IMAT 2019 Q45 [Boiling Points]

Practice Question Solving
1

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?

2

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

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

1 Like
4

Aren’t HI and HBr permanent dipoles??
I don’t understand why are they considered as induced dipole in option E.

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there is instantaneous dipole-induced dipole (london dispersion) forces in everything, even noble gasses @Moon

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So why is D not an answer?
Nobody is explaning about it

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D is wrong because iodine has more electrons, meaning larger electron cloud. the larger the cloud of electrons the higher the probability electrons will accumulate on one side or the other forming an instantaneous dipole, which then induces the formation of dipoles on molecules adjacent to it. this is london dispersion forces.