Chemical Reactions and Equations · Lesson 4 of 9
Decomposition Reaction
“One substance splits apart, provided heat, light or electricity pays the energy bill.”
• Recognise decomposition reactions and compare them with combination reactions. • Explain thermal decomposition using ferrous sulphate, calcium carbonate and lead nitrate. • Explain decomposition of water by electricity and the gas-volume relationship. • Explain decomposition of silver chloride and silver bromide by light. • Distinguish endothermic from exothermic reactions.
A combination reaction builds a single product from two or more reactants. A decomposition reaction moves in the opposite structural direction: one reactant breaks into two or more simpler products. Breaking the reactant often requires an input of energy.
Decomposition Reaction
A Decomposition Reaction is a reaction in which a single reactant breaks down to form two or more simpler products.
Thermal Decomposition
Thermal Decomposition is decomposition caused by supplying heat.
Heating Ferrous Sulphate Crystals
Ferrous sulphate crystals are green. On heating, hydrated ferrous sulphate first loses water and its appearance changes. With continued heating it decomposes to ferric oxide, sulphur dioxide and sulphur trioxide. The sulphur-containing gases have a characteristic burning-sulphur odour.
The key pattern is one reactant giving several products. Ferric oxide remains as a solid while sulphur dioxide and sulphur trioxide leave as gases.
Heating Calcium Carbonate
Limestone is calcium carbonate. On strong heating, calcium carbonate decomposes into calcium oxide and carbon dioxide. Calcium oxide is quicklime and is used, among other applications, in the manufacture of cement.
Heating Lead Nitrate
Heating lead nitrate produces brown fumes. The fumes are nitrogen dioxide. Lead oxide and oxygen are also formed. The appearance of the brown gas makes this decomposition reaction especially easy to recognise.
Decomposition By Electricity
Water can be decomposed using electric current. Carbon electrodes are placed in acidified water and connected to a battery. Gas bubbles form at both electrodes and are collected in inverted test tubes. The two gases are hydrogen and oxygen.
Why Is One Gas Volume Double The Other?
The balanced equation shows that two molecules of hydrogen are produced for every one molecule of oxygen. Under comparable conditions this is observed as about twice the volume of hydrogen compared with oxygen. The volume relationship therefore follows directly from the balanced equation: hydrogen : oxygen = two : one.
Decomposition By Light
White silver chloride turns grey when exposed to sunlight because it decomposes into silver metal and chlorine gas. Silver bromide undergoes a similar change. These light-driven decomposition reactions were used in black-and-white photography.
Endothermic Reactions
An Endothermic Reaction absorbs energy from its surroundings.
Decomposition reactions may need heat, light or electricity to break down the reactant. The chapter also describes mixing barium hydroxide with ammonium chloride and noticing a cooling effect. A temperature decrease indicates that energy is being absorbed from the surroundings.
| Energy Source | Example |
|---|---|
| Heat | Calcium carbonate → calcium oxide + carbon dioxide |
| Electricity | Water → hydrogen + oxygen |
| Light | Silver chloride → silver + chlorine |
Combination And Decomposition Compared
| Feature | Combination | Decomposition |
|---|---|---|
| Reactant-product pattern | Two or more reactants form one product | One reactant forms two or more products |
| Typical idea | Building a more complex product | Breaking a substance into simpler products |
| Energy behaviour | May release energy | Often requires an energy supply |
Quiz
Which statement defines a decomposition reaction?
What are the brown fumes formed when lead nitrate is heated?
Which gas is collected in roughly twice the volume during electrolysis of water?
Why does silver chloride turn grey in sunlight?
Which statement best describes an endothermic reaction?
Practice Problems
- Write one balanced decomposition equation caused by heat, one by electricity and one by light.
- Explain the sequence of changes observed when ferrous sulphate crystals are heated.
- Balance the lead nitrate decomposition equation and verify Pb, N and O atoms.
- Use the balanced electrolysis equation to explain why hydrogen is collected in about twice the volume of oxygen.
- Compare combination and decomposition reactions using one equation for each.
- Explain how a temperature decrease in the barium hydroxide and ammonium chloride activity indicates energy absorption.
Key Takeaways
• Decomposition reactions break one reactant into two or more products. • Thermal decomposition is caused by heat and includes the decomposition of ferrous sulphate, calcium carbonate and lead nitrate. • Electricity decomposes water into hydrogen and oxygen in a two-to-one volume relationship. • Sunlight decomposes silver chloride and silver bromide to form silver. • Reactions that absorb energy are endothermic and the required energy may be supplied as heat, light or electricity.