# Hess's Law and change of enthalpy

## Main Question or Discussion Point

The enthalpy of combustion of solid carbon to form carbon dioxide is -393.7 kj/mol carbon, and the enthalpy of combustion of carbon monoxide to form carbon dioxide is -283.3 kj/mol CO. Use these data to calculate change of enthalpy for the reaction

2C(s) + O2(g) -> 2CO(g)

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I'm quite lost . . . after trying this problem for about 20 mins now . . . I still don't see how to use Hess's Law to formulate this answer

What i tried to do was to "form" the equations by the words in the problems.

He are my formulations below; however, I can't seem to get the equation mentioned above.

2CO + O2 -> 2CO2
C + O2 -> CO2

when I rearrange these . . . I can't seem to get 2C(s) + O2(g) -> 2CO(g)

thanks

Last edited:

EUREKA!!! I GOT IT :) :)

2 (C + O2 -> CO2) change in Enthalpy = 2(-394) kj/mol
2CO2 -> O2 + 2CO change in Enthalpy = 2(239.3) kj/mol

= 2C + O2 -> 2CO change in Enthalpy = 221 kj/mol + or - 1 kj/mol

:D ;) :)