Calculating Enthalpy Change from Bond Energies

Specification Point 3.7C (Paper 2C Only):

Use bond energies to calculate the enthalpy change during a chemical reaction

Bond energy: The energy required to break a bond

Method:

• Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
• Add together the bond energies for all the bonds in the products – this is the ‘energy out’
• Calculate the energy change: energy in – energy out

Equation:

Energy change     =     energy in     –     energy out

Example: An exothermic reaction

Hydrogen and Chlorine reacts to form Hydrogen Chloride gas:

H – H   +   Cl – Cl    →    2    x   (H – Cl)

The table below shows the bond energies relevant to this reaction:

Energy In    =     436     +     243    =    679 KJ / Mole

Energy Out     =    2    x    432    =    864 KJ / Mole

Energy Change    =    679    –    864    =    -185 KJ / Mole

*The energy change is negative, showing that energy is released to the surroundings so is an exothermic reaction

Example: An Endothermic reaction

Hydrogen Bromide decomposes to form Hydrogen and Bromine:

2   x   ( H – Br )    →    H – H    +    Br – Br

The table below shows the bond energies relevant to this reaction:

Energy In    =     2     x     366    =    732 KJ / Mole

Energy Out     =    436    +    193    =    629 KJ / Mole

Energy Change    =    732    –    629    =    +103 KJ / Mole

*The energy change is positive, showing that energy is taken in from the surroundings so is an endothermic reaction