Calculating Delta H using Hess's Law for Thermochemistry Homework

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Discussion Overview

The discussion revolves around calculating the enthalpy change (delta H) using Hess's Law for a thermochemistry homework problem. Participants explore the feasibility of deriving a specific reaction from given equations, focusing on the application of Hess's Law and the implications of potential typos in the provided data.

Discussion Character

  • Homework-related, Debate/contested, Exploratory

Main Points Raised

  • One participant presents a problem involving three reactions and seeks to find the delta H for a specific reaction, B + D → E + 2C.
  • Another participant expresses uncertainty about the correctness of the original equations and suggests a possible typo in the reaction involving E and A.
  • A third participant asserts that there is no solution based on the given data, indicating that the final equation cannot be formed from the initial reactions with non-zero coefficients.
  • Some participants propose that correcting the reaction to E + A → 2D or 3B → 2C might allow for a valid calculation of delta H.
  • Concerns are raised about the specification of reaction enthalpies in kJ/mol rather than kJ, suggesting another potential error.
  • One participant reflects on a professor's claim that a solution exists despite their inability to derive it, expressing confusion over the professor's stated answer of -175 kJ/mol.

Areas of Agreement / Disagreement

Participants generally agree that there may be a typo in the provided reactions, but there is no consensus on the existence of a solution to the original problem. Multiple competing views remain regarding the validity of the given equations and the implications for calculating delta H.

Contextual Notes

Limitations include potential typos in the provided reactions, which affect the ability to derive the desired equation. The discussion also highlights the ambiguity in specifying reaction enthalpies.

ace123
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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
 
Last edited:
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:
 
Last edited:
Can mark this as solved. I would do it myself but it's not letting me for some reason
 

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