7.2 Classification of Chemical Reactions

Chemical reactions can be classified in the following ways:

7.2.1 Direction of Reaction

Irreversible Reactions

In irreversible reactions, the reactants convert to products, and the products cannot revert to reactants. This occurs if one of the products is removed. For example, when calcium carbonate is heated, it breaks into solid calcium oxide and gaseous carbon dioxide. If the carbon dioxide is removed, the calcium oxide will not convert back to calcium carbonate. These reactions are represented with a right-facing arrow (→).

Example:

CaCO3 (s) → CaO (s) + CO2 (g)

Reversible Reactions

In reversible reactions, the reactants react to form products, and these products can revert back to the reactants. The forward reaction forms products, and the backward reaction converts the products back to reactants. These reactions are written with a double-headed arrow (⇌).

Example:

CH3CH2OH + CH3COOH ⇌ CH3COOCH2CH3 + H2O

Here, ethanol and acetic acid react to form ethyl acetate and water, and the reverse reaction can occur.

7.2.2 Heat Change and Reactions

Chemical reactions are classified based on whether they absorb or release heat.

Exothermic Reactions

In exothermic reactions, heat is released during the formation of products. The heat change (ΔH) is negative.

Example:

N2 (g) + 3H2 (g) → 2NH3 (g) + 92 kJ

Endothermic Reactions

In endothermic reactions, heat is absorbed during the formation of products. The heat change (ΔH) is positive.

Example:

N2 (g) + O2 (g) + 180 kJ → 2NO (g)

7.2.3 Electron Transfer

Reactions can be divided into redox and non-redox reactions based on electron transfer.

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons. In oxidation, an atom loses electrons, and in reduction, an atom gains electrons.

Example:

Na → Na+ + e− (oxidation)

Cl− + e− → Cl (reduction)

Oxidation number is the charge an atom would have if electrons were transferred completely. The oxidation number helps identify whether an element is oxidized or reduced.

Non-Redox Reactions

Non-redox reactions do not involve electron transfer. Examples include neutralization and precipitation reactions.

Types of Chemical Reactions

1. Addition Reaction

Two or more substances combine to form a new compound.

Example:

N2 + 3H2 → 2NH3

2. Decomposition Reaction

A compound breaks down into simpler substances.

Example:

PCl5 (heat) → PCl3 + Cl2

3. Substitution or Displacement Reaction

An element or group displaces another from a compound.

Example:

Zn + H2SO4 → ZnSO4 + H2

4. Combustion Reaction

A substance reacts with oxygen to produce oxides and release heat.

Example:

CH4 + 2O2 → CO2 + 2H2O + Heat

Non-Redox Reactions

1. Neutralization Reaction

An acid reacts with a base to form salt and water.

Example:

HCl + NaOH → NaCl + H2O

2. Precipitation Reaction

When two soluble compounds form an insoluble product (precipitate).

Example:

NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)

In precipitation reactions, a solid (precipitate) is formed when an insoluble compound is produced.