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The chart compares the boiling points of alkanes and primary alcohols containing from one to four carbon atoms - Leaving Cert Chemistry - Question c - 2006
Question c
The chart compares the boiling points of alkanes and primary alcohols containing from one to four carbon atoms. (i) Give two reasons why each of these alcohols has ... show full transcript
Worked Solution & Example Answer:The chart compares the boiling points of alkanes and primary alcohols containing from one to four carbon atoms - Leaving Cert Chemistry - Question c - 2006
Step 1
Give two reasons why each of these alcohols has a higher boiling point than the corresponding alkane.
Answer
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Alcohols have a higher relative molecular mass compared to their corresponding alkanes, which contributes to stronger van der Waals forces, leading to higher boiling points.
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Alcohols contain a polar hydroxyl group (-OH) that allows for intermolecular hydrogen bonding, significantly raising the boiling point compared to alkanes that lack this feature.
Step 2
Explain why the difference in boiling points between methane and methanol is 226.5 K while the difference in boiling points between butane and butanol is only 119 K.
Answer
The greater difference in boiling points between methane and methanol (226.5 K) can be attributed to the strong hydrogen bonding in methanol due to its hydroxyl group, leading to an increase in boiling temperature. In contrast, butane has a longer non-polar hydrocarbon chain, which contributes to weaker hydrogen bonding in butanol, resulting in a smaller difference (119 K) between their boiling points.
Step 3
Describe, in general terms, the solubilities of methane, methanol, butane and butanol in water.
Answer
- Methane: virtually insoluble
- Methanol: completely soluble (miscible) in all proportions
- Butane: slightly soluble in water
- Butanol: somewhat soluble, but less than methanol.
In general, methane is insoluble due to its non-polar structure, while methanol is completely miscible due to hydrogen bonding. Butane is only slightly soluble, while butanol's solubility is limited by its larger carbon chain.