Quick Notes Bond Enthalpies

  • The overall enthalpy change that occurs in a reaction can be found by subtracting the sum of all the bond enthalpies of bonds formed (in products) from the sum of all bond enthalpies of bonds broken (in reactants).
  • A bond enthalpy is the amount of energy released or absorbed when a particular bond is made or broken.
    • Energy is released by two atoms when a bond is formed between them.
    • That same amount of energy is absorbed by the two atoms when the bond is broken.
  • Mean bond enthalpies are used to account for the different environments a bond might be in.

Full Notes Bond Enthalpies

When a bond is made between two atoms (atomic bond), the overall energy of those two atoms lowers – they are more stable when bonded.

bond making exothermic enthalpy a-level chemistry


To break an atomic bond, we have to give the bonded atoms energy – so they can ‘overcome’ the bond and break it.

bond breaking endothermic enthalpy a-level chemistry


Energy is required to break a bond; energy is released when a bond is formed.

Every bond requires different amounts of energy to break, so it releases different amounts of energy when it’s formed.

A bond enthalpy refers to the change in heat energy that occurs when that bond is either made (energy released) or broken (energy absorbed). Remember enthalpies are per mol (mol-1), meaning bond enthalpies refer to the heat energy change that occurs when one mole’s worth of a bond is made or broken.

Mean Bond Enthalpies

The exact enthalpy change that occurs when a particular type of bond is broken or formed is not always the same. This is because the chemical environment of a bond can alter how ‘stable’ a bond is and therefore changes the bond enthalpy. A carbon-hydrogen bond in methane will have a slightly different bond enthalpy compared to a carbon-hydrogen bond in methanoic acid.

It’s not realistic to use specific bond enthalpies for specific molecules, instead we use mean bond enthalpies. A mean bond enthalpy is just an average value for a bond enthalpy taken from different compounds containing that bond.


bond enthalpies environments methane bromomethane dibromoethane energy


We can use bond enthalpies to calculate the enthalpy change for a reaction.


calculating enthalpy change from bond data a-level chemistry sum-of