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Explain the trends in boiling points shown in the graph - HSC - SSCE Chemistry - Question 25 - 2003 - Paper 1
Question 25
Explain the trends in boiling points shown in the graph. Boiling point (°C) Molecular weight
Worked Solution & Example Answer:Explain the trends in boiling points shown in the graph - HSC - SSCE Chemistry - Question 25 - 2003 - Paper 1
Step 1
Explain the overall trend: Increases with Molecular Weight
Answer
As the molecular weight increases, the boiling points of alkanes, alkanols, and alkanolic acids tend to increase. This trend is due to the fact that larger molecules have greater surface areas, which leads to stronger London dispersion forces. These intermolecular forces require more energy (in the form of heat) to break, resulting in higher boiling points.
Step 2
Alkanes vs Alkanols: Compare their boiling points
Answer
Alkanols generally have higher boiling points compared to alkanes of similar molecular weights due to the presence of hydrogen bonds between alkanol molecules. These bonds are stronger than the Van der Waals forces present in alkanes, thus requiring more energy to overcome.
Step 3
Alkanols vs Alkanolic Acids: Discuss the differences
Answer
Alkanolic acids exhibit even higher boiling points than alkanols due to the ability of carboxylic acid groups to form dimeric structures through hydrogen bonding. This results in an increased effective molecular size and stronger intermolecular attractions, thereby elevating the boiling point further.
Step 4
Abrupt Change: Explain any anomalies in the graph
Answer
While the graph shows a general upward trend, any deviations from expected patterns can be attributed to specific structural features of the molecules in question or the presence of branching in alkanes, which can lower boiling points despite increased molecular weight.