Question 17: [Inorganic > Group 7]:
The standard enthalpy changes of formation of HCl and HI are –92kJmol-1 and +26kJmol-1 respectively.
Which statement is most important in explaining this difference?
A Chlorine is more electronegative than iodine.
B The activation energy for the H2/Cl2reaction is much less than that for the H2/I2 reaction.
C The bond energy of HI is smaller than the bond energy of HCl.
D The bond energy of I2is smaller than the bond energy of Cl2.
Reference: Past Exam Paper – November 2002 Paper 1 Q17
Solution 17:
Answer: C.
Elimination is the best way forward. Option A is valid information but irrelevant to the situation since electronegativity is related to intermolecular forces yet enthalpy of formation regards intra-molecular forces, hence eliminated.
Option B talks about activation energy which again is relevant to the scenario however is redundant when attempting explain the enthalpy of formation, because the activation energy only shows the energy taken in not the energy released, hence eliminated.
Option C is correct since it directly verifies the question. If HI has a smaller bond energy than HCl then less energy will be released when the bond is formed, causing it to be endothermic and hence positive.
Option D, if true, would argue against the question since a smaller bond energy in I2 would mean less energy required to break the bond, thus more exothermic, hence negative which is not the case in the question.
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