Question about enthalpy level diagrams

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Adurelm
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#1
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#1
I've just started my undergrad in chemistry and I don't really understand one of the questions I've been set. This is the question:

"Diamond and graphite are two allotropes of carbon. Calculate the enthalpy change associated with the transformation of graphite into diamond and state if the transformation is endothermic or exothermic. For your calculation, use the combustion enthalpies ∆combH of graphite (-393KJ/mol) and diamond (-396KJ/mol)."

I'd lay out my attempts to answer the question so far to give everyone something to go off, but I'm not sure what to do beyond:

C (graphite) + O2 --> CO2
C (diamond) + O2 --> CO2

And that's about it.

Also, I'm only familiar with drawing Hess cycles for these calculations, but the question requests an enthalpy level diagram (so something like this). But I'm not sure how to draw one of these given the data.

Basically, I'm hoping someone can walk me through this Q, since it'll help me to answer other questions like it.
Last edited by Adurelm; 1 month ago
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scimus63
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#2
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if you reverse the second equation so you have:
C(gra) + O2 ------------------- CO2 delta H = -393
CO2 ---------------C(dia) + o2 deltaH = 396
combine these 2 equations to give what you need
Cgra -----------------------C dia delta H = 3kj

Hess's law cycle would just be

C(gra) --------------------------C(dia)

YOU NEED TO DRAW THE ARROW GOING DOWN HERE - i CANT DO IT ON MY COMPUTER THEN ADD THE ENTHALPY CHANGES

CO2
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scimus63
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sorry for the other part

since reaction is endothermic, graphite is more stable than diamond, so just have




------------------------- diamond

3kj put in axes to show energy on the vertical scale, difference here is 3kj. No access on the link in your questions, its denied!
----------------------- graphite
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Adurelm
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#4
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(Original post by scimus63)
if you reverse the second equation so you have:
C(gra) + O2 ------------------- CO2 delta H = -393
CO2 ---------------C(dia) + o2 deltaH = 396
combine these 2 equations to give what you need
Cgra -----------------------C dia delta H = 3kj

Hess's law cycle would just be

C(gra) --------------------------C(dia)

YOU NEED TO DRAW THE ARROW GOING DOWN HERE - i CANT DO IT ON MY COMPUTER THEN ADD THE ENTHALPY CHANGES

CO2
Cheers. The explanation of how to get the enthalpy change of the reaction via Hess' cycle makes sense, though I'm still a bit unsure of two things here. First - how would you know to reverse and combine the equations? And how would you represent this on the enthalpy level diagram (here's the link that wasn't working earlier) - mainly, I'm not sure of this point because I'm not sure how these diagrams even work
Last edited by Adurelm; 1 month ago
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scimus63
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There are lots of ways to calculate enthalpy changes, drawing Hess's law cycles is what is mostly taught at A-level. But you can do them without these cycles.

To do this:
write the equation for the enthalpy change you need to calculate, in this case:
C(gra) --------------------------C(dia)

Then simply take the combustion enthalpy equations you have and rearrange them so that the Cgra is a reactant and Cdia is a product, as below:
C(gra) + O2 ------------------- CO2 delta H = -393

CO2 ---------------C(dia) + o2 deltaH = 396

You should be able to cancel out everthing except what is in the desired equation - sometimes you may need to multiply equations to get the equation you want but its one way of working out enthalpy changes and its quick for this one.

graphite is more stable than diamond, so just draw as in your link picture! sorry cant do it on this computer
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scimus63
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#6
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https://www.science-revision.co.uk/e...0diagrams.html

look here, its like the two diagrams at the bottom of the page for an endothermic process, just put graphite and diamond as the reactants and products
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