Spontaneity

You react “spontaneously” to some situations. Don’t you? Let’s say for example when your mom asks who wants the bigger piece of cake? Your hands go up in spontaneity. Don’t they? However, spontaneity in chemistry is a little different concept. It is a vital concept in Thermodynamics of chemical reactions. Let us now study all about it.

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What is Spontaneity?

According to the First Law of Thermodynamics, the total energy in an isolated system always remains constant. This law also explains the relationship between the work done by the system and the heat absorbed without putting any limitation on the direction of heat flow.

However, all processes which occur naturally usually proceed spontaneously in one direction only. What does spontaneity actually mean here? What are the various factors that determine the direction in a spontaneous change? Let’s find out!

A spontaneous process is an irreversible process. However, you can actually reverse it by the application of some external agents. The entropy of any system is the amount of randomness in it.

How to Predict the Spontaneity of a Reaction?

Generally, total entropy change is the essential parameter which we can use to describe the spontaneity of any process. Since most of the chemical reactions are either in a closed system and open system; we can say there is a change in enthalpy along with a change in the entropy.

Since enthalpy also increases or decreases the randomness by affecting the molecular motions, we can say that entropy change alone cannot be responsible for the spontaneity of such a process. Therefore, to explain the spontaneity of a process, we use the Gibbs energy change.

Browse more Topics under Thermodynamics

• Introduction to Thermodynamics
• Thermodynamic Processes
• First Law of Thermodynamics
• Second Law of Thermodynamics
• Reversible and Irreversible Process
• Carnot Engine
• Heat Engines and Heat Pumps

Gibbs Energy

Gibbs energy is a state function. It is an extensive property. The general expression for Gibbs energy change at constant temperature is as follows:

ΔG_sys  =  ΔH_sys  –  TΔS_sys

Where,

• ΔG_sys = Gibbs energy change of the system
• ΔH_sys = system’s enthalpy change
• ΔS_sys  = system’s entropy change
• T = System’s Temperature

This is the Gibbs equation. For a spontaneous process, the total entropy change, ΔS_total is always positive.

ΔS_total  =  ΔS_sys  +  ΔS_surr

Where,

• ΔS_total = total entropy change for the process
• ΔS_sys = entropy change of the system
• ΔS_surr = 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 that the system loses, the environment gains. Hence, the entropy change of the surrounding is given as,

• ΔH_surr = change in surrounding’s enthalpy
• ΔH_sys = change in system’s enthalpy

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

TΔS_sys – ΔH_sys > 0

ΔH_sys –  TΔS_sys < 0

Using the Gibbs equation, it can be said that,

ΔG_sys < 0

Thus, we can see that any process is spontaneous when the Gibbs energy change of the system is negative.

Solved Example For You

Q: How is enthalpy of a system in case of exothermic and endothermic reactions?

Ans: In exothermic reactions, enthalpy of the system is negative. Thus, all such reactions are spontaneous. In endothermic reactions, Gibbs free energy is negative only when we keep the temperature very high. in other cases, the entropy change is very elevated.