Specification Point 3.7C (Paper 2C Only):
Use bond energies to calculate the enthalpy change during a chemical reaction
Bond Energies
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
Author: Jamie
Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.