[cartmanike]

Using Hess’s law, calculate the ΔH value for the following reaction: (8 marks)

FeO(s) + CO(g) → Fe(s) + CO2(g)

Using these three reactions:

A) Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)           ΔH = -25.0 kJ   
B) 3Fe2O3(s) + CO(g) → 2Fe3O4(s) + CO2(g)           ΔH = -47.0 kJ
C) Fe3O4(s) + CO(g) → 3FeO(s) + CO2(g)      ΔH = +38.0 kJ

So I understand the basic concept of Hess's law. You have to reverse the given equations if necessary, as well as multiply or divide them if need be. What I don't understand is how to get past the point where you're done with all the "single" molecules (only appear once in given and once in equation).

For example.
In equation C, there are 3 FeO. That is the only place other than the target that FeO appears. It is also in the products, rather than the reactants. This means I need to divide equation C by 3 to end up with 1FeO, and also reverse it.
So, [3FeO(s) + CO2(g) --> Fe3O4(s) + CO(g)] /3
FeO(s) + 1/3CO2 --> 1/3 Fe3O4(s) + 1/3 CO(g)


Also, for equation A, there is 2Fe, and I need 1Fe. It is on the right side of the equation, so no reversing. I need to divide it by 2.
So, [Fe2O3(s) + 3CO(g) --> 2Fe(s) + 3CO2(g)] /2
1/2Fe2O3(s) + 3/2CO(g) --> Fe(s) + 3/2CO2(g)

I will change the delta H's later.

This is where I get stuck. I already have manipulated equations A and C, and I'm left with B. However, I still need to change the CO and CO2 that are in EVERY ONE OF THE EQUATIONS? Please help me out, I'm not sure what to do.

[bakerbg]

Hi, I worked out this question and found the answer to be -17.33 Kj mol-. I find it challenging to write the energy cycle in this format.When I check the web to confirm my answer, I found the same question being answered at : answers.yahoo.com.   I don't know how to insert the exact link.

[cartmanike]

Yeah, I saw that one too. I also saw a few where the answer was -11...

I just don't quite understand how to get there

[bakerbg]

I got -11KJ too when I use a different set of data . I have constructed a lovely energy cycle which I will work to post but not until this weekend - I have two final exams today and Tomorrow!!

[bakerbg]

insert the relevant data and solve for H2.