How the physical properties of hydrocarbons change with molecule size
2 marks · HardExplain why hydrocarbons with larger molecules are less useful as fuels than those with smaller molecules.
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Also accept: Larger molecules do not burn as easily; Larger molecules are less flammable / harder to ignite / burn with more difficulty; Larger molecules are more viscous and do not flow/pour as easily (making use difficult)
Common mistakes: Explain only 'because they are bigger' with no fuel link; Confuse with chemical composition
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Marking points
M1 · 1 mark · States that larger molecules burn less easily or are less flammable (or harder to ignite/use as fuel).
M2 · 1 mark · Relates reduced usefulness to increased viscosity / flow less easily (do not pour/run as fuel) OR links to difficulty in burning completely as a fuel.
Worked solution
Hydrocarbons with larger molecules are less useful as fuels because they burn less easily and are more viscous, making them difficult to handle and deliver as fuels.