In a combination reaction, two or more substances form a single product. In a decomposition reaction, one substance breaks down into two or more, usually with energy supplied as heat, light or electricity.
Many different reactions have the same pattern in their equations. Grouping them by pattern lets you recognise a reaction type from its equation alone. The first two types are reverses of each other.
In a combination reaction, two or more substances react to form a single product. In the equation, there are two or more formulae on the left and exactly one on the right.
When calcium oxide (quick lime) is added to water, the two combine to form calcium hydroxide (slaked lime):
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
“Heat” is written as a product because the reaction releases heat; the beaker becomes hot. The next session covers this energy change.
Carbon burning in oxygen (C + O₂ → CO₂) and hydrogen burning in oxygen (2 H₂ + O₂ → 2 H₂O) have the same pattern.
Reverse patterns. Combination forms one product from several reactants. Decomposition breaks one reactant into several products.
Burning coal is written C + O₂ → CO₂. Which type of reaction is it, and how can you tell from the equation?
It is a combination reaction. The equation has two reactants, carbon and oxygen, and one product, carbon dioxide.
The pattern “several formulae on the left, one on the right” is enough to identify the type.
A decomposition reaction is the reverse pattern. One substance breaks down into two or more simpler substances. The equation has one formula on the left and two or more on the right.
Breaking a compound apart means breaking the chemical bonds that hold its atoms together. For most compounds, that requires more energy than is released when the products form. So most decomposition reactions need energy to be supplied. They are grouped by the form that energy takes.
Heat (thermal decomposition). Heating green ferrous sulphate crystals first drives off their water, then breaks the ferrous sulphate into ferric oxide, sulphur dioxide and sulphur trioxide. The solid turns reddish-brown, and sulphur dioxide has the smell of burning sulphur. Heating calcium carbonate (limestone) gives calcium oxide and carbon dioxide: CaCO₃ → CaO + CO₂. This reaction is used in industry to make quick lime for cement. Heating lead nitrate gives off brown nitrogen dioxide gas.
Light. White silver chloride exposed to sunlight turns grey as it decomposes: 2 AgCl → 2 Ag + Cl₂. The grey colour is finely divided silver. Black-and-white photographic film used light-sensitive silver compounds that decompose in this way.
Electricity (electrolysis). Passing an electric current through water containing a little acid decomposes it into hydrogen and oxygen: 2 H₂O → 2 H₂ + O₂. The acid is added because pure water conducts electricity very poorly. Hydrogen collects at one electrode and oxygen at the other.
In the electrolysis of water, one tube collects gas at twice the rate of the other. Which gas is which, and why is the ratio 2 : 1?
The faster tube collects hydrogen. The slower tube collects oxygen.
The balanced equation is 2 H₂O → 2 H₂ + O₂. For every 2 molecules of hydrogen formed, 1 molecule of oxygen forms.
At the same temperature and pressure, equal volumes of gases contain equal numbers of molecules. So twice as many molecules of hydrogen take up twice the volume. The 2 : 1 ratio in the equation appears as a 2 : 1 ratio of gas volumes.
Combination joins substances; decomposition separates them. Many combination reactions release heat. Most decomposition reactions need energy supplied. In combination, several reactants give one product. In decomposition, one reactant gives several products.
The next session looks at the energy change in more detail: why some reactions release heat and others take it in.
1In a combination reaction…
2Sort each reaction into its type.
3A decomposition reaction caused by heating is called ____ decomposition.
4Why do most decomposition reactions need energy to be supplied?
5Silver chloride turning grey in sunlight is a decomposition caused by…
6In the electrolysis of water, why is the volume of hydrogen twice the volume of oxygen?
7Combination and decomposition are reverse patterns because…
Next: exothermic and endothermic reactions — where the energy goes.
Noescio