IMAT 2019 Q45 [Boiling Points]

Practice Question Solving
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45. Why does hydrogen iodide (HI) have a higher boiling point than hydrogen bromide (HBr)?
[Atomic numbers: bromine = 35; iodine = 53]

A) The iodide ions in HI are larger than the bromide ions in HBr.

B) The H-I covalent bond is stronger than the H-Br covalent bond.

C) There are hydrogen bonds between HI molecules but not between HBr molecules.

D) The permanent dipole-permanent dipole forces between HI molecules are stronger than those between HBr molecules.

E) The induced dipole-induced dipole (dispersion) forces between HI molecules are stronger than those between HBr molecules.

The answer is E. Can anybody explain why D is wrong?

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I think D would be the other way round, this relies on the polarity of the bond, and as Br is higher up the table vs Iodine that HBr bond would be more polar → stronger permanent dipole forces

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You’re right! I thought Iodine was higher up the table than Bromine!

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