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Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · 8.7 Check the specification (PDF) (opens in a new tab)
A hydrocarbon is a compound containing carbon and hydrogen only. Many fuels are hydrocarbons. During , the fuel reacts chemically with oxygen: atoms in the reactants are rearranged to make new substances.
For combustion to be complete, there must be sufficient oxygen. Every carbon atom from the hydrocarbon becomes part of carbon dioxide, while the hydrogen atoms become part of water. The general word equation is:
Thus, the complete combustion of a hydrocarbon produces carbon dioxide and water. Water may leave the flame as water vapour because the products are hot.
In complete combustion, atoms from a hydrocarbon and oxygen are rearranged into carbon dioxide and water, while energy is transferred to the surroundings.
Complete combustion also gives out energy, so it is an exothermic reaction. Some of the energy heats the surroundings and may also be transferred as light from the flame. This energy transfer is why hydrocarbons are useful as fuels—for example, they can provide heating or power engines.
Energy is not another chemical substance made by the reaction. A fuller word representation can show the transfer separately:
Methane is the hydrocarbon found in natural gas. Its complete combustion can be represented by:
This equation is balanced because it has one carbon atom, four hydrogen atoms and four oxygen atoms on each side. It therefore shows conservation of atoms as well as identifying the two products.
The products can be investigated experimentally. Water vapour can condense as the hot gases cool, producing droplets. Carbon dioxide can be passed through limewater; the limewater turning cloudy is evidence that carbon dioxide is present. Together, these observations support the prediction that complete combustion of a hydrocarbon forms water and carbon dioxide.
Complete combustion occurs when a hydrocarbon burns in sufficient oxygen.
Energy is given out to the surroundings, so the reaction is exothermic.
Carbon atoms end up in carbon dioxide; hydrogen atoms end up in water.
State both chemical products: carbon dioxide and water. Do not omit the energy released.
Write oxygen on the reactant side. Energy is transferred to the surroundings; it is not a chemical substance to balance in an equation.
If asked for a balanced symbol equation, change coefficients in front of formulae, never the small numbers within a formula.
Link the word “complete” to a sufficient supply of oxygen.
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Last updated
Hydrocarbon
A compound containing carbon and hydrogen atoms only.
Combustion
A chemical reaction in which a substance reacts with oxygen, usually transferring energy to the surroundings.
Complete combustion
The reaction of a fuel with sufficient oxygen so that a hydrocarbon forms carbon dioxide and water.
Exothermic reaction
A reaction that transfers energy from the reacting chemicals to the surroundings.
Put your knowledge into practice — try past paper questions for Chemistry
Hydrocarbon
A compound containing carbon and hydrogen atoms only.
Combustion
A chemical reaction in which a substance reacts with oxygen, usually transferring energy to the surroundings.
Complete combustion
The reaction of a fuel with sufficient oxygen so that a hydrocarbon forms carbon dioxide and water.
Exothermic reaction
A reaction that transfers energy from the reacting chemicals to the surroundings.