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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 broken in the reactants - this is the 'energy in'
• add together the bond energies for all the bonds formed in the products - this is the 'energy out'

The 'energy in' is an endothermic change, as the energy is being used to break bonds. The 'energy out' is an exothermic change, as the energy is released as new bonds are formed. Therefore the energy change is:

• energy change = energy in - energy out

Example

Hydrogen and chlorine react to form hydrogen chloride gas:

��−H + ���−C�� → 2 × (��−C��)

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

Energy in = 436 + 243 = 679 kJ mol^-1

Energy out = (2 × 432) = 864 kJ mol^-1

Energy change = in - out

= 679 - 864

= -185 kJ mol^-1

The energy change is negative. This shows that the reaction is exothermic, as the 'energy out' is larger than the 'energy in'.

Example

Hydrogen bromide decomposes to form hydrogen and bromine:

2 × (��−B��) → ��−H + ����−B��

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

Energy in = 2 × 366 = 732 kJ mol^-1

Energy out = 436 + 193 = 629 kJ mol^-1

Energy change = in - out

= 732 - 629

= +103 kJ mol^-1

The energy change is positive. This shows that the reaction is endothermic, as the 'energy out' is smaller than the 'energy in'.