How do I calculate bond energies?

Using bond energies, how to calculate enthalpy?

• Using these bond energies, ΔH°: C-C: 348 kJ C=C: 612 kJ C-H: 412 kJ C-O: 360 kJ C=O: 743 kJ H-H: 436 kJ H-O: 463 kJ O=O: 498 kJ Calculate the value of ΔH° of reaction for H2C=CH2(g) + H-O-H(g) --> CH3-CH2-O-H(g) a) -13 kJ b) -45 kJ c) -124 kJ d) +224 kJ e) -508 kJ

• Answer:

  You're prob in my Chem class haha.. Well, you subtract the ΔH° of products from that of the reactants. [3(412)+2(412)+348+360+463] - [4(412)+612+2(463)] = 45 I don't know why the answer is positive but the correct answer is -45. It must have something to do with exothermicity or something.