# Calculating bond enthalpiesWatch

Announcements
This discussion is closed.
#1
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.
0
14 years ago
#2
It might help if you give us some of the values and workings that you used to obtain your 170kJ/mol
0
14 years ago
#3
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)
0
14 years ago
#4
(Original post by 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.
0
14 years ago
#5
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
0
14 years ago
#6
Looks fine to me
0
X
new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• University of Bristol
Wed, 23 Oct '19
• University of Exeter
Wed, 23 Oct '19
• University of Nottingham
Wed, 23 Oct '19

### Poll

Join the discussion

Yes I know where I'm applying (120)
64.86%
No I haven't decided yet (39)
21.08%
Yes but I might change my mind (26)
14.05%