Spontaneous. reaction :
A process is said to be spontaneous if it occurs on its own without intervention of any extemal agency of any kind.
All natural processes are .spontaneous..
Entropy increases in all spontaneous processes.
Change in entropy `(triangleS)` = positive" is a condition but is not necessary and sufficient condition for the spontaneous nature of a reaction.
Change in enthalpy `(triangleH) ` =negative may be a condition but not a necessary and sufficient condition for the spontaneous nature of a reaction..
At this juncture, "Gibbs introduced another thermodynamic function which involves both enthalpy (H) and entropy (S) functions. This is known as free energy function (G). This is given by the equation G=H-TS. .G. is now referred as .Gibbs energy. (or) Gibbs function.
`triangleG_(sys)` is negative. for spontaneous reactions (or) processes. Thus the spontaneity of a reaction and the algebraic signs of `triangleH, triangleS, triangleG` and the magnitude of T can be related as follows:
Change in entropy `(triangleS)`= positive is a condition but is not necessary and sufficient condition for the spontaneous nature of a reaction.
For a spontaneous process change in entropy is positive.
`triangleS=+ve` (spontaneous)
`triangleS=-ve` (backward reaction is spontaneous)
`triangleS=0` (Equilibrium reaction)
`triangleH=-ve, triangleS=+ve, triangleG=-ve` then the reaction
Sponteneous at all T values (Reaction is irreversible)
