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9701 MCQ Solution 17
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/Cl2
reaction 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 I2
is 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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