heat of formation and combustion problem

[inaccensa]

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

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[knightstale4]

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

 

[thatguy687]

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

Combustion of acetylene is just the opposite of the formation of acetylene so just reverse the reaction and you get -226kj/mol.