What is the heat of reaction for CH4 + 2O2 -> CO2 + 2H2O, if the heat of combustion of H2O = 5 J, CO2 = 10 J, O2 = 0, and CH4 = 25 J?

Correct Answer: 5 J
Heat of reaction = 25 + 0 – 10 – 2*5 = 5 J.