Question

Explain why enthalpy function can not determine the spontanity of a chemical reaction.

Answer 1

Enthalpy change is an integral component of all physical and chemical changes. For a macroscopic objects proceed by decrease of energy to undergo a spontaneous process because state of lowest energy correspond to maximum stability. Similarly chemical reactions which are exothermic in nature are spontaneous. But this is not always true, as spontaniety of a chemical reaction depends not only on enthalpy change function, but also on entropy change and temperature of the reaction. The Gibbs free energy change is given by the relation, , the nature of determines the spontaniety of a chemical reaction. There are four possibilities by which we can predict the spontaniety of a reaction.

(i) For exothermic reaction: In case of exothermic reaction, . Now if (more disorder), then free energy change, , so the reaction is always spontaneous. But if (less disorder), then spontaniety exclusively depends on temperature. In that case, the reaction is spontaneous if , i.e reaction is spontaneous () at low temperature.

(ii) For endothermic reaction: In case of endothermic reaction, . Now if (more disorder), then spontaniety exclusively depends on temperature. In that case, thereaction is spontaneous if , i.e reaction is spontaneous () at high temperature. But if (less disorder), the reaction will be non-spontaneous at all temperatures.

Therefore, enthalpy function alone cannot determine the spontaniety of a chemical reaction.