Chapter 1 · Question 7

What are decomposition reactions? Explain thermal, electrolytic, and photolytic (photochemical) decomposition with one chemical equation for each type.

Answer Revealed

Direct Answer:

A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more simpler substances. Thermal decomposition uses heat:

2\text{FeSO}_4\text{(s)} \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3\text{(s)} + \text{SO}_2\text{(g)} + \text{SO}_3\text{(g)}

(ferrous sulphate decomposes into ferric oxide, sulphur dioxide, and sulphur trioxide). Electrolytic decomposition uses electricity:

2\text{H}_2\text{O(l)} \xrightarrow{\text{Electric current}} 2\text{H}_2\text{(g)} + \text{O}_2\text{(g)}

(water decomposes into hydrogen and oxygen). Photolytic decomposition uses light:

2\text{AgCl(s)} \xrightarrow{\text{Sunlight}} 2\text{Ag(s)} + \text{Cl}_2\text{(g)}

(silver chloride decomposes into silver and chlorine).

Simple Explanation

Decomposition is the reverse of combination — you start with one substance and it breaks apart into two or more simpler ones. Three kinds of energy can do this: heat (heating ferrous sulphate turns it from green to brown and releases gases), electricity (passing current through water splits it into hydrogen and oxygen gases), and light (leaving white silver chloride in the sun turns it grey because it breaks down into silver metal and chlorine gas).

Exam-Ready Structure

Decomposition reactions are characterised by the breakdown of one substance into simpler products, and the energy source classifies them into subtypes: 1. Definition: A decomposition reaction is a chemical reaction in which a single reactant (a compound) splits into two or more simpler products. General form:

AB \rightarrow A + B

. Decomposition reactions are generally endothermic because energy is required to break the chemical bonds. 2. Thermal decomposition (uses heat): The compound decomposes on heating. Example — Ferrous sulphate crystals (green,

\text{FeSO}_4 \cdot 7\text{H}_2\text{O}

) on heating first lose water of crystallisation and then decompose:

2\text{FeSO}_4\text{(s)} \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3\text{(s)} + \text{SO}_2\text{(g)} + \text{SO}_3\text{(g)}

(Equation 1.19). The green colour changes to brown (ferric oxide) with a characteristic odour of burning sulphur. Another example:

\text{CaCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}

— limestone decomposes to quicklime and carbon dioxide. 3. Electrolytic decomposition (uses electricity): When electric current is passed through a compound, it decomposes. Example — Electrolysis of water:

2\text{H}_2\text{O(l)} \xrightarrow{\text{Electric current}} 2\text{H}_2\text{(g)} + \text{O}_2\text{(g)}

. In this experiment, the volume of hydrogen collected at the cathode is double that of oxygen collected at the anode because water (

\text{H}_2\text{O}

) has two hydrogen atoms for every oxygen atom. 4. Photolytic or photochemical decomposition (uses light/light energy): The compound decomposes when exposed to light, particularly sunlight. Example:

2\text{AgCl(s)} \xrightarrow{\text{Sunlight}} 2\text{Ag(s)} + \text{Cl}_2\text{(g)}

(Equation 1.22). White silver chloride turns grey due to the formation of silver metal. Similarly,

2\text{AgBr(s)} \xrightarrow{\text{Sunlight}} 2\text{Ag(s)} + \text{Br}_2\text{(g)}

. This reaction of silver halides is used in black and white photography.

Key Points

Decomposition reaction: single reactant breaks into simpler products ( $AB \rightarrow A + B$ )
Thermal: $2\text{FeSO}_4 \xrightarrow{\text{Heat}} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$ (green → brown, uses heat)
Electrolytic: $2\text{H}_2\text{O} \xrightarrow{\text{Electric current}} 2\text{H}_2 + \text{O}_2$ (uses electricity; H₂ volume is double O₂ volume)
Photolytic: $2\text{AgCl} \xrightarrow{\text{Sunlight}} 2\text{Ag} + \text{Cl}_2$ (white → grey, uses light energy)
Decomposition reactions are endothermic — they require or absorb energy to proceed

What are decomposition reactions? Explain thermal, electrolytic, and photolytic (photochemical) decomposition with one chemical equation for each type.