Chemical Reactions

Introduction

Question 1

State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include

(a) Colour change

(b) Effervescence or gas evolved

(d) Formation of a precipitate

With reference to each of the above indications, state the external indication seen during

(i) Addition of dilute acid to an active metal

(ii) Addition of dilute hydrochloric acid to silver nitrate

(iii) Addition of water to quicklime

(iv) Thermal decomposition of mercury [II] oxide

Answer

A chemical reaction is a chemical change in which matter changes into a new substance or substances.

(i) Gas is evolved

(ii) Formation of a precipitate

(iii) Evolution of heat

(iv) Colour change — Red mercury [II] oxide changes to silver coloured mercury.

Main reactions

Question 2

State why a direct combination reaction is called a - 'synthesis reaction'.

Answer

As in a direct combination reaction two or more elements or compounds combine to form a new product, hence it is called a synthesis reaction.

Question 3

Differentiate between -

(a) Direct combination reaction & a decomposition reaction.

(b) Displacement reaction & a neutralization reaction.

Answer

(a) Difference between direct combination reaction & a decomposition reaction

Direct combination reaction	Decomposition reaction
It is a reaction in which two or more elements or compounds combine to give a new product.	It is a reaction in which a chemical compound decomposes into two or more simpler substances.
Example: Fe + S ⟶ FeS	Example: CuCO₃ ⟶ CuO + CO₂

(b) Difference between displacement reaction & a neutralization reaction

Displacement reaction	Neutralization reaction
It is a chemical reaction in which an element placed higher in the activity series displaces the element placed below it from a solution of its compound.	It is a reaction between two compounds - base and acid to give two new compounds - salt and water by interchange of radicals.
Example: Zn + CuSO₄ ⟶ ZnSO₄ + Cu	Example: NaOH + HCl ⟶ NaCl + H₂O

Question 4

Classify the following reactions into

(a) Direct combination

(b) Decomposition

(d) Double decomposition

The reactions are

(i) Zinc hydroxide on heating gives zinc oxide & water

(ii) Zinc reacts with copper [II] sulphate to give zinc sulphate & copper

(iii) Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide

(iv) Molten zinc at high temperatures, burns in air to give zinc oxide.

Answer

(i) Zinc hydroxide on heating gives zinc oxide & water — Decomposition

Zn(OH)₂ $\xrightarrow{\Delta}$ ZnO + H₂O

(ii) Zinc reacts with copper [II] sulphate to give zinc sulphate & copper — Displacement

Zn + CuSO₄ ⟶ ZnSO₄ + Cu

(iii) Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide — Double decomposition

ZnSO₄ + 2NH₄OH ⟶ (NH₄)₂SO₄ + Zn(OH)₂

(iv) Molten zinc at high temperatures, burns in air to give zinc oxide — Direct combination

2Zn + O₂ ⟶ 2ZnO

Types of reactions

Question 5

Give balanced equations for -

(i) A direct combination reaction involving two elements, one of which is a non-metal.

(ii) A thermal decomposition reaction involving heat on limestone [calcium carbonate].

(iii) An electrolytic decomposition reaction involving a neutral liquid.

(iv) A displacement reaction involving a metal above hydrogen in the activity series with copper [II] sulphate solution.

(v) A double decomposition neutralization reaction involving an acid & a base.

(vi) A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.

Answer

(i) C + O₂ ⟶ CO₂

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

(iii) 2H₂O $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ + O₂

(iv) Zn + CuSO₄ ⟶ ZnSO₄ + Cu

(v) HCl + NaOH ⟶ NaCl + H₂O

(vi) AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Question 6

State what is meant by 'reactivity series of metals'. With reference to -

(a) Water

(b) Acids

explain with suitable examples how the reactivity of the metals could be differentiated.

Answer

Reactivity series of metals is a series of arrangement of metals in decreasing order of their chemical reactivity. Metal reactivity series is listed below:

Activity Series
K
Na
Ca
Mg
Al
Zn
Fe
Pb
*[H]
Cu
Hg
Ag
Pt
Au

Differentiating reactivity of the metals with reference to Water & Acids:

Reaction with	Metals	Observation	Equation
Water	K & Na	React violently with cold water	2K + 2H₂O ⟶ 2KOH + H₂
	Ca	Reacts less vigorously with cold water	Ca + 2H₂O ⟶ Ca(OH)₂ + H₂
	Mg to Fe	React smoothly with water [steam]	3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂
	Below [H]	Do not react with water or steam displacing hydrogen from it.
Acids	K & Na	React explosively with HCl or dil. H₂SO₄	2K + 2HCl ⟶ 2KCl + H₂
	Ca to Fe	React less vigorously with decreasing vigour with dil. HCl or dil. H₂SO₄	Ca + 2HCl ⟶ CaCl₂ + H₂
	Below [H]	Do not react with dil. acids, displacing hydrogen from it.

Question 7

A chemical reaction may be 'reversible' in nature. State the meaning of the term in italics. Give a reason why a catalyst is used in certain chemical reactions.

Give a balanced equation for the following -

(a) A reversible catalytic reaction involving -

(i) nitrogen as one of the reactants

(ii) sulphur dioxide as one of the reactants.

Answer

A reversible reaction is one in which the products formed react together to form the original reactants depending on the conditions of the reaction.

Example : N₂ + O₂ $\xrightleftharpoons {3000\text{\degree C}}$ 2NO

A catalyst is used to alter the rate of the reaction but it does not take part in the reaction.

(a)(i) N₂ + 3H₂ $\xrightleftharpoons {400 - 500\text{\degree C/Fe}}$ 2NH₃

(a)(ii) 2SO₂ + O₂ $\xrightleftharpoons {\text{V}_2\text{O}_5/450\text{\degree C}}$ 2SO₃

Question 8

State which type of chemical reactions proceed with -

(a) Evolution of heat energy

(b) Absorption of heat energy.

State in each of the following reactions whether heat is evolved or absorbed

(i) water is added to quicklime

(ii) two neutral gases on passage through an electric arc give nitric oxide

(iii) two neutral gases combine to give - a basic gas.

Answer

(a) Exothermic reaction

(b) Endothermic reaction

(i) Exothermic reaction, heat is evolved.
CaO + H₂O ⟶ Ca(OH)₂ + Δ

(ii) Endothermic reaction, heat is absorbed.
N₂ + O₂ $\xrightleftharpoons {3000\text{\degree C}}$ 2NO - Δ

(iii) Exothermic reaction, heat is evolved.
N₂ + 3H₂ $\xrightleftharpoons {400 - 500\text{\degree C/Fe}}$ 2NH₃ + Δ

Question 9

Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide.

(a) Limestone

(b) Lead carbonate

(d) Calcium hydroxide.

Answer

(a) Thermal decomposition of limestone

CaCO₃ $\xrightarrow\Delta$ CaO + CO₂

(b) Thermal decomposition of lead carbonate

PbCO₃ $\xrightarrow\Delta$ PbO + CO₂

2Ca(NO₃)₂ $\xrightarrow\Delta$ 2CaO + 4NO₂ + O₂

(d) Thermal decomposition of calcium hydroxide.

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

Question 10

State the meaning of the term 'oxide. Give a balanced equation for formation of the following oxides -

(a) Sulphur dioxide from a non-metal

(b) Zinc oxide from a metal

Answer

Oxides are binary compounds of a metallic or non-metallic element with oxygen.

(a) Sulphur dioxide from a non-metal :

S + O₂ ⟶ SO₂

(b) Zinc oxide from a metal :

2Zn + O₂ ⟶ 2ZnO

2Pb₃O₄ $\xrightarrow\Delta$ 6PbO + O₂

Question 11

Give two examples each of the following oxides -

(a) Acidic oxides

(b) Basic oxides

(d) Neutral oxides.

State which of the following oxides i.e. (a) to (d) -

(i) React with water to give a base

(ii) React with a base to give salt & water

(iii) React with acids & bases to give salt & water.

Answer

(a) Acidic oxides — SO₂ , CO₂

(b) Basic oxides — K₂O, CaO

(d) Neutral oxides — NO, CO

(i) React with water to give a base — (b) Basic oxides

(ii) React with a base to give salt & water — (a) Acidic oxides

(iii) React with acids & bases to give salt & water — (c) Amphoteric oxides

Question 12

Give one example each of -

(a) A peroxide

(b) A mixed oxide

Answer

(a) A peroxide — Na₂O₂

(b) A mixed oxide — Pb₃O₄

Objective Type Questions

Question 1

Select the correct answer from A, B, C, D and E for each statement given below:

A: Iron

B: Carbonic acid

C: Hydrogen

D: Oxygen

E: Carbon monoxide

The product formed during direct combination reaction of carbon dioxide & water.
The neutral gas obtained on thermal decomposition of potassium nitrate.
The displaced product of the displacement reaction of sodium with cold water.
The catalyst used in the catalytic reaction involving the reactants nitrogen & hydrogen.
A neutral oxide which does not react with an acid or a base to give salt & water.

Answer

B: Carbonic acid
D: Oxygen
C: Hydrogen
A: Iron
E: Carbon monoxide

Question 2

Complete the statements by filling in the blank with the correct word/s:

Direct combination reaction of phosphorus pentoxide with water gives ............... [H₃PO₃/H₃PO₄].
Decomposition of silver salts in the presence of sunlight is an example of ............... [double decomposition / photochemical decomposition].
The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a ............... [promoter/catalyst].
The reaction of coke with steam to give water gas is an example of an ............... [exothermic/endothermic] reaction.
The metal which reacts with steam and the reaction is reversible is ............... [calcium/iron]

Answer

Direct combination reaction of phosphorus pentoxide with water gives H₃PO₄.
Decomposition of silver salts in the presence of sunlight is an example of photochemical decomposition.
The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a promoter.
The reaction of coke with steam to give water gas is an example of an endothermic reaction.
The metal which reacts with steam and the reaction is reversible is iron.

Question 3

Give a balanced equation for each of the following types of reactions:

A thermal decomposition reaction in which a compound decomposes to give two new compounds.
A reaction of direct combination i.e. synthesis in which two gases combine to give another gas - which turns lime water milky.
A thermal decomposition reaction in which a metallic nitrate decomposes to give - a basic oxide.
A catalytic, reversible, exothermic reaction.
A displacement reaction in which a metal above hydrogen in the reactivity series, displaces another metal from the solution of its compound.

Answer

CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂
2CO + O₂ ⟶ 2CO₂
2Ca(NO₃)₂ $\xrightarrow{\Delta}$ 2CaO + 4NO₂ + O₂
N₂ + 3H₂ $\xrightleftharpoons {400 - 500\text{\degree C/Fe}}$ 2NH₃ + Δ
Zn + CuSO₄ ⟶ ZnSO₄ + Cu

Question 4

Differentiate between the following:

Thermal decomposition & thermal dissociation.
Neutralization reaction & a precipitation reaction.
Electrolytic decomposition & photochemical decomposition.
A catalyst & a promoter.
An acidic oxide & a basic oxide.

Answer

Difference between thermal decomposition & thermal dissociation.

Thermal decomposition	Thermal dissociation
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.	A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.
Thermal decomposition is an irreversible reaction.	Thermal dissociation is a reversible decomposition reaction.
For example: CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂	For example: NH₄Cl ⇌ NH₃ + HCl

Difference between neutralization reaction & a precipitation reaction.

Neutralization reaction	Precipitation reaction
A reaction between two compounds - base and acid to give two new compounds - salt and water - by interchange of radicals is called a Neutralization 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.
Example: NaOH + HCl ⟶ NaCl + H₂O	FeCl₃ + 3NaOH ⟶ 3NaCl + Fe(OH)₃ [reddish brown ppt.]

Difference between electrolytic decomposition & photochemical decomposition.

Electrolytic decomposition	Photochemical decomposition
A decomposition reaction which is brought about by passage of electric current is known as Electrolytic decomposition.	A decomposition reaction which occurs in the presence of light is known as Photochemical decomposition.
For example: 2H₂O $\xrightarrow{\text{electric current}}$ 2H₂ + O₂	For example: 2AgCl $\xrightarrow{\text{sunlight}}$ 2Ag + Cl

Difference between catalyst & a promoter.

Catalyst	Promoter
A catalyst is used to alter the rate of the reaction but it does not take part in the reaction.	A promoter is a substance which is added to the catalyst to increase its efficiency.

Difference between an acidic oxide & a basic oxide.

Acidic oxide	Basic oxide
Acidic oxide are oxides of non-metals e.g. SO₂, CO₂	Basic oxide are oxides of metals e.g. K₂O, CaO
React with water to give an acid.	React with water to give a base.
React with bases [alkalis] to give salt and water	React with acids to give salt and water.

Question 5

Match the chemical reactions in List I with the appropriate answer in List II.

List I	List II
1. XY $\xrightleftharpoons {\text{heat}}$ X+Y	A: Displacement reaction
2. XY ⟶ X + Y	B: Double decomposition
3. X⁺Y^- + A⁺B^- ⟶ X⁺B^- + A⁺Y^-	C: Endothermic reaction
4. X + YZ ⟶ XZ + Y	D: Thermal dissociation
5. X + Y $\xrightarrow{\text{heat}}$ XY - Δ	E: Decomposition reaction

Answer

List I	List II
1. XY $\xrightleftharpoons {\text{heat}}$ X+Y	D: Thermal dissociation
2. XY ⟶ X + Y	E: Decomposition reaction
3. X⁺Y^- + A⁺B^- ⟶ X⁺B^- + A⁺Y^-	B: Double decomposition
4. X + YZ ⟶ XZ + Y	A: Displacement reaction
5. X + Y $\xrightarrow{\text{heat}}$ XY - Δ	C: Endothermic reaction