Question

Why are heat capacities of hydrogen halides similar to one another?

Answer 1

Increasing the number of carbons linearly increases the number of low-energy vibrational modes and each mode increases the heat capacity by a small amount.

Answer 2

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$\huge\mathfrak{Because}$

Halides are all gases with the following properties:

HF is a colourless fuming gas - b.pt = 20oC

HCl is a colourless gas - b.pt = -85oC

HBr is a colourless gas - b.pt = -69oC

HI is a colourless gas - b.pt = -35oC

The abnormality of HF is due to the presence of hydrogen bonding.

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