AS-Level Energy and Enthalpy

  • The energy levels of two individual atoms lower when they bond together.

  • A certain amount of energy is released by two atoms when a bond is formed, and that same amount of energy is absorbed by the two atoms when the same bond is broken.

  • The amounts of energy released or absorbed for a particular bond is called its bond enthalpy.

  • Mean bond enthalpies are used to account for the different environments a bond may be in.

  • The overall enthalpy change that occurs in a reaction can be found by subtracting the sum of all bond enthalpies formed from the sum of all bond enthalpies broken.


Bond Enthalpies


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




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


This is why energy is required to break a bond, but 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 energy that occurs when either that bond is made (energy released) or is broken (energy absorbed).


The exact energy 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 change the enthalpy of the bond. A carbon-hydrogen bond in methane will have a slightly different bond enthalpy compared to a carbon-hydrogen bond in methanoic acid.










It is not realistic, however, to use specific bond enthalpies for specific molecules, so 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.




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