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Spontaneity of Reactions


Formally, spontaneous changes were defined as changes that could occur within systems when the systems are left to themselves, i.e., when no external aid (by way of applying heat, electricity, pressure, etc) is applied.

By the above definition, spontaneous changes were only thought to be exothermic changes. However, several endothermic changes have been found to be spontaneous as well.

To explain this, it became clear that enthalpy change alone is not enough to explain whether a change will be spontaneous or not. A modern view of spontaneous change was first propounded by J. Willard Gibbs, an American mathematician. He defined spontaneous change as a change that is capable of doing useful work at constant temperature and pressure.

A non-spontaneous change will not do work, but have work done on it. If a process is spontaneous, its reverse will be non-spontaneous. Examples of spontaneous change include certain physical and chemical changes.

Physical spontaneous change: running of water downhill; flow of heat from hot to cold regions and diffusion of gases.

Chemical spontaneous change: reactions in voltaic cells; the dissolution of zinc in aqueous copper(II) sulphate,

Zn + CuSO4 → ZnSO4 + Cu + Heat;

the reaction of a highly electropositive metal with dilute acid to liberate hydrogen,

Mg + H2SO4 → MgSO4 + H2 + Heat; and the dissolution of ammonium chloride in water (this is endothermic change).     

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