Chemical Combination and Decomposition
Chemical combination is a reaction in which two or more elements or simple compounds join to form a single, more complex substance, and chemical decomposition is a reaction in which one compound breaks down into two or more simpler substances. In brief: hydrogen and oxygen combine to make water (2H2 + O2 → 2H2O), and electrolysis decomposes water back into hydrogen and oxygen (2H2O → 2H2 + O2).
Combination and decomposition are opposite types of reaction. Chemists use combination to make useful compounds, and decomposition to analyze compounds and to obtain elements from them. This article describes both, shows the methods that cause decomposition (heat, electricity and light), compares them in a table, works through a mass calculation and gives examples from industry and history.
What is chemical combination?
In a combination reaction (also called synthesis), two or more reactants form one product: A + B → AB. The reactants can be elements, such as magnesium and oxygen, or compounds, such as calcium oxide and water. The product has new properties.
- 2Mg + O2 → 2MgO (magnesium burns brightly to form a white powder).
- Fe + S → FeS (iron and sulfur form a black compound when heated).
- N2 + 3H2 → 2NH3 (the Haber process for ammonia).
- CaO + H2O → Ca(OH)2 (quicklime forms slaked lime and releases heat).
What is chemical decomposition?
In a decomposition reaction, one reactant splits into two or more products: AB → A + B. The reaction needs an input of energy to break the bonds in the compound, which is supplied as heat, electricity or light. The products can be elements or simpler compounds.
- 2HgO → 2Hg + O2 (heat). Mercury(II) oxide gives mercury and oxygen.
- CaCO3 → CaO + CO2 (heat). Limestone gives quicklime and carbon dioxide.
- 2H2O → 2H2 + O2 (electricity). Electrolysis of water.
- 2AgCl → 2Ag + Cl2 (light). Silver chloride darkens in sunlight, which is the basis of traditional photography.
- 2H2O2 → 2H2O + O2 (catalyst or light). Hydrogen peroxide releases oxygen.
How are compounds decomposed?
| Type | Energy source | Example |
|---|---|---|
| Thermal decomposition | Heat | Copper(II) carbonate → copper(II) oxide + carbon dioxide |
| Electrolysis | Electric current | Molten sodium chloride → sodium + chlorine |
| Photolysis | Light | Silver bromide → silver + bromine |
| Catalytic decomposition | Catalyst (speeds up the reaction) | Hydrogen peroxide with manganese(IV) oxide |
Combination and decomposition compared
| Feature | Combination | Decomposition |
|---|---|---|
| General form | A + B → AB | AB → A + B |
| Number of reactants | Two or more | One |
| Number of products | One | Two or more |
| Typical energy change | Often exothermic | Often endothermic |
| Example | 2Na + Cl2 → 2NaCl | 2NaCl → 2Na + Cl2 |
Worked example: heating limestone
Limestone (calcium carbonate) is heated in a kiln to make quicklime (calcium oxide) for cement and agriculture. How much quicklime and carbon dioxide are formed from 500 kg of pure calcium carbonate?
- Equation. CaCO3 → CaO + CO2.
- Molar masses. CaCO3 = 40.08 + 12.01 + 3 × 16.00 = 100.09 g/mol. CaO = 40.08 + 16.00 = 56.08 g/mol. CO2 = 44.01 g/mol.
- Mass of CaO. 500 × 56.08 ÷ 100.09 = 280.1 kg.
- Mass of CO2. 500 × 44.01 ÷ 100.09 = 219.9 kg.
- Check. 280.1 + 219.9 = 500.0 kg. The mass is conserved.
A 56% fraction of the mass remains as solid quicklime (280.1 ÷ 500), and 44% is released as gas. This explains why the solid left after heating is lighter than the limestone and why lime kilns release carbon dioxide.
Where are these reactions used?
- Metal production: aluminum is obtained by electrolytic decomposition of molten aluminum oxide, and many metals are produced by reduction of oxides.
- Fertilizer: combining nitrogen and hydrogen makes ammonia, the starting material of most nitrogen fertilizers.
- Building: heating limestone gives quicklime, used in cement and mortar.
- Safety: airbags inflate in milliseconds through the decomposition of a nitrogen-containing compound, which releases nitrogen gas.
- Cooking: baking soda decomposes when heated and releases carbon dioxide, which makes cakes rise.
How were these reactions discovered?
In 1774 Joseph Priestley obtained oxygen by heating mercury(II) oxide with sunlight focused by a lens, which is a famous example of decomposition. In 1800 William Nicholson and Anthony Carlisle decomposed water by electrolysis soon after Alessandro Volta invented the battery. Humphry Davy used electrolysis in 1807 and 1808 to isolate potassium, sodium, calcium, magnesium and other elements for the first time.
Which mistakes are common?
Mistake 1: Treating decomposition as a physical change
Decomposition forms new substances by breaking bonds. Melting or boiling does not.
Mistake 2: Forgetting the energy requirement
Decomposition does not happen spontaneously for stable compounds. Heat, electricity or light must be supplied.
Mistake 3: Expecting the products of decomposition to be elements
Often they are simpler compounds, such as calcium oxide and carbon dioxide.
Mistake 4: Assuming that combination always releases heat
Most do, but some combination reactions are endothermic and need an energy input.