CALCULATION OF ENTHALPY CHANGE USING BOND ENERGY

It is possible to calculate the enthalpy change of a reaction if the bond enthalpies of the compounds are known. Consider the reaction between hydrogen and fluorine to form hydrogen fluoride.

H2(g) + F2(g) = 2HF(g) The equation for the reaction is: This could be represented as:

H-H + F-F = H-F + H-F In order to perform the reaction it is necessary to break: 1x H-H bond – value 436 kJ mol-1 1 x F-F bond – value 158 kJ mol-1 Notice that breaking bonds is an endothermic process – values are positive

This gives a total of 436 + 158 = 594 kJ At this stage all molecules have been broken up into free atoms in the gas phase. The atoms are now put together into the product molecules.

H-H + F-F = H-F + H-F In order to complete the reaction it is necessary to make: 2 x H-F bonds – value -562 kJ each Total energy released is -1124 kJ When a bond is formed energy is released so this is an exothermic process – values are negative

The overall energy change for the reaction is simply the difference between the energy needed to break bonds and the energy given out when bonds are made. Energy in = 594kJ Energy out = 1124 kJ

Energy change = 594 – 1124 kJ Energy change = -530 kJ