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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.


Steps:

  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


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