top of page

Calculating Enthalpy of Reaction Using Bond Energies: A Step-by-Step Guide

We can calculate the enthalpy of reaction from the combination of bond energies.

When bond are broken, energy is absorbed(endothermic). When bonds are created, energy is released (exothermic).

ΔH = ΣBEreactants -ΣBEproducts

sum of bond energies of reactants - sum of bond energies of products

Note: To do these problems, we need to be able to draw the Lewis dot structures of molecules correctly. The equation also needs to be balanced.


  1. Balance the equation

  2. Draw out Lewis dot structures for reactants and products

  3. Find bond energies for all the bonds in the reaction in the data table given by the professor or at the end of the textbook

  4. Subtract the sum of bond energies of the products from the sum of the bond energies of reactants to determine the enthalpy of reaction

Problem: Calculate the enthalpy of reaction for

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

Reactants = 4C-H bonds + 2 O=O bonds = 4(413kJ/mol)+2(498kJ/mol) = 2648 kJ/mol

Products = 2C=O bonds + 4O-H bonds = 2(799kJ/mol)+4(467 kJ/mol)= 3466kJ/mol

ΔH = sum of bond energies of reactants - sum of bond energies of products = 2648 kJ/mol-3466kJ/mol = -818kJ/mol

Need help with your chemistry class? Our Online Chemistry Tutor is ready to help you!





Ready For Chemistry Tutoring?

I tutor all levels of chemistry including general and organic chemistry.

Click To Learn More

What subject are you taking?
Regents Chemistry
General Chemistry
Organic Chemistry

Join our email list 

bottom of page