Properties of hydrocarbons — AQA GCSE Chemistry

The physical properties of hydrocarbons change predictably with molecular size. As the carbon chain gets longer, the molecules become larger and have stronger intermolecular forces. This raises the boiling point, making larger hydrocarbons less volatile. Larger molecules also flow less easily, so viscosity increases, and they become less flammable. For example, methane (CH₄) is a small gas molecule that vaporises and ignites easily, while a long-chain hydrocarbon such as fuel oil is a viscous liquid that is harder to vaporise and burn. Fractions collected lower down in a fractionating column consist of larger molecules, meaning they are more viscous, have higher boiling points, and are less flammable.

Hydrocarbons are used as fuels because burning them transfers energy usefully, but their physical properties decide which fuel suits which job. Short-chain hydrocarbons such as methane and propane have low boiling points, so they are gases that vaporise readily and burn with a clean flame; they suit domestic heating and cooking. Longer-chain hydrocarbons have higher boiling points and are more viscous, so they are liquids that are easier to store and transport but harder to ignite; they suit petrol, diesel and heavier fuel oils. Flammability also falls as molecular size rises, so a fuel must be matched to its use: a gas for a quick, controllable flame, a liquid for a slower, sustained release of energy.

As the molecules in a hydrocarbon fraction get larger, three properties change in a regular way. Boiling point increases because larger molecules have stronger intermolecular forces between them, so more energy is needed to separate them. Viscosity increases because the longer molecules become tangled and slide over one another less easily, so the liquid flows more slowly. Flammability decreases because larger molecules are harder to vaporise and do not mix as readily with oxygen, so they ignite less easily and burn with a smokier flame. For example, methane is a gas that ignites instantly, while diesel is a viscous liquid that needs warming and fine spraying before it burns efficiently.

Combustion is an exothermic reaction: when a hydrocarbon fuel burns in oxygen, energy is transferred to the surroundings, usually as heat and light. The reaction needs a supply of oxygen, and the products are carbon dioxide and water when the fuel burns completely. For example, burning methane in plenty of air releases energy and forms carbon dioxide and water. The energy released makes hydrocarbons valuable as fuels for heating, cooking, electricity generation and transport. If oxygen is limited, combustion is incomplete and can produce carbon monoxide and soot, releasing less energy and creating a hazard, so a good air supply matters for safe and efficient burning.

In combustion, oxygen reacts with the elements in the fuel. The carbon in the hydrocarbon is oxidised to carbon dioxide, and the hydrogen is oxidised to water. Oxidation here means gain of oxygen, so both elements combine with oxygen. For example, in methane, CH₄, the carbon becomes CO₂ and the hydrogen becomes H₂O. Writing a balanced equation shows that every carbon atom needs one oxygen molecule to form carbon dioxide and every four hydrogen atoms need one oxygen molecule to form two water molecules. This is why complete combustion needs plenty of oxygen and why the products are carbon dioxide and water.

Complete combustion happens when a hydrocarbon burns in a plentiful supply of oxygen. The carbon atoms in the fuel are oxidised to carbon dioxide, and the hydrogen atoms are oxidised to water. For example, burning methane in excess oxygen gives carbon dioxide and water. The reaction releases energy, which is why hydrocarbons are useful as fuels. If oxygen is limited, combustion is incomplete and carbon monoxide or soot may form instead, so the products named here apply specifically to complete combustion. Testing the products confirms the idea: bubbling the gases through limewater turns it milky, showing carbon dioxide, while water vapour condenses as colourless droplets that turn anhydrous copper(II) sulfate from white to blue.

To balance a complete combustion equation, start from the general form: hydrocarbon plus oxygen gives carbon dioxide plus water. Balance carbon first by placing the correct number of CO₂ molecules, then balance hydrogen using H₂O molecules, and finally balance oxygen by adjusting the number of O₂ molecules. For example, for propane, C₃H₈, the balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check that every element has the same number of atoms on each side. If the oxygen count is odd, double the whole equation so that all coefficients are whole numbers, as happens with 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. State symbols may be added when required.