Heat of Combustion Problems?

[Zenzenzen]

Hi, I was wondering if someone could help me on this enthalpy problem? The only problem I have with enthalpy is when combustion is involved.

It's question 1(e) in this: http://www.xtremepapers.me/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_2.pdf

All help appreciated

 

[libra94]

remember that 1) ΔHf = ΔHc (reactants) - ΔHc (products)

and 2) ΔHr= ΔHf(products) -ΔHf(reactants)

for this question we use the first equation i.e. ΔHf = ΔHc (reactants) - ΔHc (products)

2C + 2H2 ---> C2H4

so ΔHc reactants= 2C + 2H2
2C----2* -393.7 = -787.4
2H2---2*-285.9 = -571.8

total ΔHc(reactants) = -787.4 + (-571.8)= -1359.2 kJmol-1

ΔHc(product) C2H4 = -1411.0

ΔHf = ΔHc (reactants) - ΔHc (products)
-1359.2 - (-1411.0)
= +51.8 kJmol-1 !!!

 

[libra94]

lol that was -571.8

 

[Zenzenzen]

Oh thanks! I knew the second equation, just not the first one.

A question: in the equation ΔHf = ΔHc (reactants) - ΔHc (products) the Hf is for the product in the equation 2C + 2H2 ---> C2H4 right?
Say, what if the equation had been C2H4 --> 2C + 2H2 ? Is this even possible, and if so how would we do it?

 

[libra94]

yes the ΔHf is for the product!

well i think its not possible becuz, here actully the bond is breaking not forming, which is endothermic

 

[Zenzenzen]

mmkay. thanks.

 

[libra94]

mmkay. thanks.

ur welcome
any more doubts??

 

[nisurju]

@libra..ur way of slovin this problem is so different from what i do(maybe mine is weird)
can we solve it anyway as we like or there is a fixed way of doing it?
we learnt it by another way!

 

[nisurju]

can u also please give me a list of all the oxidising,reducing,dehdrating,dehydragenating agents?

 

[nisurju]

mmkay. thanks.

southpark fan!! lol