How the physical properties of hydrocarbons change with molecule size
3 marks · NormalExplain why hydrocarbons with larger molecules burn less easily than hydrocarbons with smaller molecules.
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Also accept: Larger molecules are less volatile and have stronger intermolecular forces, making it harder to evaporate and burn.; Large hydrocarbons have higher boiling points and do not vaporise as easily, so they do not ignite or combust as readily as small ones.
Common mistakes: Confusing ease of burning with colour or viscosity.
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Marking points
M1 · 1 mark · Identifies that larger molecules have stronger intermolecular forces.
M2 · 1 mark · Mentions that large hydrocarbons are less volatile / have higher boiling points / evaporate less easily.
A1 · 1 mark · Links to burning: large molecules find it harder to vaporise, so burn less easily.
Worked solution
Larger hydrocarbon molecules have stronger intermolecular forces, which makes them less volatile and harder to evaporate.
Because they do not evaporate easily, it is harder for them to form a gas that can burn, so they burn less easily than smaller hydrocarbon molecules.