What is the heat of reaction for C2H4 + 3O2 -> 2CO2 + 2H2O, if heat of formation of C2H4, O2, CO2, and H2O are 10 J, 5 J, 6 J and 2 J respectively?

Correct Answer: -4 J
Heat of reaction = 2*2 + 2*6 – 10 – 2*5 = -4 J.