Enthalpy change

The standard enthalpy changes of formation of HCl and HI are –92 kJ/ mol and +26 kJ/ mol 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.

The answer is C, why ?

Reply 1

UpsidedownLandMan

13

No idea about the technicalities, but in an exam i would say C beacause:

A) I have no idea about the link between electronegativity and enthalpy
B) Is wrong
and
D)is true, but has nothing at all to do with these numbers- the question only talks about HCl and HI, not the actual formation reactions.

(I've had to do heaps of elimination type MC questions, MC really let me down last year)

Reply 2

charco

Study Forum Helper

18

Original post by Ari Ben Canaan

The standard enthalpy changes of formation of HCl and HI are –92 kJ/ mol and +26 kJ/ mol 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.

The answer is C, why ?

The enthalpy of formation of HCl and HI is represented by the equations:

H₂ + Cl₂ --> 2HCl

and

H₂ + I₂ --> 2HI

The enthalpy change of these reactions can be found by looking at the bonds broken and formed in terms of their energies (remembering that bond breaking is endothermic and bond formation is exothermic):

ΔH(rxn) = ΔH(reactant bonds) - ΔH(product bonds)

So if the HI bond energy is much smaller than the HCl bond energy (option C) then the HI reaction will be more endothermic...

... whereas if the bond energy of I2 were smaller than the bond energy of Cl2 (option D) the reaction forming HI would be more exothermic.

And as we know that the reaction forming HI is more endothermic, the correct answer is C