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Chemical Reactions — Question 4

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Question 4

Explain the following types of chemical reactions giving two examples for each of them:

(a) combination reaction

(b) decomposition reaction

(c) displacement reaction

(d) double displacement reaction

Answer

(a) Combination Reaction — A reaction in which two or more substances combine to form a single substance is called a combination reaction. It is also called a synthesis reaction.

Examples:

(i) Two elements combine to form a compound.

When iron and sulphur (both elements) are heated together they combine to form a compound iron sulphide.

Fe (s) + S (s) →Δ\xrightarrow{\enspace\Delta\enspace} FeS (s)

(ii) An element and a compound can also combine to form a product.

Carbon monoxide, a compound, burns in presence of oxygen, an element, to form a single product, carbon dioxide.

2CO (g) + O2 (g) →Δ\xrightarrow{\enspace\Delta\enspace} 2CO2 (g)

(b) Decomposition Reaction — A reaction in which a compound breaks up on heating into two or more simpler substances is called a decomposition reaction.

Examples:

(i) Calcium carbonate decomposes on strong heating to form two compounds, calcium oxide and carbon dioxide.

CaCO3 (s) →Δ\xrightarrow{\enspace\Delta\enspace} CaO (s) + CO2 (g)

(ii) Mercuric oxide when heated decomposes to form mercury and oxygen.

2HgO (s) →Δ\xrightarrow{\enspace\Delta\enspace} 2Hg (s) + O2 (g)

(c) Displacement Reaction — A reaction in which a more active element displaces a less active element from a compound is called a displacement reaction.

Examples:

(i) Zinc is more reactive than copper so it displaces copper from copper sulphate solution to form zinc sulphate.

Zn (s) + CuSO4 (aq) ⟶ ZnSO4 (aq) + Cu (s)

(ii) Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution.

2KBr + Cl2 ⟶ 2KCl + Br2

(d) Double Displacement Reaction — A chemical reaction in which two compounds in their aqueous state exchange their ions or radicals to form new compounds is called a double decomposition reaction.

AB + CD ⟶ CB + AD

Here AB and CD are reactants. They exchange their ions to form CB and AD which are the products.

Examples:

(i) When a solution of silver nitrate is added to a solution of sodium chloride a precipitate of silver chloride is formed.

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

(ii) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed.

FeSO4 (aq) + 2NaOH (aq) ⟶ Fe(OH)2 ↓ + Na2SO4 (aq)

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Chemical Reactions — Interactive Study Guide

Quick Concept Map

CHANGES: Physical (reversible, no new substance) vs Chemical (irreversible, new substance)
REACTION TYPES: Combination | Decomposition | Displacement | Double Displacement
ENERGY: Exothermic (releases heat) vs Endothermic (absorbs heat)
REDOX: Oxidation (gain O / lose H) + Reduction (lose O / gain H) happening together

Four Types of Reactions — Quick Reference

TypePatternEasy Memory AidExample
CombinationA + B → ABJoining togetherMg + O2 → MgO
DecompositionAB → A + BBreaking apartCaCO3 → CaO + CO2
DisplacementA + BC → AC + BBully kicks out the weakFe + CuSO4 → FeSO4 + Cu
Double DisplacementAB + CD → AD + CBExchange partnersNaCl + AgNO3 → AgCl + NaNO3

Signs of a Chemical Reaction

  1. Change in colour
  2. Evolution of gas (bubbles)
  3. Change in temperature (hot or cold)
  4. Formation of precipitate (insoluble solid)
  5. Change in smell
  6. Change in state
  7. Sound or light produced

Oxidation vs Reduction

OxidationReduction
Gain of oxygenLoss of oxygen
Loss of hydrogenGain of hydrogen
Example: C + O2 → CO2 (C is oxidised)Example: CuO + H2 → Cu + H2O (CuO is reduced)

Memory aid: OIL RIG — Oxidation Is Loss (of hydrogen), Reduction Is Gain (of hydrogen)

Self-Check Questions

  1. Is dissolving salt in water a physical or chemical change? Why?
  2. Classify: Iron + Copper sulphate → Iron sulphate + Copper. What type of reaction is this?
  3. What is the difference between exothermic and endothermic reactions? Give one example of each.
  4. In the reaction CuO + H2 → Cu + H2O, which substance is oxidised and which is reduced?
  5. Write word equations for one example of each: combination, decomposition, displacement, double displacement.

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