The Student Room Group

Calculating bond enthalpies

This is probably very basic but I'm doing something majorly wrong somewhere.

Trying to calculate the bond enthalpy of the Carbon/Carbon bond in benzene, using the value for enthalpy of combustion and other bond enthalpy values. I thought all it involved was working out the enthalpy of combustion as normal but making it equal to the combustion value and working out the C-C bond from there. But, I get quite a low answer (~170 iirc). I'm sure I'm making a basic mistake somewhere.

Cheers.
Reply 1
It might help if you give us some of the values and workings that you used to obtain your 170kJ/mol
Ah ... I have a few similar q's if anyone can help with how to do them, please:

1) Calculate a value for the mean bond enthalpy of the H-O bond in water.

Would this just be 464*2 = 928 kJmol^-1 ?

2) Use the following data to calculate mean bond enthalphy values for the C-H and C-C bonds in methane:

CH4 ---> C + 4H (deltaH = 1648)
C2H6 ----> 2C + 6H (deltaH = 2820)
BarelyLegal
Ah ... I have a few similar q's if anyone can help with how to do them, please:

1) Calculate a value for the mean bond enthalpy of the H-O bond in water.

Would this just be 464*2 = 928 kJmol^-1 ?



What is 464? I think that is wrong, you shouldn't be multiplying by two.


2) Use the following data to calculate mean bond enthalphy values for the C-H and C-C bonds in methane:

CH4 ---> C + 4H (deltaH = 1648)
C2H6 ----> 2C + 6H (deltaH = 2820)


dissociation enthalpy of methane is equivalent to the dissociation of 4 C-H bonds so divide it by 4 to get the mean.

Dissociation of ethan is 6 C-H bons and one C-C, multiply above answer by 6 and subtract from enthalpy of dissociation = enthalpy of C-C bond.
That's great, thanks.

The 464 was "the relative emount of energy needed to break bonds" of O-H.

Can you check out my other q please because I'm not sure whether the answer is correct or not: http://www.thestudentroom.co.uk/t88550.html
Looks fine to me