O₂ 2NO

Nitric oxide is further oxidized to nitrogen dioxide.

2NO +O₂ 2NO₂

The nitrogen dioxide dissolves in atmospheric moisture in the presence of oxygen of the air and forms nitric acid which is washed down by the rain and combines with the salt present on the surface of the earth.

4NO₂+2H₂O +O₂ 4HNO₃

Question 3

Ammonia is used in the Ostwald process.

(a) Give the source of reactants used in the process.

(b) Name the catalyst used in the process.

(c) Name the oxidising agent used in this process.

(d) What is the ratio of ammonia and air taken in this process?

(e) Why is quartz used in the process?

Solution 3

(a) A mixture of air and dry ammonia in the ratio of 10:1 by volume

(b) Platinum gauze

(c) Oxygen

(d) ammonia : dry air :: 1 : 10

(e) Quartz is acid resistant. When packed in layers, it helps in dissolving nitrogen dioxide uniformly in water.

Question 4

(a) Write a balanced chemical equation for the laboratory preparation of nitric acid.

(b) In the preparation of nitric acid from KNO₃, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why?

(c) Conc. Nitric acid prepared in the laboratory is yellow in colour .Why? How is this colour removed?

(d) Give reasons for the following:

In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitrate should not be heated very strongly above 200^oC.

Solution 4

(a)Chemical equation is:

KNO₃ +H₂SO₄ KHSO₄ +HNO₃

(b) Concentrated hydrochloric acid cannot replace Conc. Sulphuric acid for the preparation of nitric acid because hydrochloric acid is volatile acid and hence nitric acid vapours will carry HCl vapours.

(c) Conc. Nitric acid prepared in the laboratory is yellow in colour due to the dissolution of reddish brown coloured nitrogen dioxide gas in acid. This gas is produced due to the thermal dissociation of a portion of nitric acid.

4HNO₃ 2H₂O + 4NO₂ + O₂

The yellow colour of the acid is removed:

If dry air or CO₂ is bubbled through the yellow acid, the acid turns colourless because it drives out NO₂ from warm acid which is further oxidized to nitric acid.

By addition of excess of water, nitrogen dioxide gas dissolves in water and thus the yellow colour of the acid is removed.

(d)The temperature of the mixture of concentrated sulphuric acid and sodium nitrate should not exceed 200^oC because sodium sulphate formed at higher temperature forms a hard crust which sticks to the walls of the retort and is difficult to remove. At higher temperature nitric acid may also decompose.

NaNO₃ + NaHSO₄ Na₂SO₄ + HNO₃

Question 5(a)

Nitric acid cannot be concentrated beyond 68% by the distillation of dilute solution of HNO₃. State the reason.

Solution 5(a)

Nitric acid forms a constant boiling mixture with water containing 68% acid. This mixture boils constantly at constant boiling point without any change in its composition. At this temperature, the gas and the water vapour escape together. Hence the composition of the solution remains unchanged. So nitric acid cannot be concentrated beyond 68% by distillation of dilute solution of HNO₃.

Question 5(b)

What is passive iron? How is passivity removed?

Solution 5(b)

Iron becomes inert when reacted with nitric acid due to the formation of extremely thin layer of insoluble metallic oxide which stops the reaction.

Passivity can be removed by rubbing the surface layer with the sand paper or by treating with strong reducing agent.

Question 6

Name the product formed when :

(a) Carbon and conc. Nitric acid is heated

(b) Dilute HNO₃ is added to copper.

Solution 6

(a) When carbon and conc. Nitric acid is heated the products formed are Carbon dioxide, Nitrogen dioxide and water.

C + 4HNO₃ CO₂ + 2H₂O +4NO₂

(b) Copper when reacts with dilute HNO₃ forms Copper nitrate, Nitric oxide and water.

3Cu + 8 HNO₃ 3Cu(NO₃) ₂ +4H₂O + 2NO

Question 7

Give two chemical equations for each of the following:

(a) Reactions of nitric acid with non-metals.

(b) Nitric acid showing acidic character.

(c) Nitric acid acting as oxidizing agent.

Solution 7

(a) Reaction of nitric acid with non-metals:

C + 4HNO₃ CO₂ + 2H₂O + 4 NO₂

S + 6 HNO₃ H₂SO₄ + 2H₂O + 6 NO₂

(b) Nitric acid showing acidic character:

K₂O + 2HNO₃ 2KNO₃ + H₂O

ZnO + 2HNO₃ Zn(NO₃)₂ + H₂O

(c) Nitric acid acting as oxidizing agent

P₄ +20HNO₃ 4H₃PO₄ + 4H₂O + 20NO₂

3Zn +8HNO₃ 3Zn(NO₃)₂ +4H₂O +2NO

Question 8

Write the balanced equations and name the products formed when :

(a) Sodium hydrogen carbonate is added to nitric acid.

(b) Cupric oxide reacts with dilute nitric acid.

(c) Zinc reacts with dilute nitric acid.

(d) Concentrated nitric acid is heated.

Solution 8

(a) When Sodium hydrogen carbonate is added to nitric acid sodium nitrate, carbon dioxide and water is formed.

NaHCO₃ + HNO₃ NaNO₃ +H₂O +CO₂

(b) When Cupric oxide reacts with dilute nitric acid, it forms Copper nitrate.

CuO +2HNO₃ Cu(NO₃)₂ +H₂O

(c) Zinc reacts with nitric acid to form Zinc nitrate, nitric oxide and water.

3 Zn +8HNO₃ 3Zn(NO₃)₂ +4H₂O +2NO

(d) 4HNO₃ 2H₂O + 4NO₂ + O₂

Question 9

How will you prepare the following from nitric acid?

(a) Sodium nitrate

(b) Copper nitrate

(c) Lead nitrate

(d) Magnesium nitrate

(e) Ferric nitrate

(f) Aqua regia

Solution 9

(a) Sodium nitrate:

NaOH + HNO₃ NaNO₃ +H₂O

Sodium hydroxide reacts with nitric acid to form sodium nitrate.

(b) Copper nitrate:

CuO + 2HNO₃ Cu(NO₃)₂ + H₂O

Copper oxide reacts with nitric acid to form copper nitrate.

(c) Lead nitrate:

Pb +4HNO₃ Pb(NO₃)₂ +2H₂O +2NO₂

Lead reacts with conc. nitric acid to form lead nitrate.

(d) Magnesium nitrate:

Mg +2HNO₃ Mg(NO₃)₂ +H₂

Magnesium with dil. nitric acid to form magnesium nitrate.

(e) Ferric nitrate:

Fe + 6HNO₃ Fe(NO₃)₃ +3H₂O + 3NO₂

Iron reacts with conc. nitric acid to form ferric nitrate.

(f) Aqua regia:

HNO₃ + 3HCl NOCl +2H₂O +2[Cl]

Nitric acid reacts with hydrochloric acid to form a mixture called aqua regia.

Question 10

Write the equation for following conversions A, B , C and D.

Solution 10

A: Copper can be converted into copper nitrate.

3Cu + 8HNO₃ 3Cu(NO₃)₂ + 4H₂O+ 2NO

B:2Cu(NO₃)₂ 2CuO + 4NO₂ + O₂

C:2Cu+ O₂ 2CuO

D:By reduction

2CuO + C 2Cu + CO₂

Question 11

Correct the following , if required:

(a) HNO₃ is a strong reducing agent.

(b) NaNO₃ gives NO₂ and O₂ on heating.

(c) Constant boiling nitric acid contains 80% nitric acid by weight.

(d) Nitric acid remains colourless even when exposed to light.

Solution 11

(a) HNO₃ is strong oxidizing agent.

(b) NaNO₃ gives NaNO₂ and oxygen on heating.

(c) Constant boiling nitric acid contains 68% nitric acid by weight.

(d) Nitric acid turns yellow solution when exposed to light.

Chapter 10 - Study of Compounds C. Nitric Acid Exercise Ex. 10

Question 1

Choose the correct answer:

(a) The nitrate salt which does not give a mixture of NO₂ and O₂ on heating is:

(i) AgNO₃

(ii) KNO₃

(iii) Cu(NO₃)₂

(iv) Zn(NO₃)₂

(b) The chemical used in the brown ring test is:

(i) CuSO₄

(ii) FeSO₄

(iii) Fe₂(SO₄)₃

(iv) ZnSO₄

(c) Lead nitrate decomposes on heating to give:

(i) NO

(ii) N₂O

(iii) NO₂

(iv) N₂O₅

Solution 1

(a)KNO₃

(b) FeSO₄

(c) NO₂

Question 2

Name:

(a) A nitrate of metal which on heating does not give nitrogen dioxide.

(b) A nitrate which on heating leaves no residue behind.

(c) A metal nitrate which on heating is changed into metal oxide.

(d) A metal nitrate which on heating is changed into metal.

(e) A solution which absorbs nitric oxide.

(f) The oxide of nitrogen which turns brown on exposure to air. How is it prepared?

Solution 2

(a) Sodium nitrate

2NaNO₃ 2NaNO₂ +O₂

(b) A nitrate which on heating leaves no residue behind- Ammonium nitrate.

(c) A metal nitrate which on heating is changed into metal oxide- Calcium nitrate

(d) A metal nitrate which on heating is changed into metal- Silver nitrate

(e) A solution which absorbs nitric oxide- Freshly prepared ferrous sulphate

(f) The oxide of nitrogen which turns brown on exposure to air. - nitric oxide

By catalytic oxidation of ammonia.

4 NH₃ + 5 O₂ 4 NO + 6 H₂O + Heat

Question 3

Mention three important uses of nitric acid. Give the property of nitric acid involved in the use.

Solution 3

Three important uses of Nitric acid and the property of nitric acid involved is:

Question 4

(a) Explain with the help of a balanced equation, the brown ring test for nitric acid.

(b) Why is freshly prepared ferrous sulphate solution used for testing the nitrate radical in the brown ring test?

Solution 4

(a) Brown ring test

Procedure:

(i) Add freshly prepared saturated solution of iron (II)sulphate to the aq. solution of nitric acid.

(ii) Now add conc. Sulphuric acid carefully from the sides of the test tube, so that it should not fall drop wise in the test tube.

(iii) Cool the test tube in water.

(iv) A brown ring appears at the junction of the two liquids.

(b)A freshly prepared ferrous sulphate solution is used because on exposure to the atmosphere, it is oxidized to ferric sulphate which will not give the brown ring.

Question 5

From the following list of substances, choose one substance in each case which matches the description given below:

Ammonium nitrate, Calcium hydrogen carbonate, copper carbonate, lead nitrate, potassium nitrate, sodium carbonate, sodium hydrogen carbonate, zinc carbonate.

(a) A nitrate which gives off only oxygen when heated.

(b) A nitrate which on heating decomposes into dinitrogen oxide (nitrous oxide) and steam.

(c) A nitrate which gives off oxygen and nitrogen dioxide when heated.

Solution 5

(a) Potassium nitrate

(b) Ammonium nitrate

(c) Lead nitrate

Question 6

The action of heat on the blue crystalline solid X gives a reddish brown gas Y, a gas which re-lights a glowing splint and leaves a black residue. When gas Z, which has a rotten egg smell, is passed through a solution of X, a black ppt. is formed.

a. Identify X, Y and Z.

b. Write the equation for action of heat on X.

c. Write the equation between solution X and gas Z.

Solution 6

a. X is copper nitrate.

Y is nitrogen dioxide.

Z is hydrogen sulphate.

b. 

c. 

Question 7

X, Y and Z are three crystalline solids which are soluble in water and have common anion.

To help you to identify X, Y and Z you are provided with the following experimental observations. Copy and complete the corresponding inferences in (a) to (f).

(a) A reddish -brown gas is obtained when X, Y and Z are separately warmed with concentrated sulphuric acid and copper turning added to the mixture.

Inference 1: The common anion is the _____ion.

(b) When X is heated, it melts and gives off only one gas which re-lights a glowing splint.

Inference2: The cation in X is either _____ or _____.

(c) The action of heat on Y produces a reddish brown gas and yellow residue which fuses with glass of the test tube.

Inference3: The metal ion present in Y is the____ ion.

(d) When Z is heated, it leaves no residue. Warming Z with sodium hydroxide solution liberates a gas which turns moist red litmus paper blue.

Inference4: Z contains the ____ cation.

(e) Write the equations for the following reactions:

(1)X and concentrated sulphuric acid (below 200^oC). (One equation only for either of the cations given in Inference 2)

(2)Action of heat on Y.

(3) Concentrated nitric acid is added to copper turnings kept in a beaker.

Solution 7

(a) Nitrate.

(b) Sodium or potassium

(c) Lead

(d) Ammonia

(e) (1)KNO₃ + H₂SO₄ KHSO₄ + HNO₃

     (2) 2Pb(NO₃)₂ 2PbO + 4NO₂ +O₂

     (3) Cu +4HNO₃ Cu(NO₃)₂ +H₂O +2NO₂

Question 8

a. Dilute nitric acid is generally considered a typical acid except for its reaction with metals. In what way is dilute nitric acid different from other acids when it reacts with metals?

b. Write the equation for the reaction of dilute nitric acid and conc. nitric acid with copper.

Solution 8

a. Dilute nitric acid is generally considered a typical acid except for its reaction with metals because it does not liberate hydrogen. It is a powerful oxidising agent, and nascent oxygen formed oxidises hydrogen in water.

b. 

 i. Reaction of dilute nitric acid with copper:

  3Cu + 8HNO₃→ 3Cu(NO₃) + 4H₂O + 2NO

 ii. Reaction of conc. nitric acid with copper:

 Cu + 4HNO₃→ Cu(NO₃) + 2H₂O + 2NO₂

Question 9

Explain why:

a. Only all-glass apparatus should be used for the preparation of nitric acid by heating concentrated sulphuric acid and potassium nitrate.

b. Nitric acid is kept in a reagent bottle for a long time.

Solution 9

a. The glass apparatus is purposely used because nitric acid vapours are highly corrosive in nature and corrode cork, rubber etc. if used as a stopper.

b. Pure nitric acid is unstable to heat or sunlight. In the presence of sunlight, it decomposes even at room temperature.

 Nitric acid stored in a bottle turns yellow. This colour is due to dissolved NO₂ in HNO₃. To avoid decomposition, nitric acid is normally stored in coloured bottles.

Question 10

The figure given below illustrates the apparatus used in the laboratory preparation of nitric acid.

a. Name A (a liquid), B (a solid) and C (a liquid). (Do not give the formulae).

b. Write an equation to show how nitric acid undergoes decomposition.

c. Write the equation for the reaction in which copper is oxidised by concentrated nitric acid.

Solution 10

a. A (a liquid): Conc.  sulphuric acid

B (a solid): Sodium nitrate

C (a liquid): Nitric acid

b. 

c. 

i. Reaction of dilute nitric acid with copper:

  3Cu + 8HNO₃→ 3Cu(NO₃) + 4H₂O + 2NO

ii. Reaction of conc. nitric acid with copper:

  Cu + 4HNO₃→ Cu(NO₃) + 2H₂O + 2NO₂

Question 11

a. A dilute acid B does not normally give hydrogen when reacted with metals but does give a gas when reacts with copper. Identify B. Write the equation with copper.

b. Complete the table:

c. What is the property of nitric acid which allows it to react with copper?

Solution 11

a. The dilute acid is nitric acid.

 Reaction of dilute nitric acid with copper:

 3Cu + 8HNO₃ →3Cu(NO₃) + 4H₂O + 2NO

b. 

c. Its oxidising property allows it to react with copper.

Question 2012

a. Name the gas produced when copper reacts with conc. HNO₃.

b. State your observation: Zinc nitrate crystals are strongly heated.

c. Correct the statement: Magnesium reacts with nitric acid to liberate hydrogen gas.

d. Iron is rendered passive with fuming HNO₃. Give reason.

e. Give the balanced equation for dilute nitric acid and copper carbonate.

Solution 2012

a. Nitrogen dioxide gas is produced when copper reacts with conc. HNO₃.

b. When zinc nitrate crystals are strongly heated, they decompose into yellow-coloured zinc oxides and nitrogen dioxides, and oxygen gas is liberated.

c. Very dilute (about 1%) acid reacts with magnesium at room temperature to give magnesium nitrate and hydrogen gas.

d. Iron is rendered passive with fuming HNO₃. This is due to the formation of insoluble metallic oxide which stops the reaction.

e. 

Question 2013

a. Identify the gas evolved when

 i. Sulphur is treated with conc. nitric acid.

 ii. A few crystals of KNO₃ are heated in a hard glass test tube.

b. State two relevant observations for: Lead nitrate crystals are heated in a hard glass test tube.

c. Give a balanced equation for: Oxidation of carbon with conc. HNO₃.

Solution 2013

a. 

 i. When sulphur is treated with conc. nitric acid, it produces nitrogen dioxide gas.

 ii. When a few crystals of KNO₃ are heated in a hard glass test tube, it decomposes to form KNO₂, and O₂ gas is librated.

b. First, it decomposes with slight decrepitation, and second, it is reddish brown in colour when hot. After cooling, it turns yellow and fuses in glass.

c. 

Question 2014

a. Fill in the blank:

 Cold dil. nitric acid reacts with copper to form (hydrogen, nitrogen dioxide, nitric oxide).

b. Give balanced equations for the following:

 i. Laboratory preparation of nitric acid.

 ii. Action of heat on a mixture of copper and nitric acid.

Solution 2014

a. Nitric oxide

b. 

 i. 

 ii. Reaction of dilute nitric acid with copper:

3Cu + 8HNO₃→ 3Cu(NO₃) + 4H₂O + 2NO Reaction of conc. nitric acid with copper:

Cu + 4HNO₃→ Cu(NO₃) + 2H₂O + 2NO₂

Question 2015 (a)

(a) Identify the acid

 (i) The acid which is used in the preparation of a non-volatile acid.

 (ii) The acid which is prepared by catalytic oxidation of ammonia. 

Solution 2015 (a)

(i) Sulphuric acid 

(ii) Nitric acid 

Question 2015 (b)

State one relevant observation for each of the following:

When crystals of copper nitrate are heated in a test tube. 

Solution 2015 (b)

Reddish brown nitrogen dioxide gas is released, and the residue left behind is black copper oxide. 

Question 2015 (c)

Explain the following:

(i) Dilute nitric acid is generally considered a typical acid but not so in its reaction with metals.

(ii) Concentrated nitric add appears yellow when it is left standing in a glass bottle.

(iii) An all glass apparatus is used in the laboratory preparation of nitric acid.

Solution 2015 (c)

(i) Dilute nitric acid is generally considered a typical acid but not in its reaction with metals, because the action of nitric acid on metals depends on the temperature and concentration of nitric acid. These conditions are not required in case of hydrochloric acid or sulphuric acid.

(ii) Although pure concentrated nitric acid is colourless, it appears yellow when left standing in a glass bottle due to the dissolution of reddish brown nitrogen dioxide gas in the acid. Nitrogen dioxide is produced because of the thermal decomposition of a portion of nitric acid.

 4HNO₃ → 2H₂O + 4NO₂ + O₂

(iii) An all-glass apparatus is used in the laboratory preparation of nitric acid, because nitric acid vapour corrodes rubber and cork.

Chapter 7 - Metallurgy Exercise Ex. 7C

Question 1

State the position of aluminium in the periodic table.

Solution 1

Position in the Periodic Table: Period 3,Group IIIA(13)

Question 2

(a) Give the chemical names and formulae of the main ores of (i) aluminium, (ii) iron and (iii) zinc.

(b) Which impurities are present in bauxite?

(c) What is red mud, how is it removed?

Solution 2

(a)

(i) Ores of aluminium

(ii)Ores of iron

(b) Bauxite ore contains approximately 60% aluminium oxide. The rest being sand, ferric oxide and titanium oxide.

(c) Red mud consists of ferric oxide, sand etc. left after bauxite dissolves in NaOH forming sodium aluminate and is removed by filtration.

(i)Ores of aluminium

Question 3

In order to obtain 1 tonne of aluminium, the following inputs are required: 4 tonnes of bauxite, 150 kg of sodium hydroxide and 600 kg of graphite. The aluminium compound in bauxite is aluminium oxide and the main impurity is iron (III) oxide. Aluminium is obtained by the electrolysis of aluminium oxide dissolved in cryolite.

a. When bauxite is treated with sodium hydroxide solution, what happens to

 i. the aluminium oxide

 ii. the iron (III) oxide

b. 

 i. Name the process used for the purification of bauxite.

 ii. Write the equation for the action of heat on aluminium hydroxide.

c.  

 i. Write the formula of cryolite.

 ii. Write down the word which correctly completes the following sentence. By dissolving aluminium oxide in cryolite a (conducting/non-conducting) solution is produced.

 iii. Why is so much graphite required for the electrolytic process?

 iv. Write the equation for the reaction which takes place at the cathode.

 v. What is cathode made up of?

Solution 3

a. 

i. Aluminium oxide dissolves in sodium hydroxide and forms sodium meta aluminate leaving behind insoluble impurities consisting of ferric oxide which is removed by filtration.

b.  

i. The process used for the purification of bauxite is Baeyer's process.

ii. Action of heat on aluminium hydroxide:

c.  

i.  Formula of cryolite is

ii. By dissolving aluminium oxide in cryolite, a  conducting solution is produced.

iii. Thick graphite rods are used as the anode. The anode has to be replaced from time to time, as it gets oxidised by evolved oxygen.

iv. Reaction at the cathode:

v. The cathode is made of carbon.

Question 4

Aluminium is extracted from its chief ore, bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.

a. Write three balanced equations for the purification of bauxite.

b. Name a chemical used for dissolving aluminium oxide. In which state of subdivision is the chemical used?

c. Write an equation for the reaction which takes place at the anode during the extraction of aluminium by the electrolytic process.

d. Mention one reason for the use of aluminium in thermite welding.

Solution 4

a. Balanced equations for the purification of bauxite:  

b. Chemicals used for dissolving aluminium oxide: Fluorspar and cryolite Alumina 20%, cryolite 60%, fluorspar 20%

C.At anode: Al - 3e^- → Al³⁺

d. Aluminium is a good reducing agent used for thermite welding.

Question 5

a. A to F below relate to the source and extraction of either zinc or aluminium:

A. Bauxite

B. Coke

C. Cryolite

D. Froth floatation

E. Sodium hydroxide solution

F. Zinc blende

 i. Write down the three letters each from the above list which are relevant to

1. Zinc

2. Aluminium

 ii. Fill in the blanks using the most appropriate words from A to F.

1. The ore from which aluminium is extracted must first be treated with ………………. so that pure aluminium oxide can be obtained.

2. Pure aluminium oxide is dissolved in ….. to make a conducting solution.

 iii. Write the formula of cryolite.

Solution 5

a. 

 i. 

1. Zinc

 Zinc blende, Froth flotation, Coke

2. Aluminium

 Cryolite, Bauxite, Sodium hydroxide solution

b. 

 ii. 

1. The ore from which aluminium is extracted must first be treated with sodium hydroxide solution so that pure aluminium oxide can be obtained.

2. Pure aluminium oxide is dissolved in cryolite  to make a conducting solution.

 iii. Formula of cryolite is Na₃AlF₆.

Question 6

Explain with reasons:

(a) In the electrolytic reduction of alumina, the graphite anode is gradually consumed.

                                                               or

      Why the anode has to be replaced in this process?

(b) Roasting is carried out on sulphide ores and not on carbonates ores.

(c) Carbon can reduce lead oxide but not aluminium oxide.

(d) Electrolytic reduction is done to obtain aluminium.

(e) Why 'food containing iron salts' should not be cooked in aluminium utensils?

(f) A neutral gas other than oxygen is formed at the anode during electrolysis of fused alumina.

Solution 6

(a) In the electrolytic reduction of alumina, the graphite (anode) is oxidized by oxygen to CO and further forms CO₂, so it is consumed and has to be replaced from time to time.

2C + O₂ 2CO

2CO + O₂ 2CO₂

(b) Roasting provides oxygen to convert metallic sulphides into metallic oxide and SO₂ which takes place when heated in excess of air.

Carbonate is converted into oxide by loss of CO₂ which takes place in the absence of air and when heated strongly.

(c) Aluminium has a great affinity towards oxygen and so cannot be reduced by carbon or carbon monoxide or hydrogen whereas lead oxide can be easily reduced to metal lead by carbon.

PbO + C Pb + CO

(d) Aluminium oxide is a very stable compound because of its great affinity for oxygen. It is not reduced easily by common reducing agents such as carbon or hydrogen. Hence, electrolytic reduction is done to obtain aluminium.

(e) Aluminium comes before iron in the metal activity series so it can displace iron from iron salts; thus, food containing iron salts should not be cooked in aluminium utensils.

(f) An anode is made of carbon which gets oxidised in the presence of oxygen to form carbon monoxide which is a neutral gas. 

2C + O₂ → 2CO

An anode is made of carbon which gets oxidised in the presence of oxygen to form carbon monoxide which is a neutral gas. 

2C + O₂ → 2CO

Question 7

For each substance listed below, explain its significance in the extraction of aluminium:

a. Bauxite

b. Sodium hydroxide

c. Cryolite

d. Graphite

Solution 7

a. Bauxite Aluminium is extracted from bauxite ore. It contains 60% Al₂O₃

b. Sodium hydroxide The ore from which aluminium is extracted must first be treated with sodium hydroxide solution so that pure aluminium oxide can be obtained.

c. Cryolite It lowers the fusion temperature and enhances conductivity.

d. Graphite Thick graphite rods are used as the anode in electrolytic reduction.

Question 8

Distinguish between electrolytic methods of reduction and refining.

Solution 8

Electrolytic Reduction

(i)It is removal of oxide or halide from a metal.

(ii)Oxides of highly active metals like Na,K,Ca,Mg,Al are reduced by electrolytic reduction of their fused salts.

(iii)Oxides of these metals have great affinity for oxygen than carbon and cannot be reduced by carbon or CO or hydrogen.

Electrolytic refining of metals is the separation of residual impurities like Si and phosphorus.

(i)Presence of other metals and non-metals like Si and phosphorus.

(ii)Unreduced oxides and sulphides of metals.

It depends upon:

(i)Nature of metal

(ii)Purpose for which metal is to be obtained.

(iii)Nature of impurities present.

Impure metal is made anode while a thin sheet of pure metal is made cathode and electrolyte used is a salt of solution of a metal to be refined.

Question 9

Give three ways in which the metal zinc differs from the non-metal carbon. At least one of the differences must be a chemical difference.

Solution 9

The three ways in which metal zinc differs from the non-metal carbon is:

1. Zinc has a valency 2 and carbon has valency 4.

2. Zinc does not form hydride but carbon does (CH₄).

3. Oxides of zinc are amphoteric (ZnO) whereas oxides of carbon are acidic (CO₂) and neutral (CO).

Question 10

a. Aluminium is a more active metal than iron but suffers less corrosion. Why?

b. Explain and give reasons why aluminium vessels should not be cleaned with powders containing alkalis.

Solution 10

a. When aluminium is exposed to the atmospheric air, it combines with oxygen and a film of aluminium oxide (Al₂O₃) is formed at the surface. This hard, tightly adhering film of aluminium oxide prevents corrosion.

b. Aluminium vessels should not be cleaned with powders containing alkalis because aluminium reacts with alkalis to produce meta aluminate.

Question 11

Fill in the blanks:

(a) During the concentration of bauxite ore, aluminium goes in ………. (soluble/insoluble) part because of its …………… (acidic/basic/amphoteric) nature.

(b) In Hoope's process, pure aluminium is collected at the …………… (top/bottom) of the electrolytic cell.

Solution 11

(a) During the concentration of bauxite ore, aluminium goes in the soluble part because of its amphoteric nature.

(b) In Hoope's process, pure aluminium is collected at the top of the electrolytic cell.

Chemistry Class X Chapter 6 - Electrolysis Exercise Intext 1

Question 1

Fill in the blanks:

(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when ____ or when _____.

(b) Molten lead bromide conducts electricity. It is called an____. It is composed of lead ____ and bromide _____. The lead ions are ____charged and are called_____. The bromide ____ are _____ charged and are called ______.

(c) Substances which conduct electricity in the solid state are generally ______.

(d) The electron releasing tendency of zinc is _____than that of copper.

(e) A solution of HCl gas in water conducts electricity because _____, but a solution of HCl gas in toluene does not conduct an electric current because_____.

(f) Pure water consists entirely of ………….. (ions/molecules).

(g) We can expect that pure water …………… (will/will not) normally conduct electricity.

(h) Electrolysis is the passage of……………. (electricity/electrons) through a liquid or a solution accompanied by a ………….. (physical/chemical) change.

Solution 1

(a) Powdered sodium chloride (common salt) does not conduct an electric current, but it does so when dissolved in water or when melted.

(b) Molten lead bromide conducts electricity .It is called an electrolyte. It is composed of lead ions and bromide ions. The lead ions are positively charged and are called cations. The bromide ions are negatively charged and are called anions.

(c) Substances which conduct electricity in the solid state are generally metals.

(d) The electron releasing tendency of zinc is more than that of copper.

(e) A solution of HCl gas in water conducts electricity because it ionizes , but a solution of HCl gas in toluene does not conduct an electric current because it does not ionize in toluene.

 (f) Pure water consists entirely of molecules.

 (g) We can expect that pure water will not normally conduct electricity.

 (h) Electrolysis is the passage of electricity through a liquid or a solution accompanied by a chemical change.

Question 2

Define the following terms:

(a) Electrolysis

(b) Non-electrolyte

(c) Cation and an anion

(d) Weak electrolyte

Solution 2

(a) Electrolysis: It is the process of decomposition of a chemical compound in aqueous solutions or in molten state accompanied by a chemical change using direct electric current.

(b) Non-electrolyte: It is a compound which neither in solution nor in the molten state allows an electric current to pass through it.

(c) Cation and anion: Atoms which carry positive charge are called cations.

Atoms which carry negative charge are called anions.

(d) Weak electrolyte: Electrolytes which allow small amount of electricity to flow through them and are partially dissociated in fused or aqueous solution are called weak electrolyte.

Question 3

What is the difference between:

(a) Modern explanation and Arrhenius explanation for the theory of electrolysis

(b) Electrolytic dissociation and ionization

(c) A cation and an anion

(d) Electrolytic dissociation and thermal dissociation

(e) Strong electrolyte and weak electrolyte

Solution 3

(a) Difference between Modern explanation and Arrhenius explanation for the theory of electrolysis:

Arrhenius considered that water ionizes electrolytes but Modern theory explained that electrolytes are ionic even in solid state and their ions are held by strong electrostatic forces which make them immobile. Water renders these ions mobility by breaking the electrostatic forces.

(b) Difference between electrolytic dissociation and ionization :

(c) A cation and anion:

(d) Electrolytic dissociation and thermal dissociation:

Electrolytic dissociation is the dissociation of an electrovalent compound into ions in the fused state or in aqueous solution state.

Thermal dissociation: Reversible breakdown of a chemical compound into simpler substances by heating it. The splitting of ammonium chloride into ammonia and hydrogen chloride is an example. On cooling, they recombine to form the salt.

(e)

Question 4

Name:

(a) A salt which is a weak electrolyte

(b) A base which is a weak electrolyte

(c) An inert electrode and an active electrode

(d) A positively charged non-metallic ion

(e) The electrode at which reduction occurs

(f) A non-metallic element which is a conductor of electricity

Solution 4

(a) Sodium carbonate

(b) NH₄OH

(c) An inert electrode: graphite and Active electrode: silver

(d) H⁺

(e) Electrode is cathode

(f) Graphite

Question 5

Electrolysis is a redox process. Explain.

Solution 5

Electrolysis is a redox process. The reaction at the cathode involves reduction of cations as they gain of electrons while the reaction at anode involves oxidation of anions as they loss of electrons to become neutral.

Example: Dissociation of sodium chloride during electrolysis.

NaCl Na⁺ + Cl^-

Cathode : Na⁺ + e- Na(reduction)

Cl^- - e^- Cl(oxidation)

Cl + Cl Cl₂

Overall reaction:2NaCl 2Na + Cl₂

Question 6

Classify the following substances under three headings:

a. strong electrolytes

b. weak electrolytes

c. non-electrolytes

Acetic acid, ammonium chloride, ammonium hydroxide, carbon tetrachloride, dilute hydrochloric acid, sodium acetate, dilute sulphuric acid.

Solution 6

a. dilute hydrochloric acid, dilute sulphuric acid, sodium acetate

b. acetic acid, ammonium hydroxide

c. carbon tetrachloride

Question 7

Explain why:

a. Cu, though a good conductor of electricity, is a non-electrolyte.

b. Solid sodium chloride does not allow electricity to pass through.

Solution 7

a. Copper metal is a solid and has no mobile ions, whereas an electrolyte should dissociate into oppositely charged ions to conduct the electric current.

b. In solid sodium chloride, Na⁺ and Cl^- ions are not mobile to conduct the electric current.

Question 8

Choose A, B, C or D to match the descriptions

(i) to (v) below. Some letters may be repeated.

A. non-electrolyte

B. strong electrolyte

C. weak electrolyte

D. metallic conductor

 i. Molten ionic compound

 ii. Carbon tetrachloride

 iii. An aluminium wire

iv. A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules.

 v. A sugar solution with sugar molecules and water molecules.

Solution 8

 i. Molten ionic compound - strong electrolyte

 ii. Carbon tetrachloride - non-electrolyte

 iii. Aluminium wire - metallic conductor

iv. A solution containing solvent molecules, solute molecules and ions formed by the dissociation of solute molecules. - weak electrolyte

 v. A sugar solution with sugar molecules and water molecules. - non-electrolyte

Chapter 6 - Electrolysis Exercise Intext 2

Question 1

Name two substances in each case:

(a) Contain only molecules

(b) Contain only ions

(c) Contain ions as well as molecules

Solution 1

(a) Glucose, Kerosene

(b) NaCl and NaOH

(c) CH₃COOH and NH₄OH

Question 2

Select the ion in each case that would get selectively discharged from the aqueous mixture of the ions listed below:

Solution 2

(a) OH^-

(b)Ag⁺

Question 3

(a) Among Zn and Cu, which would occur more readily in nature as metal and which as ion?

(b) Why cannot we store AgNO₃ solution in copper vessel?

(c) Out of Cu and Ag, which is more active?

Solution 3

(a) Zn occurs readily as ion whereas Cu occurs more readily as metal in nature.

(b) Copper is above silver in the electrochemical series and is thus more reactive than silver. So, copper displaces silver from silver nitrate. Hence, we cannot store AgNO₃ solution in copper vessel.

Cu +AgNO₃ Cu(NO₃)₂ + 2Ag

(c) Copper is more active than Ag.

Question 4

(a) How would you change a metal like Cu into ions?

(b) How would you change Cu²⁺ ions to Cu?

Solution 4

(a) By treating its salt with a more reactive metal.

(b) By supplying two electrons to Cu⁺²

Cu⁺² + 2e^- Cu

Question 5

A solution of caustic soda (NaOH) in water or when fused, conducts an electric current. What is the similarity in these two cases?

Solution 5

In the aqueous state, the slightly negatively charged oxygen atoms of the polar water molecule exerts a pull on the positively charged sodium ions. A similar pull is exerted by the slightly charged hydrogen atoms of the water on the negatively charged chloride ions. Thus the ions become free in solution. These free ions conduct electricity.

In the molten state, the high temperatures required to melt the solid weakens the bond between the particles and the ions are set free.

Question 6

During electrolysis of an aqueous solution of sulphuric acid between platinum electrodes, two types of anions migrate towards the anode but only one of them is discharged.

(a) Name the two anions.

(b) Name the main product of the discharge of anion at the anode and write the anode reaction.

(c) Name the product at the cathode and write the reaction.

(d) Do you notice any change in colour. State why?

(e) Why this electrolysis is considered as an example of catalysis.

Solution 6

(a) Two anions are and OH^-.

(b) OH^- is discharged at anode and the main product of the discharge of OH^- is O₂

Reaction is :

OH^- OH +e^-

4OH 2H₂O + O₂

(c) The product formed at cathode is hydrogen. The reaction is :

H⁺ + e^- H

H + H H₂

(d) No change in colour is observed.

(e) Dilute sulphuric acid catalyse the dissociation of water molecules into ions, hence electrolysis of acidified water is considered as an example of catalysis.

Question 7

Copper sulphate solution is electrolysed using a platinum anode. Study the diagram given alongside and answer the following questions:

a. Give the names of the electrodes A and B.

b. Which electrode is the oxidising electrode?

Solution 7

a. A = Platinum anode, B = Platinum or copper cathode

b. A = Platinum anode

Question 8

To carry out the so-called 'electrolysis of water', sulphuric acid is added to water. How does the addition of sulphuric acid produce a conducting solution?

Solution 8

The addition of sulphuric acid causes dissociation of water into H⁺ ions and OH^- ions.

Question 9(a)

Choosing only words from the following list, write down the appropriate words to fill in the blanks (i) to (v) below: Anions , anode, cathode, cations , electrode, electrolyte, nickel , voltameter.

The electroplating of an article with nickel requires an (i) ____ which must be solution containing (ii) ____ ions. The article to be plated is placed as the (iii) ____ of the cell in which the plating is carried out. The (iv) ____ of the cell is made from pure nickel. The ions that are attracted to the negative electrode and discharged are called (v) _____.

Solution 9(a)

(a) Electrolyte

(b) Nickel

(c) Cathode

(d) Anode

(e) Cations

Question 9(b)

When a molten ionic compound is electrolysed, the metal is always formed at …………… and the non-metal is formed at …………..

Solution 9(b)

cathode, anode

Question 9(c)

Electrolysis of acidulated water is an example of ………… (Reduction/ oxidation/ redox reaction/synthesis)

Solution 9(c)

Electrolysis of acidulated water is an example of redox reaction.

Question 10

Explain the following :

(a) A solution of cane sugar does not conduct electricity, but a solution of sodium chloride is a good conductor,

(b) Hydrochloric acid is a good conductor of electricity.

(c) During the electrolysis of an aqueous solution of NaCl, hydrogen ion is reduced at the cathode and not the sodium ion though both Na⁺ and H⁺ ions are present in the solution.

(d) On electrolysis of dilute copper (II) sulphate solution, copper is deposited at the cathode but hydrogen gas evolves there. Explain why.

(e) When a dilute aqueous solution of sodium chloride is electrolysed between platinum electrodes, hydrogen gas is evolved at the cathode but metallic sodium is not deposited. Why?

(f) Zinc can produce hydrogen on reacting with acids but copper cannot. Explain.

Solution 10

(a) Cane sugar is a compound which does not have ions even in solution and contains only molecules. Hence, it does not conduct electricity. On the other hand, sodium chloride solution contains free mobile ions and allows electric current to pass through it. This makes it a good conductor of electricity.

(b) Hydrochloric acid is a strong electrolyte and dissociates completely in aqueous solution. The solution contains free mobile ions which allow electric current to pass through it. Hence, hydrochloric acid is a good conductor of electricity.

(c) Hydrogen is placed lower in the electrochemical series and sodium is placed at a higher position. This is because H⁺ ions are discharged more easily at the cathode than Na⁺ during electrolysis and gains electrons more easily.

Therefore, H⁺ ion is reduced at the cathode and not Na⁺ ion.

(d)Copper is placed below hydrogen in the activity series. Cu²⁺ on reduction is discharged as metallic copper in preference to hydrogen.

(e) Since hydrogen is much below sodium in the activity series, hydrogen is discharged at the cathode in preference to sodium.

(f)Zinc is more reactive than hydrogen, so it displaces hydrogen from acids, but copper is less reactive than hydrogen, so it does not liberate hydrogen from acids.

Chapter 6 - Electrolysis Exercise Ex. 6

Question (2008)

(a) Here is an electrode reaction :

Cu Cu⁺² +2e^-

At which electrode (anode or cathode) would such a reaction take place? Is this an example of oxidation or reduction?

(b) A solution contains magnesium ions (Mg⁺²), iron (II) ions (Fe⁺²) and copper ions (Cu⁺²).On passing an electric current through this solution, which ions will be first to be discharged at the cathode? Write the equation for the cathode reaction.

(c) Why is carbon tetrachloride, which is a liquid a non-electrolyte?

(d) During the electrolysis of molten lead bromide, which of the following takes place?

 A. Bromine is released at the cathode

 B. Lead is deposited at the anode

 C. Bromine ions gain electrons

 D. Lead is deposited at the cathode

Solution (2008)

(a) The reaction takes place at anode. This is an example of oxidation.

(b) Cu⁺² will discharge easily at cathode.

Reaction at cathode:

Cu⁺² +2e^- Cu

(c) Carbon tetrachloride is a non-electrolyte because it is a covalent compound. It does not ionize and hence do not conduct electricity.

(D) During the electrolysis of molten lead bromide. Lead is deposited at cathode.

Question (2016)

Give reasons why :

(a) Sodium Chloride will conduct electricity only in fused or aqueous solution state.

(b) In the electroplating of an article with silver, the electrolyte sodium argentocyanide solution is preferred over silver nitrate solution.

(c) Although copper is a good conductor of electricity, it is a non-electrolyte.

Solution (2016)

(a) Electrostatic forces of attraction between ions in the solid state are very strong. These forces weaken in the fused or solution state. Hence, ions become mobile.

(b) If silver nitrate solution is used directly instead of double cyanide of silver and sodium, silver will deposit very fast and its deposition will not be smooth and uniform. 

(c) Copper has no mobile electrons in the solid state and an electrolyte should dissociate into oppositely charged ions to conduct electricity.

Hence, copper is a non-electrolyte. 

Question 1

Give reasons for the following:

(a) Electrolysis of molten lead bromide is considered to be a reaction in which oxidation and reduction go side by side i.e, a redox reaction.

(b) The blue colour of aqueous copper sulphate fades when it is electrolyzed using platinum electrodes.

(c) Lead bromide undergoes electrolytic dissociation in the molten state but is a non-electrolyte in the solid state.

(d) Aluminium is extracted from its oxide by electrolytic reduction and not by conventional reducing agents.

(e) The ratio of hydrogen and oxygen formed at the cathode and anode is 2:1 by volume.

(f) In the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid for acidification.

(g) Ammonia is unionized in the gaseous state but in the aqueous solution, it is a weak electrolyte.

(h) A graphite anode is preferred to other inert electrode during electrolysis of fused lead bromide.

(i) For electroplating with silver, silver nitrate is not used as electrolyte.

(j) Carbon tetrachloride is a liquid but does not conduct electricity.

(k) Potassium is not extracted by electrolysis of its aqueous salt solution.

Solution 1

(a) During electrolysis of lead bromide, there is loss of electrons at anode by bromine and gain of electrons at cathode by lead. Thus oxidation and reduction go side by side. Therefore, it is a redox reaction.

(b) The blue colour of copper ions fades due to decrease in Cu⁺² ions and finally the solution becomes colourless as soon as Cu⁺² ions are finished.

(c) Lead bromide dissociate into ions in the molten state whereas it does not dissociate in solid state. The ions become free when lead bromide is in molten state but in the solid state the ions are not free since they are packed tightly together due to electrostatic force between them. Therefore, lead bromide undergoes electrolytic dissociation in the molten state.

(d) Aluminium has great affinity towards oxygen, so it is not reduced by reducing agent. Therefore it is extracted from its oxide by electrolytic reduction.

(e) As per electrolytic reactions, 4H⁺¹ are needed at cathode and 4OH^- at the anode and two molecules of water are produced at the anode. Hence for every two molecules of water, two molecules of hydrogen and one molecule of oxygen are liberated at the cathode and anode respectively.

(f) This is because HNO₃ is volatile.

(g) Ammonia is a covalent compound. Therefore, it is unionized in the gaseous state but in the aqueous solution it gives NH₄OH which is a weak electrolyte and dissociates into ions.

(h) Graphite is unaffected by the bromine vapours.

(i) Silver nitrate is not used as electrolyte for electroplating with silver because the deposition of silver will be very fast and hence not very smooth and uniform.

(j) Carbon tetrachloride is a liquid and does not conduct electricity because it is a covalent compound and there are no free ions present and contain only molecules.

(k) Potassium is not extracted from its aqueous salt solution by electrolysis as it can react with water.

Question 2(a)

Copy and complete the following table which refers to two practical applications of electrolysis

Solution 2(a)

Question 2(b)

Write the equation taking place at the anode for Q. 2(a).

Solution 2(b)

 i. Ag - e^- → Ag⁺

 ii. Cu - e^-→ Cu²⁺

 iii. Cl^- - e^- → Cl

Cl + Cl →Cl₂

Question 3

(a) Draw a labeled diagram to show how iron is electroplated with copper.

(b) Which solution is preferred as electrolyte, CuSO₄ or FeSO₄?

(c) Describe what happens to the iron object and the copper rod.

Solution 3

(a)  

(b) CuSO₄ is preferred as an electrolyte.

(c) The copper anode continuously dissolves as ions in solution and is replaced periodically. The electrolyte dissociates into Cu⁺² ions which migrate towards the iron object taken as the cathode and are deposited as neutral copper atoms on the cathode.

Electrolyte: Aqueous solution of nickel sulphate

Dissociation: CuSO₄   Cu²⁺ + SO₄^2- 

  H₂O   H⁺ + OH^-

  Electrodes:

Cathode: Article to be electroplated

Anode: Block of pure copper

Electrode reactions: 

Reaction at cathode: Cu²⁺ + 2e^-→ Cu (deposited)

Reaction at anode: Cu - 2e^-→ Cu²⁺

Question 4

Element X is a metal with a valency 2. Element Y is a non-metal with a valency 3.

(a) Write equations to show how X and Y form ions?

(b) If Y is a diatomic gas, write the equation for the direct combination of X and Y to form a compound.

(c) If the compound formed between X and Y is melted and an electric current passed through the molten compound, the element X will be obtained at the _____ and Y at the _________of the electrolytic cell.(Provide the missing words)

Solution 4

(a) X X²⁺ +2e^-,Y + 3e^- Y^3-

(b) Y₂ + 3X X₃Y₂

(c) Cathode, Anode

Question 5

Write two applications of electrolysis in which the anode diminishes in mass.

Solution 5

 i. Electroplating of metals

 ii. Electrorefining of metals

Question 6

(a) What kind of particles will be found in a liquid compound which is a non- electrolyte?

(b) If HX is a weak acid, what particles will be present in its dilute solution apart from those of water?

(c) Cations are formed by ____ (loss/gain) of electrons and anions are formed by _________(loss/gain) of electrons. (Choose the correct word to fill in blank)

(d) What ions must be present in a solution used for electroplating a particular metal?

Solution 6

(a) Non-electrolyte contains molecules.

(b) Molecules of HX and H⁺ and X^- ions.

(c) Loss

(d) The electrolyte used for the purpose must contain the ions of metal which is to be electroplated on the article.

Question 7

A strip of copper is placed in four different colourless salt solutions. They are KNO₃, AgNO₃, Zn(NO₃)₂ and Ca(NO₃)₂. Which one of the solutions will finally turn blue?

Solution 7

AgNO₃ solution

Question 2009(a)

Select the correct answer: The aqueous solution of the compound which contains both ions and molecules is

A. H₂SO₄

B. HCl

C. HNO₃

D. CH₃COOH

Solution 2009(a)

D. CH₃COOH

Question 2009(b)

Correct the following statement: Lead bromide conducts electricity.

Solution 2009(b)

Molten lead bromide conducts electricity.

Question 2009(c)

A metal article is to be electroplated with silver. The electrolyte selected is sodium argentocyanide.

 i. What kind of salt is sodium argentocyanide?

 ii. Why is it preferred to silver nitrate as an electrolyte?

 iii. State one condition to ensure that the deposit is smooth, firm and long lasting.

iv. Write the reaction taking place at the cathode.

 v. Write the reaction taking place at the anode.

Solution 2009(c)

 i. Complex salt

 ii. On using silver nitrate, the deposition of silver on the cathode is very fast and hence not very smooth and uniform because it is a strong electrolyte.

 iii. A long current for a longer time should be used.

iv. Ag⁺ + e^- → Ag

 v. Ag - e^- → Ag⁺

Question 2009(d)

Aqueous solution of nickel sulphate contains Ni²⁺ and SO₄^2- ions.

 i. Which ion moves towards the cathode?

 ii. What is the product at the anode?

Solution 2009(d)

 i. Ni²⁺ ions

 ii. Oxygen gas, when an inert electrode is used.

Question 2010(a)

Select the correct answer: A compound which during electrolysis in its molten state liberates a reddish brown gas at the anode.

 i. Sodium chloride

 ii. Copper [II] oxide

 iii. Copper [II] sulphate

iv. Lead [II] bromide

Solution 2010(a)

iv. Lead [II] bromide

Question 2010(b)

During electroplating of an article with nickel,

 i. Name

A. The electrolyte

B. The cathode

C. The anode

 ii. Give the reaction of electrolysis at

A. The cathode

B. The anode

Solution 2010(b)

 i.