Calculating Delta H using Hess's Law for Thermochemistry Homework

[ace123]

1. The problem is to calculate the delta H.

(1/2)A====> B 150 KJ/mol

3B====> 2C + D -125 KJ/mol

E + A====> D 350 KJ/mol

We have to find the delta H for: B + D====> E + 2C

2. The equation to use is just Hess's Law II

3. Now I have tried to combine these three reactions everyway I can think of but I can't get it the reaction B + D====> E + 2C

Is it even possible to get it there because my D's cancel out. The closest I get is to is this B====> 2C + E

What am I doing wrong? Any help would be appreciated.

 

[ace123]

Anybody?

edit: Did I not state the problem clear enough?
edit: Never had zero responses. Oh well

 

[eli64]

hi again,

I'm with you on this one, I can't see a solution either, sure the original equations are correct? would be helpful if it were E + A --> 2D?

 

[Gokul43201]

There is no solution from the given data. It's fairly easy to show that the final equation can not be written as a linear combination of the first 3 equations, with non-zero coefficients.

Probably a typo!

PS: E + A --> 2D will fix it, as would 3B --> 2C

 

[ace123]

Thank you I've been going mad. It was on one of my midterm's and my professor said that it was possible. I knew it couldn't be because the D's cancel out. Thank you again

Edit: The wierdest thing is that he told me the answer is -175 KJ/mol. How does he come up with an answer with no reaction.

 

[eli64]

PS: E + A --> 2D will fix it, as would 3B --> 2C

maybe use one of these, that might give his answer of -175kJ, then show him the typo!

 

[Gokul43201]

Also, in general reaction enthalpies can not be specified in kJ/mol, simply in kJ. That's at least one more typo!

 

[ace123]

Yea, we told him that part. I guess I will try the E + A --> 2D and see if I get that answer.

Edit: Yea it looks like it was a typo. It should have be E + A --> 2D. But he didn't say their was a typo and he better not mark my answer as wrong then. Anyway thanks

 

[ace123]

Can mark this as solved. I would do it myself but it's not letting me for some reason