Gibbs Free Energy Criterion

2.
4.
1 Gibbs free energy criterion

The exothermic reaction at room temperature generally can proceed spontaneously, and the reaction tends to proceed in the direction of exothermic reaction; the endothermic but increased reaction can proceed spontaneously at high temperature, and the reaction tends to proceed in the direction of increase
.

Under the condition of constant temperature and constant pressure with non-volume work, according to the first law of thermodynamics

△U=Q+W _body +W _non

Then there is

Q=△UW _body- W _non

= △ U - (- p △ V) -W _non

= △ U + p △ VW _non

=△HW _non

Since the reversible pathway system absorbs the largest heat Qr,

Qr≥ △ HW _non

due to

△S=Qr/T

therefore

T △ S≥ △ HW _non

For constant temperature processes

T ₂ =T ₁

Therefore

(T ₂ S ₂ -T _.
1 S _.
1 ) ≥ (H ₂ -H _.
1 ) -W _non

Move items and organize them

- [(H ₂ -T ₂ S ₂ ) - (H _.
1 -T _.
1 S _.
1 )] ≥-W is _{a non-}

Since H, T, and S are all state functions, (H-TS) is also a state function, so a new state function G can be defined
.

G=H-TS

G is called Gibbs free energy, or free energy for short.
It is an additive state function, and its unit is the same as enthalpy
.

-△G≥-W _non

Under constant temperature and constant pressure conditions, the reduced value of Gibbs free energy G is the maximum non-volume work done by the system to the environment, and this maximum can only be achieved in a reversible way
.

The non-volume work done by the environment to the system is W _non , then -W _non is the non-volume work done by the system to the environment
.

Gibbs free energy G is the energy of the system to do non-volume work at constant temperature and pressure, which is the physical meaning of Gibbs free energy
.

2.
4.
2 The second law of thermodynamics

Using the Gibbs free energy criterion (- △ G≥-W _non ) direction of the reaction can be determined when the constant temperature and pressure with a non-volume work
.

The primary battery reaction is the process of having non-volume work (electrical work)
.

The reactions involved in this chapter are all reactions without non-volume work.

That is to say, the direction in which Gibbs free energy decreases is the direction in which the constant temperature and constant pressure are nothing more than the spontaneous progress of the volumetric work reaction, which is a form of expression of the second law of thermodynamics

Related Links: Calculation of Entropy Change