Two heteronuclear diatomic species, HF and HI, each form a sigma bond between H and a halogen atom. Their potential–energy curves are described below.
• Curve D has its minimum potential energy of –560 kJ·mol⁻¹ at 93 pm.
• Curve E has its minimum potential energy of –297 kJ·mol⁻¹ at 160 pm.
Atomic (covalent) radii: H = 37 pm, F = 64 pm, I = 133 pm.
Average experimental bond enthalpies: HF = 567 kJ·mol⁻¹, HI = 299 kJ·mol⁻¹.
(a) Identify which curve (D or E) corresponds to HF and which corresponds to HI. Support your identification with two distinct pieces of evidence drawn from the information provided.
(b) For the bond that corresponds to curve D, indicate whether the force constant (bond stiffness) is expected to be larger or smaller than that for the bond represented by curve E. Explain your answer in terms of features of the curves.
(c) Compare the depths of the two curves with the corresponding experimental bond enthalpies supplied. Evaluate whether the curves are consistent with the data and briefly justify your evaluation.