How can Hess' law be applied to this?

[mgray10]

Hello,

I am in the ME field, but my son asked me if I could answer this chemistry question. Naturally, I was quite embarrassed when I couldn't figure it out. Here it is:

Calculate the value of delta H for the formation of 1.00mol HI(g) from the following data:

H_2(g) + Cl_2(g) -> 2HCl(g) Delta H= -184.4 kJ
HCl(g) + H_2_0(l) -> HCl(aq) Delta H= -75.07 kJ
HI(g) + H_2_O(l) -> HI(aq) Delta H= -80.30 kJ
KOH(aq) + HCl(aq) -> KCl(aq) Delta H= -57.43 kJ
KOH(aq) + HI(aq) -> KI(aq) Delta H= -57.14 kJ
Cl_2(g) + 2KI(aq) -> 2KCl(aq) + I_2(s) Delta H= -219.1 kJ

I understand the terminology, but I can't figure out how to apply Hess' law to this. With none of the reactions having HI(g) isolated to one side, I am clueless. At one point, I began to argue, with myself, that the question was a typo and should have specified HI(aq). I don't think that is the case, however.

Nonetheless, I am not looking for the answer to the question, but I am looking for a way to apply Hess' law to this. If anyone could set me in the right direction, or even make some rough representations of how to set this up, I would greatly appreciate it. You see, I have to figure this out before he does!

Thanks,
Michael

 

[25362]

These are the general rules for manipulating the equations:

1. One may reverse an equation; the sign of [Δ]H^o must also be reversed.
2. Whenever cancelling terms from both sides of an equation the substances must be in identical physical states.
3. When multiplying or dividing the coefficients by a common factor, [Δ]H^o must be likewise changed.

 

[25362]

Whatever the result you get from playing around with the above equations, the net result would be (according to a book on General Chemistry):

[½]I(s) + [½]H₂(g) [→] HI(g) [Δ]H = +25.9 kJ​