Thermochemistry

[Pisiform]

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Q- What is the enthalpy needed to convert
C(graphite) -----> C(diamond)
given:
C(graphite) + O2 ---> CO2(g) Hrxn = –393.5 kJ/mol
C(diamond) + O2 ----> CO2(g) Hrxn = –395.4 kJ/mol

Using Berkely Reveiw method I did:
.∆H..rxn.. = ∑∆H®..formation products.. - ∑∆H® ..formation reactants
= -395.4 - (-393.5)
= -1.9KJ/mol

but the answer is +1.9 KJ/mole

How to get this positive sign.
Help please
.

 

[IntelInside]

Q- What is the enthalpy needed to convert
C(graphite) -----> C(diamond)
given:
C(graphite) + O2 ---> CO2(g) Hrxn = –393.5 kJ/mol
C(diamond) + O2 ----> CO2(g) Hrxn = –395.4 kJ/mol

Using Berkely Reveiw method I did:
.∆H..rxn.. = ∑∆H®..formation products.. - ∑∆H® ..formation reactants
= -395.4 - (-393.5)
= -1.9KJ/mol

but the answer is +1.9 KJ/mole

How to get this positive sign.
Help please
.

We have to use Hess's law here so if I reverse C(diamond) + O2 -> CO2 we get a positive 395.4 kJ/mol then I add this to -393.5 because I am adding reactions.

You cant do it your way because they arent giving us heats of formation.

 

[Pisiform]

We have to use Hess's law here so if I reverse C(diamond) + O2 -> CO2 we get a positive 395.4 kJ/mol then I add this to -393.5 because I am adding reactions.

You cant do it your way because they arent giving us heats of formation.

Thanks I really appreciate it.

So when actually we use Hess's Law and when we use the above (mine) equation

 

[IntelInside]

we can use the hess's law equation all the time. You can only use yours when we have the heats of formation

 

[Pisiform]

we can use the hess's law equation all the time. You can only use yours when we have the heats of formation

Thanks man, you are the best 😍

 

[Salient]

You should know, off the top of your head, that molecules in their most natural state have an enthalpy of formation of zero. I remember having a few questions that involved doing it your way where the enthalpy of formation of something like O2 wasn't given and you just had to remember that it was zero. That said, if they don't give it to you and you need to know it to get the answer, it's probably zero!

Hess's law is good for combining equations for which you know the total enthalpy for (such as your problem).

There's a similar procedure involving equilibrium constants (albeit with new rules because of where the equilibrium constants come from), but I haven't come across one of those in my MCAT practice yet.

 

[Pisiform]

You should know, off the top of your head, that molecules in their most natural state have an enthalpy of formation of zero. I remember having a few questions that involved doing it your way where the enthalpy of formation of something like O2 wasn't given and you just had to remember that it was zero. That said, if they don't give it to you and you need to know it to get the answer, it's probably zero!

Hess's law is good for combining equations for which you know the total enthalpy for (such as your problem).

There's a similar procedure involving equilibrium constants (albeit with new rules because of where the equilibrium constants come from), but I haven't come across one of those in my MCAT practice yet.

cool. thanks