The energy change in a reaction can be calculated using bond energies. A bond energy is the amount of energy needed to break one mole of a particular covalent bond. Different bonds have different bond energies. These are given when they are needed for calculations.

To calculate an energy change for a reaction:

• add together 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’
• energy change = energy in - energy out

Example

Hydrogen and chlorine react to form hydrogen chloride gas:

H−H + Cl−Cl → 2 × (H−Cl)

Use the bond energies in the table to calculate the energy change for this reaction.

Energy in = 436 + 243 = 679 kJ/mol

Energy out = (2 × 432) = 864 kJ/mol

Energy change = in - out

= 679 - 864

= -185 kJ/mol

The energy change is negative. This shows that the reaction is exothermic.

Example

Hydrogen bromide decomposes to form hydrogen and bromine:

2 × (H−Br) → H−H + Br−Br

Use the bond energies in the table to calculate the energy change for this reaction.

Energy in = 2 × 366 = 732 kJ/mol

Energy out = 436 + 193 = 629 kJ/mol

Energy change = in - out

= 732 - 629

= +103 kJ/mol

The energy change is positive. This shows that the reaction is endothermic.