A student has two unknown compounds. He discovers that one of the compounds is acetic acid (C2H4O2). He notes that the second compound has a lower boiling point than acetic acid. Which of the following must be true of the second unknown?
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Correct Answer: B. The second compound is less hydrogen bonding than acetic acid
The boiling temperature of a compound is largely dependent on the steric hindrance of the molecule and its polarity, or hydrogen bonding capability. As steric hindrance and hydrogen bonding increase, boiling point increases. As steric hindrance and hydrogen bonding decrease, boiling point decreases. Initially, it may seem that answer D could be possible. However, a lower molecular weight does not guarantee a lower boiling temperature. For example, ethanol has a lower molecular weight than butane. However, ethanol’s boiling point is about 78°C whereas butane’s boiling point is -1 °C.
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