Calculating Energy Changes

  • Each chemical bond has a specific bond energy associated with it.
  • This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.
  • This energy can be used to calculate how much heat would be released or absorbed in a reaction.
  • To do this it is necessary to know the bonds present in both the reactants and products.
  • We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved.
  • 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 using the equation:

Energy change = Energy taken in – Energy given out

Example 1

Calculating Energy Changes Table 1, IGCSE & GCSE Chemistry revision notes

Example 2

Calculating Energy Changes Table 2, IGCSE & GCSE Chemistry revision notes

Need help?

AQA GCSE Chemistry Notes

Want to aim for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.