As fuels

2 marks · Hard

State the trend in boiling point and position within the fractionating column for hydrocarbons with increasing chain length, giving a reason for each.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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Also accept: As chain length increases, boiling point increases; hydrocarbons with longer chains condense lower down the column.; The boiling point increases as the size of the hydrocarbon increases, and these hydrocarbons are found nearer the bottom of the column since they condense at higher temperatures.

Common mistakes: Stating that all hydrocarbons have the same boiling point.; Reversing the trend: saying longer chains are at the top.; Not linking position to temperature/boiling point.

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Marking points

B1 · 1 mark · States or describes correctly that boiling point increases as hydrocarbon chain length increases.

B2 · 1 mark · States or describes that longer chain hydrocarbons condense lower in the column, or are found nearer the bottom; or correctly links position/temperature to chain length.

Worked solution

As the chain length of a hydrocarbon increases, its boiling point increases (B1). This means that longer chain hydrocarbons condense at higher temperatures and are collected lower down the fractionating column (B2).