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Enthalpy change

Hi, can someone please explain why the enthalpy change of a reaction is sometimes

(sum of bond enthalpies of products) - (sum of bond enthalpies of reactants)

And sometimes the other way around!

Which is correct? I keep seeing these 2 different variations: are they used in different contexts?
Reply 1
Original post by subbhy
Hi, can someone please explain why the enthalpy change of a reaction is sometimes
(sum of bond enthalpies of products) - (sum of bond enthalpies of reactants)
And sometimes the other way around!
Which is correct? I keep seeing these 2 different variations: are they used in different contexts?

Hello, what level of study are you at?

I've only used (sum of bond enthalpies of reactants) - (sum of bond enthalpies of products), unless I've misunderstood what you mean.

If you can provide examples of where you've seen the opposite, that would be great :smile:
Reply 2
Original post by Methene
Hello, what level of study are you at?

I've only used (sum of bond enthalpies of reactants) - (sum of bond enthalpies of products), unless I've misunderstood what you mean.

If you can provide examples of where you've seen the opposite, that would be great :smile:


A level chemistry!

Yes, that’s what I’ve been told to use too. 11F6F903-86BE-49E8-9CE4-E8CCFF312BF7.jpg.jpeg2DFCA172-D156-4A24-901D-C438AC6DB3AA.jpg.jpeg

Not sure why there’s 2 different types: the subscript may offer some clue as to when each is used but I don’t know
Reply 3
Ah, that makes sense... personally I never used it like this, I just draw the cycle and follow the arrows :giggle: (with more complex examples it may not be as straightforward as x-y)
With formation for example, by following the arrows you are effectively doing (-reactants) + products, hence the formula.

So to answer your question, both formulas are used, and you are correct that they are in different contexts. It's worth learning the definitions for enthalpy of formation and combustion, as they will guide you well when tackling these q's! And always draw a diagram :wink:

Hope this helps! Let me know if I wasn't clear or you have any other questions :smile:
Reply 4
I always assumed it was products - reactants. The enthalpy change for a reaction is the difference in enthalpy of the products and the reactants, switching them round would give a change the entahlpy from + to -. ΔH = ΣH_products - ΣH_reactants
Reply 5
Original post by Methene
Ah, that makes sense... personally I never used it like this, I just draw the cycle and follow the arrows :giggle: (with more complex examples it may not be as straightforward as x-y)
With formation for example, by following the arrows you are effectively doing (-reactants) + products, hence the formula.

So to answer your question, both formulas are used, and you are correct that they are in different contexts. It's worth learning the definitions for enthalpy of formation and combustion, as they will guide you well when tackling these q's! And always draw a diagram :wink:

Hope this helps! Let me know if I wasn't clear or you have any other questions :smile:


Makes sense, thank you!

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