Two hydrogen halides, HF and HI

CHEMISTRY · Multi-part · A Level · Chemistry · Bonding · Bonding

Problem

Two hydrogen halides, HF and HI, have the following measured spectroscopic data.
MoleculeBond length / pmDipole moment / D
HF921.82
HI1610.44
Useful constants: 1 D = 3.34×10−30 C⋅m3.34\times10^{-30}\,\text{C}\cdot\text{m}; e=1.60×10−19 Ce = 1.60\times10^{-19}\,\text{C}. (a) For each molecule, calculate the dipole moment expected for complete electron transfer across the bond. (b) Determine the percent ionic character for both HF and HI. (c) Using your answers to part (b), explain why hydrofluoric acid is a weak acid in aqueous solution whereas hydroiodic acid is a strong acid, even though the H–F bond has the higher percent ionic character. (d) Predict how the enthalpy of vaporization of liquid HF compares with that of liquid HI, and relate your prediction to differences in bond polarity and intermolecular forces.

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