How Do You Calculate Enthalpy Change Using Bond Energies?

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Discussion Overview

The discussion centers around calculating enthalpy change using bond energies, specifically in the context of a homework problem. Participants explore the application of Hess's law and bond energy values to derive the enthalpy change for a chemical reaction.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant calculates the total energy from bond energies but questions the validity of their approach given the provided values and options.
  • Another participant emphasizes the need for a "good faith" effort in solving the problem.
  • Some participants express uncertainty about how to construct a Hess's cycle, which is necessary for the calculation.
  • There is a discussion about the total enthalpy change being consistent across different reaction pathways, as per Hess's law.
  • A participant proposes a calculation involving bond energies, leading to a derived value for the C-C bond energy, but notes a discrepancy with the expected answer.
  • Some participants suggest that using conventional notation might improve clarity and success in the calculations.

Areas of Agreement / Disagreement

Participants generally agree on the importance of Hess's law and the need for a structured approach to the problem. However, there is disagreement regarding the specific calculations and the interpretation of bond energy values, leading to unresolved questions about the correct approach and final answer.

Contextual Notes

Limitations include potential misunderstandings of Hess's cycle, the use of bond energies, and the assumptions made in the calculations. The discussion does not resolve the discrepancies in the calculated values versus expected results.

Who May Find This Useful

Students studying thermodynamics, particularly those working on homework related to enthalpy changes and bond energies, may find this discussion relevant.

Priyadarshini
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Homework Statement


upload_2015-12-27_11-25-13.png


Homework Equations

The Attempt at a Solution


6C-H bonds= 410 x 6 = 2460 kJ/mol
3C-C bonds
H of formation= +53.3 kJ/mol
H of atomisation = +717 kJ/mol
H-H = 436 kJ/mol

as H of formation is 53.3, can't I just do:
2460+3x=53.3?
But then I am not using a lot of the values provided and the answer in not in the options.[/B]
 
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You have to make a "good faith" effort.
 
Bystander said:
You have to make a "good faith" effort.
Actually, I don't know what to do at all. I know that I have make a Hess's cycle so that I can form equations. But I can't figure out the Hess's cycle here.
 
What is Hess's cycle?
 
Bystander said:
What is Hess's cycle?
The total enthalpy change is the same for all the routes the reaction can take to produce the product.
 
Correct. What route are you taking?
 
Bystander said:
Correct. What route are you taking?
Snapshot.jpg

Let the C-C bond energy be x
(3*-717) + (-436*3) + (410*6) + 3x = 53.3
-2151-1308+2460+3x=53.3
-999+3x=53.3
3x=1052.3
x=350.767
approx. 351 kJ/mol

But the answer should be 315kJ/mol.
 
Were you to use conventional notation, you might be more successful.
 
Bystander said:
Were you to use conventional notation, you might be more successful.
What do you mean by conventional notation?
 

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