4.16 use average bond energies to calculate the enthalpy change during a simple chemical reaction.

To work out the enthalpy change you do:
 the sum of bond energies in the reactants - the sum of bond energies in the products

Given average bond energies, you can easily work out the sum of bond energies.
For example, if we are told that
C-H = 413
O=O= 498
C=O= 746
H-O=464

Work out the enthalpy change for this reaction: CH4 + 2O2 > CO2 + 2H2O

((4 x C-H) + (2 x O=O)) - ((2x C=O) + (4 x H-O))

Then we substitute in:

(4 x 413) + (2 x 498) - (2 x 746) + (4 x 464) = 2648KJ - 3348KJ = 700KJ enthalpy change

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