Chemical Changes and Reactions

Exercise 2(A)

Question 1

(a) What is a chemical reaction?

(b) State the conditions necessary for a chemical change or reaction.

Answer

(a) A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

(b) Conditions necessary for a chemical change or reaction are :

1. Mixing (close contact) — Some chemical reactions take place when two substances are mixed in their solid state.
   Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

2. Solution — In some cases, a chemical reaction occurs when substances are mixed either in molten or aqueous state.
   AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

3. Heat — Some chemical reactions occur only on heating.
   CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ ↑ (g)

4. Light — Some chemical reactions take place by the action of light. They are called photochemical reaction.
   2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ + O₂ ↑

5. Electricity — Some reactions are caused or accompanied by the passage of an electric current. They are called electrochemical reaction.
   2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

6. Pressure — Some chemical reaction take place only when the involved substances are subjected to high pressure.
   N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

7. Catalyst — Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
   2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

Question 2

Define the following terms:

(a) Chemical change

(b) Chemical bond

(c) Effervescence

(d) Precipitate

Answer

(a) A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

(b) A chemical bond is the force that holds the atoms of a molecule together, as in a compound.

(c) Effervescence is the formation of gas bubbles in a liquid during a reaction.

(d) Certain chemical reactions are characterized by the formation of insoluble solid substances called precipitate.

Question 3

Give an example of a reaction where the following are involved:

(a) Heat

(b) Light

(c) Electricity

(d) Close contact

(e) Solution

(f) Pressure

(g) Catalyst

Answer

(a) Heat

CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ ↑ (g)

(b) Light

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(c) Electricity

2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

(d) Close contact

Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

(e) Solution

NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

(f) Pressure

N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

(g) Catalyst

2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

Question 4

Define :

(a) Photochemical reaction

(b) Electrochemical reaction.

Give one example in each case.

Answer

(a) A chemical reaction that takes place by the action of light is called a photochemical reaction.

Example —

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Reactions that are caused or accompanied by the passage of an electric current are called electrochemical reaction.

Example — On passing current through through molten sodium chloride, sodium and chloride are obtained separately.

2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

Question 5

Give an example of each of the following chemical changes.

(a) A photochemical reaction involving 1. silver salt 2. water

(b) A reaction involving 1. blue solution 2. formation of a dirty green precipitate

(c) Two gases combine to form a white solid.

(d) Two solids combine to form a liquid.

(e) A reaction where color change is noticed.

Answer

(a) A photochemical reaction involving 1. silver salt
2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑ 2. water
Cl₂ + H₂O $\xrightarrow{\text{sunlight}}$ HCl + HClO

(b) A reaction involving 1. blue solution

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

2. formation of a dirty green precipitate

$\underset{\text{ ferrous sulphate}}{\text{FeSO}_4} + \underset{\text{ sodium hydroxide}}{2\text{NaOH}} \longrightarrow \underset{\text{ dirty green ppt. }}{\text{Fe(OH)}_2}↓ + \underset{\text{ sodium sulphate}}{\text{Na}_2\text{SO}_4}$

(c) Two gases combine to form a white solid.

NH₃ (g) + HCl (g) ⟶ NH₄Cl (s)

(d) Two solids combine to form a liquid.

C (s) + 2S (s) ⟶ CS₂ (l)

(e) A reaction where color change is noticed.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

Question 6

Write the chemical reaction where the following changes are observed.

(a) Gas is evolved

(b) Colour change is noticed

(c) Precipitate is formed

(d) Physical state of reactants is changed

Answer

(a) Gas is evolved

Zn(s) + H₂SO₄ (aq) ⟶ ZnSO₄ (aq) + H₂ (g)

(b) Colour change is noticed

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

(c) Precipitate is formed

$\underset{\text{ ferrous sulphate}}{\text{FeSO}_4 \text{ (aq.)}} + \underset{\text{ sodium hydroxide}}{2\text{NaOH}\text{ (aq.)}} \longrightarrow \underset{\text{ dirty green ppt.}}{\text{Fe(OH)}_2↓} + \underset{\text{ sodium sulphate}}{\text{Na}_2\text{SO}_4\text{ (aq.)}}$

(d) Physical state of reactants is changed

C (s) + 2S (s) ⟶ CS₂ (l)

Question 7

Give a reason for the following :

(a) Silver nitrate solution is kept in coloured bottles.

(b) Molybdenum is used in the manufacture of ammonia.

(c) A blue solution of copper sulphate changes to green when a piece of iron is added to this solution.

(d) Colourless concentrated sulphuric acid in a test tube changes to blue on adding a small piece of copper to it.

Answer

(a) Silver nitrate solution is kept in brown bottles in the laboratory because it decomposes in the presence of light. It becomes black and forms silver, nitrogen dioxide and oxygen.

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Molybdenum acts as a promoter and increases the efficiency of the catalyst iron, hence it is used in the manufacture of ammonia.

(c) When a piece of iron is added to a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4}\text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

(d) When a small piece of copper is added to concentrated sulphuric acid, a blue solution of copper sulphate is formed.

$\underset{\text{ brown}}{\text{Cu (s)}} + \underset{\text{ conc.}}{2\text{H}_2\text{SO}_4\text{(aq)}} \longrightarrow \underset{\text{ blue}}{\text{CuSO}_4\text{(aq)}} + 2\text{H}_2\text{O (l)} + \text{SO}_2\text{(g)}$

Exercise 2(B)

Question 1(a)

Which of the following is not a characteristic of a chemical change?

(i) It is irreversible

(ii) No net energy change is involved

(iii) New substance is formed

(iv) Involves absorption or liberation of energy

Answer

No net energy change is involved
Reason — Characteristics of a chemical change are :

1. It's irreversible.

2. A new substance is formed.

3. Involves absorption or liberation of energy.

Question 1(b)

A reaction of a type: AB + CD ⟶ AD + CB, involves

(i) no chemical change

(ii) decomposition of AB and CD

(iii) exchange of ions of AB and CD

(iv) combination of AB and CD

Answer

exchange of ions of AB and CD
Reason — In the type of reaction called double displacement two compounds exchange their positive and negative radicals.

Example : AB + CD ⟶ AD + CB

Question 1(c)

The reaction:

BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq) is

(i) displacement reaction

(ii) neutralisation reaction

(iii) decomposition reaction

(iv) double displacement reaction

Answer

Double displacement reaction
Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

It is represented as XY + AB ⟶ XB + AY

Question 1(d)

Thermal decomposition of sodium carbonate will produce

(i) carbon dioxide

(ii) oxygen

(iii) sodium hydroxide

(iv) no other product

Answer

Carbon dioxide
Reason — Sodium carbonate, on heating gives sodium oxide and carbon dioxide

Na₂CO₃ $\xrightarrow{\Delta}$ Na₂O + CO₂ ↑

Question 2

Match the following :

Answer

Question 3

Complete the following statements.

(a) The chemical change involving iron and hydrochloric acid illustrates a ............... reaction.

(b) In the type of reaction called ............... two compounds exchange their positive and negative radicals.

(c) A catalyst either ............... or ............... the rate of a chemical change but itself remains ............... at the end of the reaction.

(d) On heating, hydrated copper sulphate changes its colour from ............... to ...............

Answer

(a) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.

(b) In the type of reaction called double displacement two compounds exchange their positive and negative radicals.

(c) A catalyst either accelerates or decelerates the rate of a chemical change but itself remains unaffected at the end of the reaction.

(d) On heating, hydrated copper sulphate changes its colour from blue to white.

Question 4

When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical changes involved in the two cases.

Answer

When hydrogen burns in oxygen, hydrogen and oxygen combine together and forms water, hence, it is a direct combination reaction whereas, when electricity is passed through water, it breaks to give hydrogen and oxygen, hence, it is a decomposition reaction.

Question 5

Explain the following chemical changes by giving one example of each:

(a) Double decomposition

(b) Thermal dissociation

(c) Reversible reaction

(d) Displacement

Answer

(a) Double decomposition — This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called double displacement reaction.
AB + CD ⟶ AD + CB

Eg., AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

(b) Thermal dissociation — A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

Heat some solid ammonium chloride in a test tube. Two colourless gases, ammonia and hydrogen chloride, are produced. As these gases move up to the upper part of the test tube which is cooler, they combine to form ammonium chloride, which appears as a white sublimate on the upper cooler side of the test tube.

NH₄Cl ⇌ NH₃ ↑ + HCl ↑

(c) Reversible reaction — A reaction that can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.

$\underset{\text{ red hot iron}}{\text{3Fe}} + \underset{\text{ steam}}{4\text{H}_2\text{O}} \rightleftharpoons \underset{\text{ magnetic oxide}}{\text{Fe}_3\text{O}_4} + \underset{\text{ hydrogen gas}}{4\text{H}_2}$

(d) Displacement — A chemical change in which a more active element displaces a less active element from its salt solution is called a displacement reaction.

Mg + H₂SO₄ ⟶ MgSO₄ + H₂ ↑

Question 6

(a) What is synthesis?

(b) What kind of chemical reaction is synthesis? Support your answer with an example.

Answer

(a) A reaction in which two or more substances combine together to form a single substance is called a synthesis or combination reaction.

A + B ⟶ AB

(b) Synthesis is a direct combination reaction.

In the above reaction, substances A and B combine to give a molecule of a new substance, AB (Product).

For example, carbon burns in oxygen to form a gaseous compound, carbon dioxide.

C + O₂ $\xrightarrow{\Delta}$ CO₂ ↑

Question 7

Decomposition brought about by heat is known as thermal decomposition. What is the difference between thermal dissociation and thermal decomposition?

Answer

Difference between the two is that thermal dissociation is a reversible reaction whereas, thermal decomposition is an irreversible reaction.

Question 8

(a) Define neutralization reaction with an example.

(b) Give a balanced equation for this reaction.

(c) Give three applications of neutralization reactions.

Answer

(a) The reaction between an acid and a base that forms salt and water only is referred to as Neutralization reaction.

Reaction between an acid and an alkali to form salt and water only is an example of neutralization reaction.

(b) NH₄OH + HCl ⟶ NH₄Cl + H₂O

(c) Applications of neutralization reactions are as follows :

1. When someone is stung by a bee, formic acid enters the skin and causes pain, which can be relieved by rubbing the spot with slaked lime or baking soda, both of which are bases and thus neutralize the acid.
2. The acid which is accidentally spilt on to our clothes can be neutralised with ammonia solution.
3. If soil is somewhat acidic and thus unfavourable for the growth of certain crops, slaked lime is added to neutralise the excess acid.

Question 9

What do you understand by precipitation reaction? Explain with an example.

Answer

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt (precipitate) as one of the products is called a precipitation reaction.

Example: Take a solution of silver nitrate in a test tube and add dil. hydrochloric acid. A curdy white ppt. is formed.

AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

Question 10

(a) What are double displacement reactions?

(b) Give an example of a double displacement reaction, where gas is evolved.

Answer

(a) Double displacement reactions are those in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. This type of reaction is also known as a double decomposition reaction.

(b) FeS (s) + H₂SO₄ (aq) ⟶ FeSO₄ (aq) + H₂S ↑

Question 11

(a) What is a decomposition reaction ?

(b) Decomposition reactions can occur by (i) heat (ii) electricity and (iii) sunlight.
Give two balanced reactions for each.

Answer

(a) The chemical reaction in which a compound splits into two or simpler substances (elements or compounds) is called decomposition reaction.

(b) Two balanced chemical equations for:

(i) Heat

2HgO (s) $\xrightarrow{\Delta}$ 2Hg (l) + O₂ (g)

CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

(ii) Electricity

2NaCl (l) $\xrightarrow{\text{electricity}}$ 2Na (l) + Cl₂ (g)

2H₂O (l) $\xrightarrow{\text{electricity}}$ 2H₂ (g) + O₂ (g)

(iii) Light

2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

2H₂O₂ (l) $\xrightarrow{\text{ sunlight}}$ 2H₂O (l) + O₂ (g)

Question 12

State the type of reactions each of the following represent and balance the ones that are not balanced.

(a) Cl₂ + 2KBr ⟶ 2KCl + Br₂

(b) NaOH + HCl ⟶ NaCl + H₂O

(c) 2HgO ⟶ 2Hg + O₂ ↑

(d) Fe + CuSO₄ ⟶ FeSO₄ + Cu

(e) PbO₂ + SO₂ ⟶ PbSO₄

(f) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

(g) 2H₂O₂ ⟶ 2H₂O + O₂ ↑

(h) KNO₃ + H₂SO₄ ⟶ HNO₃ + KHSO₄

(i) CuO + H₂ ⟶ Cu + H₂O

(j) CaCO₃ ⟶ CaO + CO₂ ↑

(k) NH₄Cl ⟶ NH₃ + HCl

(l) PbO + 2HNO₃ ⟶ Pb(NO₃)₂ + 2H₂O

(m) AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Answer

(a) Cl₂ + 2KBr ⟶ 2KCl + Br₂

Displacement reaction

(b) NaOH + HCl ⟶ NaCl + H₂O

Neutralisation reaction

(c) 2HgO ⟶ 2Hg + O₂ ↑

Decomposition reaction

(d) Fe + CuSO₄ ⟶ FeSO₄ + Cu

Displacement reaction

(e) PbO₂ + SO₂ ⟶ PbSO₄

Combination reaction

(f) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

Decomposition reaction

(g) 2H₂O₂ ⟶ 2H₂O + O₂ ↑

Decomposition reaction

(h) KNO₃ + H₂SO₄ ⟶ HNO₃ + KHSO₄

Double decomposition reaction

(i) CuO + H₂ ⟶ Cu + H₂O

Displacement reaction

(j) CaCO₃ ⟶ CaO + CO₂ ↑

Decomposition reaction

(k) NH₄Cl ⟶ NH₃ ↑ + HCl

Decomposition reaction

(l) PbO + 2HNO₃ ⟶ Pb(NO₃)₂ + H₂O

Neutralisation reaction

(m) AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Double decomposition reaction

Exercise 2(C) — Multiple Choice Type

Question 1

Which of the of following is not a chemical change.

1. Rusting of iron
2. Iron is magnetised
3. Potassium is dissolved in water
4. Magnesium wire is burnt

Answer

Iron is magnetised

Reason — Magnetisation of iron is not a chemical change, as it is a temporary change.

Question 2

Heating of zinc nitrate is a ............... reaction.

1. Displacement
2. Combination
3. Decomposition
4. Redox

Answer

Decomposition reaction

Reason — Zinc nitrate undergoes thermal decomposition and produces reddish brown Nitrogen dioxide gas (NO₂), Zinc oxide (ZnO), and Oxygen gas (O₂).

$\text{Zn}(\text{NO}_3)_2 \xrightarrow{\Delta} \text{4NO}_2 + \text{2ZnO} + \text{O}_2 ↑$

Question 3

Burning of a metal is an example of :

1. Decomposition reaction
2. Combination reaction between an element and a compound
3. Direct combination reaction between two elements
4. Displacement reaction

Answer

Direct combination reaction between two elements

Reason — A reaction in which two elements react to form a single substance is called a direct combination reaction between two elements.
When a metal, for example Magnesium, is burnt, it reacts with oxygen to form a new compound magnesium oxide.

2Mg + O₂ ⟶ 2MgO

Hence, burning of a metal is a direct combination reaction between two elements.

Question 4

Formation of white precipitate on mixing sodium chloride solution and silver nitrate solution is an example of ............... reaction.

1. Combination
2. Decomposition
3. Displacement
4. Double decomposition

Answer

Double decomposition

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction. Eg.,

AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Question 5

A brown substance A on heating in air turns black forming another substance B. Substances A and B are:

1. A = Fe and B = FeO
2. A = Zn and B = ZnO
3. A = Cu and B = CuO
4. A = Pb and B = PbO

Answer

A = Cu and B = CuO

Reason — Brown copper on heating forms black copper oxide.

2Cu + O₂ ⟶ 2CuO

Question 6

Which one of these is a chemical change?

1. Water changes to steam
2. Dissolution of sugar in water
3. Combustion of LPG
4. Liquefying ammonia

Answer

Combustion of LPG

Reason — A chemical change is a permanent change and combustion of LPG is a permanent change.

Question 7

The reaction of neutralization is a :

1. Displacement reaction
2. Double decomposition
3. Combination reaction
4. Combination reaction between two compounds

Answer

Double decomposition

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

These reactions are of two types:

1. precipitation reaction
2. neutralization reaction

Question 8

FeS + H₂SO₄ ⟶ FeSO₄ + H₂S is an example of:

1. Displacement reaction
2. Double decomposition reaction
3. Decomposition reaction
4. Combination reaction

Answer

Double decomposition reaction

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

AB + CD ⟶ AD + CB

Exercise 2(C) — Very Short Type

Question 1

Name

(a) a carbonate which does not decompose on heating.

(b) a nitrate which produces oxygen as the only gas.

(c) a compound which produces carbon dioxide on heating

(d) a nitrate which produces brown gas on heating.

Answer

(a) Sodium carbonate

(b) Potassium nitrate

(c) Calcium carbonate

(d) Lead nitrate

Question 2

Which is the most reactive element in this question :

CuSO₄ + Fe ⟶ FeSO₄ + Cu

Answer

Fe, as Fe is higher in the metal reactivity series. The reactivity of the metal reactivity series decreases as one moves from top to bottom.

Exercise 2(C) — Short Answer Type

Question 1

Define :

(a) Photochemical reaction

(b) Electrochemical reaction

Give one example in each case.

Answer

(a) Photochemical reaction — It is a reaction that occurs with the absorption of light.

Example : Decomposition of silver nitrate

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Electrochemical reaction — It is a reaction that occurs with absorption of electrical energy.

Example : Acidulated water breaks into hydrogen and oxygen when electric current is passed through it.

2H₂O $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ ↑ + O₂ ↑

Question 2

Give an example of a reaction where the following are involved

(a) Evolution of heat

(b) Absorption of heat

(c) High pressure is required

Answer

(a) CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

(b) C + 2S $\xrightarrow \Delta$ CS₂

(c) N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

Question 3

Why does the blue colour of copper sulphate solution fade on adding iron fillings ?

Answer

Blue colour of copper sulphate fades away as iron displaces copper from copper sulphate solution as iron is more reactive than copper.

Question 4

Give an example of:

(a) Single displacement reaction

(b) Double displacement reaction

(c) Photochemical reaction

(d) Thermal decomposition reaction

(e) Electrolytic decomposition reaction

(f) Combination reaction between two compounds ?

Answer

(a) Single displacement reaction

CuSO₄ + Zn ⟶ Cu + ZnSO₄

(b) Double displacement reaction

BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq)

(c) Photochemical reaction

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(d) Thermal decomposition reaction

2HgO (s) $\xrightarrow{\Delta}$ 2Hg (l) + O₂ (g)

(e) Electrolytic decomposition reaction

2H₂O $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ + O₂ ↑

(f) Combination reaction between two compounds ?

NH₃ + HCl ⟶ NH₄Cl

Question 5

What do you understand by a 'chemical reaction'?

Answer

A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

Example : BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq)

Question 6

Give an example of each of the following chemical changes.

(a) A reaction involving

(i) change of state

(ii) formation of a precipitate

(b) An exothermic and an endothermic reaction involving carbon as one of the reactants.

(c) A reaction where colour change is noticed.

Answer

(a) An example of:

(i) A reaction involving change of state — Ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride.

NH₃ (g) + HCl (g) ⇌ NH₄Cl (s)

(ii) A reaction involving formation of a precipitate — Take a solution of silver nitrate in a test tube and add solution of sodium chloride, a curdy white ppt. is formed.

AgNO₃ (aq) + NaCl (aq) ⟶ AgCl ↓ + NaNO₃ (aq)

(b) Exothermic reaction involving carbon — Combustion of methane gives out large amount of energy.

CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

Endothermic reaction involving carbon — When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.

C + 2S $\xrightarrow \Delta$ CS₂

(c) A reaction where colour change is noticed — When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

Question 7

What is a chemical change? Give two examples of a chemical change?

Answer

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

Examples :

1. Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

2. NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

Question 8

Define exothermic and endothermic reactions. Give two examples in each case.

Answer

A chemical reaction in which heat is given out is called an exothermic reaction. It causes a rise in temperature.

Examples:

1. Combustion of methane gives out large amount of energy.
   CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

2. When carbon burns in oxygen to form carbon dioxide, lot of heat is produced.
   C + O₂ ⟶ CO₂ ↑ + Heat

A chemical reaction in which heat is absorbed is called an endothermic reaction. It causes a fall in temperature.

Example :

1. When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.
   C + 2S $\xrightarrow \Delta$ CS₂

2. Calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C
   CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

Question 9

Why is energy involved in a chemical change?

Answer

A chemical reaction involves the breaking up of chemical bonds between atoms resulting in absorption of energy in the form of heat, and simultaneous formation of bonds with release of energy. These two types of energies are different from each other, i.e., there is either a surplus or a deficit of energy during the reaction.

Therefore, in a chemical reaction, energy is involved.

Exercise 2(C) — Descriptive Type

Question 1

State the main characteristics of chemical reactions. Give at least one example in each case.

Answer

The main characteristics of chemical reactions are :

1. Evolution of gas
   Zn(NO₃)₂ $\xrightarrow{\Delta}$ 4NO₂ + 2ZnO + O₂ ↑

2. Change of colour
   $\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

3. Change of state
   2H₂O (l) $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ (g) + O₂ (g)

4. Formation of precipitate
   NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

Question 2

State the effects of endothermic and exothermic reactions on the surroundings with examples.

Answer

In exothermic reactions heat is given out, hence it causes a rise in temperature.

CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

In endothermic reactions heat is absorbed, hence it causes a fall in temperature.

C + 2S $\xrightarrow \Delta$ CS₂

Question 3

Complete and balance the following reactions:

(a) NaCl (aq.) + AgNO₃ (aq.) ⟶

(b) Pb(NO₃)₂ + KI ⟶

(c) CuCO₃ $\xrightarrow{\Delta}$

(d) Pb(NO₃)₂ $\xrightarrow{\Delta}$

(e) CO₂ + H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$

Answer

(a) NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

(b) Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

(c) CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ (g)

(d) Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂

(e) 6CO₂ + 6H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$ C₆H₁₂O₆ + 6O₂

Question 4

What do you observe in the following cases?

(a) Lead nitrate is heated.

(b) Silver chloride is exposed to sunlight.

(c) Hydrogen peroxide is exposed to sunlight

(d) H₂S gas is passed through copper sulphate solution

(e) Barium chloride is added to sodium sulphate solution.

(f) Water is added to quick lime

(g) Sodium chloride solution is added to silver nitrate solution.

Answer

(a) When white crystalline solid Lead nitrate is heated, it decomposes into a buff yellow residue of lead monoxide. Reddish brown gas Nitrogen dioxide and colourless gas Oxygen are evolved.

$\underset{\text{[lead nitrate (white)]}}{2\text{Pb(NO}_3)_2} \overset{\Delta}{\longrightarrow} \underset{\text{[litharge - buff yellow]}}{2\text{PbO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2} + \underset{\text{[Oxygen]}}{\text{O}_2}$

(b) When silver chloride (white) is exposed to sunlight, it undergoes photochemical decomposition forming a grey metal, silver and a greenish yellow gas, chlorine.

2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

(c) When hydrogen peroxide is exposed to sunlight, it changes to water and colourless, odourless oxygen gas is evolved.

2H₂O₂ $\xrightarrow{\text{ sunlight}}$ 2H₂O + O₂ ↑

(d) When H₂S is passed through copper sulphate solution, a black precipitate of copper sulphide is obtained

CuSO₄ + H₂S ⟶ CuS [black] + H₂SO₄

(e) White coloured precipitate of barium sulphate is formed when sodium sulphate is mixed with barium chloride.

Na₂SO₄ (aq.) + BaCl₂ (aq.) ⟶ 2NaCl (aq.) + BaSO₄ [white ppt.] ↓

(f) When calcium oxide (quick lime) combines with water, a vigorous reaction takes place with the liberation of a large amount of heat (exothermic reaction) and calcium hydroxide is formed. Heat produced boils the water and a hissing sound is produced.

CaO (s) + H₂O (l) ⟶ Ca(OH)₂ (s) + Heat

(g) When sodium chloride is added to the silver nitrate solution, a white curdy precipitate of silver chloride is formed.

NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

Question 5

Discuss the factors that affect a chemical change.

Answer

1. Certain chemical reactions occur only when electricity is passed through the reactants.
   2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

2. Some chemical reactions occur when the involved substances are subjected to high pressure.
   N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

3. Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
   2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

4. Some chemical reactions take place by the action of light.
   2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

5. Some chemical reactions take place when two substances are mixed in their solid state.
   Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

6. Some chemical reactions occur only on heating

$\underset{\text{[lead nitrate (white)]}}{2\text{Pb(NO}_3)_2} \overset{\Delta}{\longrightarrow} \underset{\text{[litharge - buff yellow]}}{2\text{PbO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2} + \underset{\text{[Oxygen]}}{\text{O}_2}$

Question 6

Balance these reactions and state what type of reactions they are:

(a) NaNO₃ ⟶ NaNO₂ + O₂

(b) AgNO₃ + Zn ⟶ Zn(NO₃)₂ + Ag

(c) Fe + HCl ⟶ FeCl₂ + H₂

(d) AgNO₃ ⟶ Ag + NO₂ + O₂

(e) NaBr + Cl₂ ⟶ NaCl + Br₂

(f) PbO + C ⟶ Pb + CO₂

(g) KClO₃ ⟶ KCl + O₂

(h) AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

(i) NH₃ + HCl ⟶ NH₄Cl

(j) N₂ + H₂ ⟶ NH₃

Answer

(a) 2NaNO₃ ⟶ 2NaNO₂ + O₂ ↑

Decomposition reaction

(b) 2AgNO₃ + Zn ⟶ Zn(NO₃)₂ + 2Ag

Displacement reaction

(c) Fe + 2HCl ⟶ FeCl₂ + H₂ ↑

Displacement reaction

(d) 2AgNO₃ ⟶ 2Ag + 2NO₂ ↑ + O₂ ↑

Decomposition reaction

(e) 2NaBr + Cl₂ ⟶ 2NaCl + Br₂

Displacement reaction

(f) 2PbO + C ⟶ 2Pb + CO₂ ↑

Displacement reaction

(g) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

Decomposition reaction

(h) AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

Double decomposition reaction

(i) NH₃ + HCl ⟶ NH₄Cl

Direct combination reaction

(j) N₂ + 3H₂ ⟶ 2NH₃ ↑

Direct combination reaction

Question 7

Explain the different types of double decomposition reactions by giving suitable equations.

Answer

Different types of double decomposition reactions are:

1. Precipitation reaction — A reaction between two compounds in aqueous solution state to give two new compounds one of which is insoluble (precipitate) is called a Precipitation reaction.
   For example, when H₂S is passed through copper sulphate solution, a black precipitate of copper sulphide is obtained.
   CuSO₄ (aq) + H₂S (g) ⟶ CuS [black ppt.] ↓ + H₂SO₄

2. Neutralization reaction — The reaction between an acid and a base that forms salt and water only is referred to as Neutralization reaction.
   For example, Sodium chloride (base) reacts with HCl (acid) to form salt and water only.
   NaOH + HCl ⟶ NaCl + H₂O

Question 8

Explain thermal dissociation and thermal decomposition reactions with examples. What is the difference between them ?

Answer

A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

For example, on application of heat, ammonium chloride decomposes into Ammonia and HCl:

NH₄Cl ⇌ NH₃ + HCl

A chemical reaction in which a compound decomposes to give two new elements / a new compound & an element / two new compounds on application of heat without any recombination on cooling is called a Thermal decomposition reaction.

For example, calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C

CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

Difference between the two is that thermal dissociation is a reversible decomposition reaction whereas, thermal decomposition is an irreversible reaction.

Question 9

Metal X was placed in lead nitrate solution. A thin layer of lead metal deposits on metal X. Which is more reactive metal X or lead ? State the type of reaction and give an example of this type.

Answer

Metal X is more active metal because it displaced the metal lead from lead nitrate. It is a displacement reaction.

CuSO₄ + Zn ⟶ ZnSO₄ + Cu

Zn is more active metal than Cu, and it displaces Cu from copper sulphate.

Question 10

What type of reaction is respiration ? Explain.

Answer

Respiration is an exothermic reaction as it involves combustion of glucose with oxygen in the cells of the body releasing heat energy that is used by cells to perform various functions such as muscle contraction, nerve impulse transmission, and protein synthesis. The equation for respiration reaction is:

C₆H₁₂O₆ + 6O₂ ⟶ + 6CO₂ ↑ + 6H₂O + Heat energy

Question 11

Discuss some decomposition reactions occurring inside our body.

Answer

Digestion of food in our body is an example of decomposition reaction.

The starch present in the food we eat decomposes into glucose and sugar. Proteins undergo decomposition to form amino acids. Fats and oils are decomposed to fatty acids and finally oxidized by respiration into carbon dioxide and water.

Starch $\xrightarrow{\text{Enzymes}}$ Glucose $\xrightarrow{\text{[O]}}$ CO₂ + H₂O + Energy

Exercise 2(C) — Structures/Application/Skill Type

Question 1

K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, H, Hg, Ag

Pick an element from the reactivity series given above and write the equations for the the following:

(a) Metal hydroxide on heating forms metal oxide and water vapour.

(b) Metal hydroxide on heating forms metal, oxygen and water vapours.

(c) Metal nitrate decomposes to give two products only.

(d) Metal nitrate on heating forms metal oxide, nitrogen dioxide and oxygen.

(e) Metal nitrate on heating forms metal, nitrogen dioxide and oxygen.

(f) Metal carbonate which is stable to heat.

(g) Metal carbonate which forms metal oxide and carbon dioxide on heating.

(h) The heating effect on bivalent metal hydrogen carbonate.

Answer

(a) Metal hydroxide on heating forms metal oxide and water vapour — Calcium hydroxide

Ca(OH)₂ $\xrightarrow{\Delta}$ CaO + H₂O

(b) Metal hydroxide on heating forms metal, oxygen and water vapours — silver hydroxide

4AgOH $\xrightarrow{\Delta}$ 4Ag + O₂ + 2H₂O

(c) Metal nitrate decomposes to give two products only — potassium nitrate

2KNO₃ $\xrightarrow{\Delta}$ 2KNO₂ + O₂ ↑

(d) Metal nitrate on heating forms metal oxide, nitrogen dioxide and oxygen — Lead nitrate

2Pb(NO₃)₂ $\xrightarrow{\Delta}$ 2PbO + 4NO₂ + O₂ ↑

(e) Metal nitrate on heating forms metal, nitrogen dioxide and oxygen — Silver nitrate

2AgNO₃ $\xrightarrow{\Delta}$ 2Ag + 2NO₂ + O₂ ↑

(f) Metal carbonate which is stable to heat — Potassium carbonate

Potassium carbonate is stable to heat

(g) Metal carbonate which forms metal oxide and carbon dioxide on heating — Magnesium carbonate

MgCO₃ $\xrightarrow{\Delta}$ MgO + CO₂ ↑

(h) The heating effect on bivalent metal hydrogen carbonate — Magnesium bicarbonate

Mg(HCO₃)₂ $\xrightarrow{\Delta}$ MgCO₃ + H₂O + CO₂ ↑

Question 2

KI + Cl₂ ⟶ KCl + I₂

(a) Balance the equation given above.

(b) Which type of reaction is it ?

(c) Which element is more reactive ?

(d) What will be the product formed if KCl reacts with I₂ ?

Answer

(a) 2KI + Cl₂ ⟶ 2KCI + I₂

(b) Displacement reaction

(c) Chlorine

(d) No reaction, because Iodine being less reactive than chlorine will not be able to displace chlorine from KCl.