The vapour pressure versus temperature graph below was obtained for four straight chain (unbranched) alkanes (P, Q, R and S) - NSC Physical Sciences - Question 3 - 2017 - Paper 2
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 FRO... show full transcript
Worked Solution & Example Answer:The vapour pressure versus temperature graph below was obtained for four straight chain (unbranched) alkanes (P, Q, R and S) - NSC Physical Sciences - Question 3 - 2017 - Paper 2
Step 1
3.1 Give a reason why alkanes are said to be SATURATED.
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Answer
Alkanes are said to be saturated because they have only single bonds between carbon atoms. This means that each carbon atom is bonded to the maximum number of hydrogen atoms, as there are no double or triple bonds present.
Step 2
3.2 Define vapour pressure.
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Answer
Vapour pressure is the pressure exerted by a vapour in equilibrium with its liquid in a closed system. It reflects the tendency of the molecules to escape from the liquid phase into the gas phase.
Step 3
3.3.1 What is the effect of an increase in temperature on vapour pressure? Choose from INCREASES, DECREASES or NO EFFECT.
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Answer
Increases
Step 4
3.3.2 Which compound has a boiling point of approximately 68 °C? Give a reason for the answer.
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Answer
Compound Q has a boiling point of approximately 68 °C because it is the temperature where the graph intercepts the dotted line, indicating the balance between the vapour and liquid phases under atmospheric pressure.
Step 5
3.3.3 Which compound has the longest chain length? Fully explain the answer.
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Answer
Compound S has the longest chain length as it is the last in the series from P to S. As we move from P to S, each compound gains a -CH₂ group, leading to the longest carbon chain in compound S. This affects properties such as boiling point and vapour pressure.
Step 6
3.4.1 Draw the structural formula of a chain isomer of P. Write down the IUPAC name of this isomer.
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Answer
The structural formula can be drawn as follows:
CH3
|
CH3-CH-CH2-CH3
|
CH2
The IUPAC name of this isomer is 2-Pentane.
Step 7
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.
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Answer
The vapour pressure of this isomer will be LOWER THAN that of compound P due to increased surface area and intermolecular forces that affect its volatility.
