What is the heat of reaction for CH4 + 2O2 -> CO2 + 2H2O, if heat of formation of CH4, O2, CO2 and H2O are 10 J, 5 J, 6 J, and 12 J respectively?

Correct Answer: 10 J
Heat of reaction = 2*12 + 6 – 2*5 – 10 = 10 J.