Given the following reactions and enthalpies:
H2O H2 + 1/2O2 +643KJ/mole
C2H6 2C + 3H2 +190.6 KJ/mole
4CO2 + 6H2O 2C2H6 + 7 O2 +7022 KJ/mole
What is the enthalpy of the following reaction?
CO2 C + O2
In this questions you have to rearrange given equations and add them to get the final desired equation.
CO2 ---> C + O2
To get above equation you need to
1)Take equation 2 and divide it by 2 and reverse it so that you get one mole of C ..accordingly - 190.6/2 KJ is the enthalpy of this reaction.
2) Divide equation 3 by 4 ...enthalpy will become 7022/4 KJ
3)Now reverse equation 1 and multiply it by 6/4 ...enthalpy will be -643 *6/4kJ
Add all of them you will get answer.
In Hess's law questions this is the only way to solve this question