Calculating Heats of Formation with 2 Datas.

iCrazzy

So I have no trouble calculating it with 3 datas given as per the picture of the revise wise method. But how would one go about calculating it(using that method) if you were only given 2 formulas equations.

Trekker47

It seems like it's the same as when there's 3 data given, here's my (hastily done) worked solution; hopefully it makes sense :P
http /i(dot)imgur(dot)com/uTphXnf(dot)png (Apparently I can't post images since I'm new, so you're gonna have to piece together the url, sorry)

iCrazzy

Any chance you can do it using that method from the revise wise.
I dont really understand how you do it. And what is up with the lines across.
Thanks

Trekker47

I did out an explanation of my method which might help; it's dead easy, the only slightly tricky bit is working out the equations if they're not given to you, like they might say "Given that the heat of combustion of carbon is -393KJ", in which case you can work out that C + O2 -> CO2.
http /i.imgur.com/OQ8tArA.png?1
I've never seen your method before so I'd need to have a couple of worked examples using the equations to learn it from, sorry about that.

iCrazzy

Trekker47 wrote: »

I did out an explanation of my method which might help; it's dead easy, the only slightly tricky bit is working out the equations if they're not given to you, like they might say "Given that the heat of combustion of carbon is -393KJ", in which case you can work out that C + O2 -> CO2.
http /i.imgur.com/OQ8tArA.png?1
I've never seen your method before so I'd need to have a couple of worked examples using the equations to learn it from, sorry about that.

Thank you, I get everything apart on how you calculated the 3rd equation

The only part I can figure out is that
Ca+C+O2

---

CaCO3

There is 3 oxygens on the left, so should there not be 3 oxygen in the right or make this to balance:
2Ca+2C+3O2

---

2CACO3

Why do you have the half

Trekker47

Well we can only find oxygen as O2, it doesn't come as just O, 3O2 would really be 6O.
Hence 1.5 O2 is actually 3O. Hope that helps

EoghanIRL

I've never seen the method in the revise wise. I wouldn't use it myself. With the other method you can check your work to see if you are right much more easily.