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### HW 8.73 (Bond Enthalpies

Posted: Tue Jan 17, 2017 10:51 am
I'm kind of confused with the concept of bond enthalpies.
Estimate the reaction enthalpy for
a) 3 C2H2(g)-->C6H6(g)
b) CH4(g)+4 Cl2(g)-->CCL4(g)+4 HCL(g)

### Re: HW 8.73 (Bond Enthalpies

Posted: Tue Jan 17, 2017 2:30 pm
To calculate the enthalpy of a reaction using bond enthalpies you would have to add together all of the bond enthalpies of new bonds formed (which will be a negative value because formation of bonds releases energy) and add this value to the total of the bond enthalpies of all of the bonds broken (which will be a positive value because breaking a bond requires an input of energy). Personally, it helps me a little more to draw out the structures of the reactants and products to see exactly which bonds are broken and which ones are formed. For example, for part a, 3 carbon-carbon triple bonds (reactants) are broken to form 3 carbon-carbon single bonds and 3 carbon-carbon double bonds in the resulting hydrocarbon ring structure.

### Re: HW 8.73 (Bond Enthalpies

Posted: Thu Jan 19, 2017 1:21 am
use the sum of bond enthalpies of all products minus the sum of bond enthalpies of all reactants.
and do not forget to multiply each bond with their moles

### Re: HW 8.73 (Bond Enthalpies

Posted: Tue Mar 14, 2017 12:12 pm
Why don't we include the C-H bonds? Is it because there are 3 moles of 2 C-H bonds broken and 6 C-H formed in the benzene ring, so they cancel out?

### Re: HW 8.73 (Bond Enthalpies

Posted: Sat Mar 18, 2017 3:37 pm
I'm not getting the correct answer for part A. I get a -369kJ/mol but the answer is -597kJ/mol? Can anyone help me with this please?

### Re: HW 8.73 (Bond Enthalpies

Posted: Sat Mar 18, 2017 3:49 pm
104781135 wrote:I'm not getting the correct answer for part A. I get a -369kJ/mol but the answer is -597kJ/mol? Can anyone help me with this please?

Never mind, I figured it out. C6H6 is benzene, and all of benzene's carbon bonds are resonance bonds, which have a different bond enthalpy than a single carbon bond or triple carbon bond.