The relationship between boiling point and the number of carbon atoms in straight chain molecules of aldehydes, alkanes and primary alcohols is investigated - NSC Physical Sciences - Question 3 - 2020 - Paper 2
Question 3
The relationship between boiling point and the number of carbon atoms in straight chain molecules of aldehydes, alkanes and primary alcohols is investigated. Curves ... show full transcript
Worked Solution & Example Answer:The relationship between boiling point and the number of carbon atoms in straight chain molecules of aldehydes, alkanes and primary alcohols is investigated - NSC Physical Sciences - Question 3 - 2020 - Paper 2
Step 1
Define the term boiling point.
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Answer
The boiling point is the temperature at which the vapor pressure of a liquid equals the external atmospheric pressure, causing the liquid to transform into vapor.
Step 2
Write down the STRUCTURAL FORMULA of the functional group of the aldehydes.
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Answer
The structural formula of the functional group of aldehydes is:
O
||
R–C–H
Where R is a hydrocarbon chain.
Step 3
The graph shows that the boiling points increase as the number of carbon atoms increases. Fully explain this trend.
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Answer
As the number of carbon atoms increases, the molecular size and surface area also increase. This leads to a greater number of electron sites, which enhances the strength of London dispersion forces.
Additionally, larger molecules tend to have more polarizability, resulting in stronger intermolecular attractions. Consequently, more energy (higher temperatures) is needed to overcome these intermolecular forces, thereby increasing the boiling point.
Step 4
Identify the curve (A, B or C) that represents the following:
3.4.1 Compounds with London forces only
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Curve C represents compounds with London forces only, as it corresponds to alkanes which only exhibit these weak intermolecular forces.
Step 5
3.4.2 The aldehydes
Explain the answer.
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Curve B represents the aldehydes. Aldehydes possess both London forces and dipole-dipole interactions due to their polar carbonyl group, which makes their boiling points higher than those of alkanes (Curve C).
Step 6
Use the information in the graph and write down the IUPAC name of the compound with a boiling point of 373 K.
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Answer
The compound with a boiling point of 373 K is Butanal.
Step 7
Write down the IUPAC name of the compound containing five carbon atoms, which has the lowest vapour pressure at a given temperature.
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The compound with five carbon atoms that has the lowest vapor pressure is Pentan-1-ol.
