Properties of hydrocarbons: boiling points

The idea

Boiling point is the temperature at which a liquid hydrocarbon changes to gas, with all molecules gaining enough energy to overcome intermolecular forces. In a homologous series, chain length controls boiling point: larger molecules have greater surface contact and stronger intermolecular forces, so more energy is needed to separate them. Fractional distillation depends on this trend, because shorter chains rise higher in the column and condense at lower temperatures.

What earns the marks

• Boiling point increases as the number of carbon atoms in the hydrocarbon chain increases.
• Larger hydrocarbon molecules have greater intermolecular forces because they have more surface contact between molecules.
• More energy is required to overcome these intermolecular forces, so the boiling point is higher.
• Smaller hydrocarbons such as methane and propane are gases at room temperature, while larger ones such as octane are liquids.
• The trend applies within a homologous series and can be used to predict the physical state at a given temperature.
• Fractional distillation separates hydrocarbons because fractions with lower boiling points condense higher up the column.

Common mistakes and how to put them right

• Stating that covalent bonds inside hydrocarbon molecules break during boiling.

  Put it right: Boiling overcomes intermolecular forces between molecules; covalent bonds remain intact.

• Saying larger molecules have weaker forces because they are heavier.

  Put it right: Larger molecules have stronger intermolecular forces because of greater surface contact, despite being heavier.

• Confusing boiling point with melting point or with the temperature at which a fraction condenses.

  Put it right: Boiling point is the temperature at which a liquid changes to gas at a stated pressure.

Exam tips

• When describing the trend, quote at least two named hydrocarbons and their approximate boiling points to support your answer.
• Use the phrase intermolecular forces rather than bonds when explaining why boiling points increase.
• If asked to predict a state, compare the boiling point with the stated temperature and state whether the substance is solid, liquid or gas.
• Link boiling point to fractional distillation by explaining that shorter chains condense at lower temperatures higher up the column.