Question 3

The vapour pressure versus temperature graph below was obtained for four straight chain (unbranched) alkanes (P, Q, R and S).
FROM P TO S, EACH COMPOUND DIFFERS FROM THE PREVIOUS COMPOUND BY A –CH₂ GROUP.
The vapour pressures are measured in mmHg. Atmospheric pressure is 760 mmHg.

3.1 Give a reason why alkanes are said to be SATURATED. (1)

3.2 Define vapour pressure. (2)

3.3 Use the information in the graph above to answer the following questions.
3.3.1 What is the effect of an increase in temperature on vapour pressure? Choose from INCREASES, DECREASES or NO EFFECT. (1)

3.3.2 Which compound has a boiling point of approximately 68 °C? Give a reason for the answer. (2)

3.3.3 Which compound has the longest chain length? Fully explain the answer. (4)

3.4 Compound P has FIVE carbon atoms.
3.4.1 Draw the structural formula of a chain isomer of P. Write down the IUPAC name of this isomer. (3)
3.4.2 How will the vapour pressure of this isomer compare with that of compound P? Choose from HIGHER THAN, LOWER THAN or EQUAL TO. (1)

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