Calculating Delta H using Hess's Law for Thermochemistry Homework

AI Thread Summary
The discussion revolves around calculating delta H for the reaction B + D → E + 2C using Hess's Law. Participants express difficulty in deriving the desired equation from the given reactions, particularly noting that the D's cancel out. It is suggested that a potential typo in the original equations may be the source of the problem, specifically that E + A should be E + A → 2D. The consensus is that without this correction, the reaction cannot be achieved as stated. Ultimately, the discussion highlights the importance of accurate reaction equations in thermochemistry calculations.
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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. :smile:
 
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Anybody?

edit: Did I not state the problem clear enough?
edit: Never had zero responses. Oh well :confused:
 
Last edited:
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?
 
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
 
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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. :biggrin:
 
Last edited:
Gokul43201 said:
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!
 
Also, in general reaction enthalpies can not be specified in kJ/mol, simply in kJ. That's at least one more typo!
 
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 :wink:
 
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Can mark this as solved. I would do it myself but it's not letting me for some reason
 

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