Gibbs Energy Change And Criteria For Equilibrium - Practice Questions & MCQ

It was introduced in order to relate H, S and to explain spontaneity. According to J. Willard Gibb's Free energy of a system is defined as the maximum amount of energy available to a system during a process that can be converted into useful work. 

or 

It is the thermodynamic quantity specially characterizing the system, the decrease in whose value during a process is equal to the useful work done by the system. 

It is denoted by G and it is given mathematically as follows: 

Here,

H = Enthalpy

T = Absolute Temperature

S = Entropy

Also, we learnt that 

Therefore, Free energy change at constant temperature and pressure is given as:

At standard condition that is, 298 K and 1 atm pressure

It is called Gibbs equation and it is used to explain criterion of spontaneity, driving force etc.

It is a state function and an extensive property. 

Gibb's Free Energy change for a Reaction

For a general reaction, it can be given as follows: 

This requires the exact same treatment as  or 

Gibb's Free Energy Change for small changes in a Reversible process

Now, 

Using equations (1) and (2), we can write

Now, 

Putting these values in the above expression (3), we have 

Note: Remember this important formula for small changes in dG values 

and Criteria of Spontaneity

Suppose we consider a system that is not isolated from its surroundings then for such a system is given as:

If we consider that q_p amount of heat is given by the system to the surroundings at constant temperature and constant pressure then 

From equation (i) and (ii)

Or

As according to Gibb-Helmholtz equation,

So,

As for the spontaneous process

Hence 

Thus for a spontaneous process must be positive.

Or must be negative.

Case I.  Suppose both energy and entropy factors oppose a process that is,

Thus, is positive for a non-spontaneous process.

  Case II.  Suppose both tendencies are equal in magnitude but opposite, that is,

Thus, the process is said to be at equilibrium.

Case III.  Suppose entropy and energy, both factors are favorable for a process, that is,

Thus, this process is spontaneous at every temperature.

• ,  Spontaneous process

• , Non-spontaneous process

• , Process in equilibrium

• In exergonic reaction

• In endergonic reaction

• Temperature also plays an important role to decide the spontaneity of a process. A process that is not spontaneous at low temperature can become spontaneous at high temperature and vice-versa.

Relationship between or  with E or :-

Free energy change in an electrochemical cell can be related to electrical work done (E) in cell as follows 

when we use standard conditions than 

Here = standard E.M.F of the cell

n = No. of moles of e- transferred 

F = Faraday's constant