Energy Calculations
The general formula:
Bonds of all the reactants – Bonds of all the products = Energy change
Example: Methane reacts with chlorine to produce chloromethane and hydrogen chloride. The equation:
CH4 + Cl2 -> CH3Cl + HCl
| Bond | C – H | C – Cl | H – Cl | Cl – Cl |
| Energy (kJ mol -1) | 413 | 346 | 432 | 243 |
You would be given a table with the bonds and the energy required to break/bond them:
Reactants:
4 C – H bonds (CH4) = 4 x 413 = 1652kJ
1 Cl – Cl bond (Cl2) = 1 x 243 = 243 kJ Total: 1652 + 243 = 1895 kJ
Products:
3 C – H bonds = 3 x 413 = 1236 kJ
1 C – Cl bond = 1 x 346 = 346 kJ
1 H – Cl (HCl) = 1 x 432 = 432 kJ Total: 2017 kJ
(Carbon can form 4 bonds. In this case, 3 of them bonds with 3 hydrogen and the last one bonds with chlorine)
Energy Change = 1895 – 2017 = -122 kJ the reaction is exothermic