How to use bond energies to determine the △H for the combustion of methane ?

Answer 1

First you must write the balanced equation for the combustion of methane. That would look like this...

CH4(g) + 2O2(g) ===> CO2(g) + 2H2O(g)

Then we want to look at all the bond energies of reactant and the products. You need to look up these bond energies. The general equation is ∑bond energies of reactants - ∑bond energies products = ∆Hrxn

So on the reactant side you have 4 C-H bonds and 2 O=O bonds. On the product side you have 2 C=O bonds and 4 O-H bonds. Look up the values and do the math.