Spontaneity

Nature tends to greater disorder

Any change that occurs spontaneously must have a final state which is more probable than its initial state

The final entropy of the universe must be greater than its initial entropy. (This is the second law of thermodynamics.)

Entropy change of the universe

It depends on both the entropy change of the system as well as the entropy change of the surroundings

Δssyst measures the entropy change of the system

The major effect of chemical changes on the entropy of the surroundings result from gain and loss of heat energy.

If a change in chemical potential energy is converted into heat energy which gets transferred to the universe through an exothermic change, it results in an increase in entropy of the surroundings

And the opposite is true for an endothermic change.

Magnitude

The magnitude of entropy change is given by:

ΔSΘsurr = - ΔHΘ/T

T is the absolute temperature.

The condition for a spontaneous change to occur is that Δsuniv is positive

ΔSΘuniv = Δsθ

surr + Δsθsyst

= -ΔHΘ/T +Δsθsyst

If a change is spontaneous depends on how it affects the entropy of the system and entropy of the surroundings through the heat energy released

If both are favorable, then a reaction will occur and vice versa

If one is favorable, the outcome will depend on which factor is the dominant one at the given temperature

Gibbs free energy change

The above mentioned condition has been traditionally considered in the terms of the Gibbs free energy change

The Gibbs free energy change ΔG is equal to – T ΔS

If this quantity is negative, then ΔSuniv must be positive so that the process can occur spontaneously

ΔGθ = -TΔS = -T (-ΔHθ/T + Δsθsyst) = ΔHθ-

TΔSθ

For Δsuniv to be positive and change to be spontaneous, ΔG has to be negative

Conditions for a change

Whether it results in a product with lower enthalpy

Whether it results in a product with greater entropy

Hence a reaction will definitely occur if both the enthalpy change is negative and the entropy change is positive

Positive/endothermic

Positive/endothermic

Negative/exothermic

Negative/exothermic

Positive/entropy increase

Negative/ entropy decrease

Positive/ entropy increase Negative/entropy increase

Depends on T

Always positive

Always negative

Depends on T

Spontaneous at high temperatures

Never spontaneous

Always spontaneous

Spontaneous at low temperatures when T.