Calculating an Enthalpy change using
Bond Enthalpies
Ex. Use the data sheet of average bond enthalpies to calculate the
overall enthalpy change for the reaction between 2 moles of hydrogen and
1 mole of oxygen. Draw an energy level diagram for the process.
1. Calculate the overall energy change for the reaction below and
state whether the reaction is exothermic or endothermic
H2 + F2  2HF
2. Methane reacts with chlorine by the following equation.
CH4 + Cl2  CH3Cl + HCl
(a)
Calculate the energy change
(b)
This reaction does not happen until ultraviolet light is
shone on it. Why do you think this is?
3. Calculate the energy change for the reaction below using a labelled
Hess Cycle.
N2 + 3H2  2NH3
4. Why are bond enthalpies given as average values?
5. Why might the value for a reaction enthalpy change you calculate
from bond data not match what you measure in an experiment?