Question

Classify the possible combinations of signs for a reaction's ΔΗ and AS values by the resulting spontaneity. spontaneous as written spontaneous as written spontaneous as written at all temperatures spontaneous in reverse at all temperatures above a certain temperature below a certain temperature

Answer 1

The basic requirement for a process to be spontaneous depends on two parameters 1.Enthalpy

2.Entropy

So we will define spontaneity by the relation

∆G = ∆H - T ∆S

Delta GGmust be negative for reaction to be spontaneous

For a system to be spontaneous ∆S should be positive. If ∆S is positive, it means that the reaction can further take place till ∆S becomes 0 and so, the reaction is spontaneous. But if ∆S is negative, the reaction is non-spontaneous.

Gibbs free energy combines effect of both parameters and if Gibbs free energy comes out negative than process is spontaneous

As we have to make Delta G negative for Reaction will be spontaneous .

Case 1 Delta H = +ve Delta S = + ve

Reaction will be spontaneous at above a certain temperature

Case 2 ∆H= - ve. ∆ S= +ve

Reaction will be spontaneous at all temperature

Case 3 ∆H=-ve ∆S=-ve

Reaction will be spontaneous below certain range of temperature

Case 4 ∆H= +ve ∆S=-ve

Always non spontaneous so spontaneous in reverse direction