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Limitations of First Law of Thermodynamics

  1. First law does not provide a clear idea about the direction of absorption or evolution of heat.
  2. The informations provided by the first law of thermodynamics are not enough to predict the spontaneity or feasibility of a process.
To deal with spontaneity of chemical processes some new terms viz., entropy (S) and Gibbs free energy (G) were introduced in thermodynamics which lead way to the formulation of second and third laws of thermodynamics.

Entropy: The degree of disorderliness or randomness is called entropy of a system. More the randomness, more will be the entropy of a system. The terms heat (q) and temperature plays a main role in arriving at the entropy of a system. The relationship between them can be formulated as
Δ S=
qrev stands for heat changes involved in a reversible process. Unlike internal energy changes involved in first law of thermodynamics the value of Δ S for an irreversible reaction is not zero. It become zero in equilibrium reactions where a state of orderliness is setup in the system.

Entropy is independent of the path followed by a reaction and hence it is state function

In a perfect crystalline state where the entities are orderly arranged the entropy is found to be zero.

Generally in physical state transformations (solidliquid, liquid gas etc.) the entropy increases.

Example for the phenomenon where entropy decreases, making amlette.
Conversion liquid water to ice.

Gibbs free energy: Since almost all the processes proceed with change in enthalpy or entropy which sometimes may not be a sufficient criteria for spontaneity of a reaction an other thermodynamic parameter viz.,free energy(G) was introduced.The term G can be related to enthalpy(H) and entropy(S) of a system as,
G = H - TS (at constant temperature)

Using the above we can arrive at the Gibbs-Helmholtz equation given by,
G = H - TS

From the above expression it is clear that if G is negative the reaction is spontaneous or feasible and not otherwise.

Effect of temperature on spontaneity of reactions is given by the following table:

Effect of temperature on spontaneity of reactions

 

Description*
-

-

-

+

+

+

+

-

-

+

+

-

-

- (at low T)

+ (at high T)

+ (at low T)

- (at high T)

+ (at all T)

Reaction spontaneous at all temperature

Reaction spontaneous at low temperature

Reaction non spontaneous at high temperature

Reaction non spontaneous at low temperature

Reaction spontaneous at high temperature

Reaction non spontaneous at all temperatures


Go (free energy change under standard conditions of temperature and pressure) can be related to equilibrium constant as,
Go = -2.303 RT log K




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