Explain the spontaneity of a reaction in terms of enthalpy change, entropy change and Gibbs energy change.

Explanation

To predict about the spontaneity of the reaction and
To redict the useful work that could be extract is the sign and magnitude of the quantities in Gibb.s equation is useful.
When a reaction is reversible it cannot proceed in both the direction with same case. In general the system would spontaneously change to configuration of lower free-energy.
At equilibrium `triangleG= 0`
`O=triangleG^@+RT_(lnK)`
`triangle_r G^@= "-RT_(lnK)=-2.303 RT log k"`
We know `triangle_r G^@=triangle_r H^@-T triangle_r S^@=-"RT log k"`
For endothermic reaction `triangle_rH^@` may be large and positive
In such a case K is much less `(K lt 1)`
In exothermic reactions `triangle_rH^@` is large and negative `(K gt 1)`
For exothermic reaction
a) K must be large
b) `triangle_rG^@` is highly negative
c) `triangle_rS^@` is highly positive
Hence with the sign and magnitude of the factors the yield of the product my be estimated.
`triangleG_(sys)` =negative for spontaneous reaction `(triangleG gt 0)`
`triangleS_(sys)` = positive for non-spontaneous reaction `(triangleG gt 0)`
`triangleG_(sys)` =zero equilibrium reactions `(triangleG=0)`