# heat of formation and combustion problem

Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 22, 2009.

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1. ### inaccensa 7+ Year Member

512
1
Sep 5, 2008
If the heat of formation of acetylene is 226.6kJ/mol, what is the heat of combustion of acetylene?

I can't seem to understand how to get the correct ans.

This is what I did

C + H = C2H2 Hf = 226kj/mol
C2H2 + 5/2 O2 =2CO2+ H2O

Bond dissociation energy
C=O =805
O-H = 464

Delta H = Products -Reactants
= 4*805 + 2*464 -226
= 4148-226
= 3922
I know this is incorrect, since heat of combustion is an exothermic rxn

3. ### knightstale4 7+ Year Member

183
4
Jun 11, 2007
You have the correct formula for the combustion of acetylene. Here's how I calculate the heat of combustion:
heat of combustion of acetylene = products- reactants
[2 x heat of formation of CO2 (g) + heat of formation of H2O (g)] -
[heat of formation of C2H4 (g) + 2.5 x heat of formation of O2 (g)]
= [(2 * -393.5 kJ) + (-285.8 kJ)] - [226.6 kJ + (2.5 *0 kJ)]
= -787 -285.8 - 226.6 = -1299.4; rounded to -1300 kJ/mol

Hope this helps.
If you are confused about something, send me a message

4. ### thatguy687

23
0
Jun 6, 2009
Buffalo, NY
Combustion of acetylene is just the opposite of the formation of acetylene so just reverse the reaction and you get -226kj/mol.