Spontaneity in Thermodynamics

According to the First Law of Thermodynamics, the total energy of an isolated system always remains constant. The first law explains about the relationship between the work done by the system or by the system and the heat absorbed without putting any limitation on the direction of heat flow. However, all processes which occur naturally tend to proceed spontaneously in one direction only. What does spontaneity mean here? What factors determine the direction in a spontaneous change?

What is spontaneity?

Let us try to understand the meaning of spontaneity. A spontaneous process is an irreversible process and it could only be reversed by some external agents. The entropy of any system is defined as the degree of randomness in it.

Predicting the spontaneity of a reaction

Generally, total entropy change is the essential parameter which defines the spontaneity of any process. Since most of the chemical reactions fall under the category of a closed system and open system; we can say there is a change in enthalpy too along with the change in entropy. Since, change in enthalpy also increases or decreases the randomness by affecting the molecular motions, entropy change alone cannot account for the spontaneity of such a process. Therefore for explaining the spontaneity of a process we use the Gibbs energy change. Gibbs energy is a state function and an extensive property. The general expression for Gibbs energy change at constant temperature is expressed as:

Gibbs Equation ⇒ 

ΔGsys = ΔHsys – TΔSsys

Where,

ΔGsys = Gibbs energy change of the system

ΔHsys = enthalpy change of the system

ΔSsys = entropy change of the system

= Temperature of the system

This is known as the Gibbs equation.

For a spontaneous process, the total entropy change, ΔStotal is always greater than zero.

ΔStotalSsys + ΔSsurr

Where,

ΔStotal= total entropy change for the process

ΔSsys = entropy change of the system

ΔSsurr = entropy change of the surrounding

The change in temperature between the system and the surrounding in the case of thermal equilibrium between system and surrounding is 0, i.e. DT= 0. Thus, enthalpy lost by the system is gained by the surrounding. Hence, the entropy change of the surrounding is given as,

spontaneity

ΔHsurr = change in enthalpy of the surrounding

ΔHsys = change in enthalpy of the system

Also, for a spontaneous process, the total change in entropy is 0, i.e. ΔStotal> 0.

Therefore;

TΔSsys – ΔHsys>0

ΔHsys– TΔSsys<0

Using the Gibbs equation, it can be said that

ΔGsys< 0

Thus it can be inferred that any process is spontaneous if the change in Gibbs energy of the system is less than zero or else the process is not spontaneous.

Therefore with the help of the above relation, spontaneity of a reaction can be easily predicted.

  • In the case of exothermic reactions, enthalpy of the system is negative thereby making all exothermic reactions spontaneous.
  • In the case of endothermic reactions, Gibbs free energy becomes negative only when the temperature is very high or the entropy change is very high.

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Practise This Question

The degree of randomness or disorder in a system is known as entropy