Given: CH4 + 2O2 ~ CO2 + 2H2O

C - H = 99 kcal O - O = 119 kcal
C = O = 192 kcal O - H = 111 kcal

Calculate the change in energy for the CH4 - O2 system.

1 answer

dHrxn = (n*BE reactants) - (n*BE products).
BE = bond energy.
For example You have broken 4 C-H bonds so that is 4*99 = ? kcal.
You have broken 2*119 O bonds = ? kcal
you have formed two C-O bonds for CO2
etc.