Gibbs Free Energy

What is Gibbs Energy?

Generally, the total entropy change is the fundamental parameter that 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.

Gibbs energy is a state function and an extensive property. Since, change in enthalpy too increases or decreases the randomness by affecting the molecular motions, entropy change alone cannot account for the spontaneity of such process. Therefore, we use the Gibbs energy change for explaining the spontaneity of a process.

Table of Contents

• Expression for Gibbs Energy Change
• The Spontaneity of A Process
• Gibbs Energy in Chemistry
• Frequently Asked Questions – FAQs

Knowledge of the Gibbs energy under one condition compared with another allows us to predict the direction of spontaneous change or movement: A spontaneous change in a system at constant temperature and pressure proceeds in the direction of decreasing free energy.

Expression for Gibbs Energy Change

General expression for Gibbs energy change at constant temperature is expressed as:

Where,

Above equation is popularly known as Gibbs equation. Gibbs’ equation relates enthalpy and entropy of the system. We know that for a spontaneous process, the total entropy change,

In case of thermal equilibrium between system and surrounding, temperature change between system and surrounding,

As discussed earlier, for the spontaneity of a process, ΔStotal > 0. Above equation becomes,

Above equation can be related to Gibbs equation as,

On the basis of above equation we can infer:

• \(\begin{array}{l}\triangle G_{sys} < 0\end{array} \)
  , the process is spontaneous
• \(\begin{array}{l}\triangle G_{sys} > 0\end{array} \)
  ,, the process is non-spontaneous

The Spontaneity of A Process

Gibbs equation helps us to predict the spontaneity of reaction on the basis of enthalpy and entropy values directly. When the reaction is exothermic, enthalpy of the system is negative making Gibbs free energy negative. Hence, we can say that all exothermic reactions are spontaneous.

In the case of endothermic reactions, when enthalpy of the system is positive, the process is spontaneous under two conditions:

• Temperature is very high to make the Gibbs energy value negative
• Entropy change is very high to make the Gibbs free energy negative.

Spontaneity can only indicate if a reaction can occur not necessarily if a reaction will occur. For example, the conversion of diamond to graphite is a spontaneous process at Standard Temperature and Pressure (STP) but it is a slow process. It will take years for the transformation to occur.

Gibbs Energy in Chemistry

The energetics of processes for systems at constant temperature and pressure, the appropriate quantity is known as the Gibbs free energy. The Gibbs free energy has a very useful property; it decreases for a spontaneous process at constant temperature and pressure. Under such conditions the decrease in Gibbs free energy equals the maximum amount of energy available for work, whereas if it increases for some transition, the change in Gibbs free energy represents the minimum amount of work required.

The transformation of a system from one state to another, at constant temperature and pressure, is spontaneous if the Gibbs free energy decreases. If the Gibbs free energy is unchanged by the transformation the two states are in equilibrium. In other words the criterion for the thermodynamic equilibrium of a system at constant temperature and pressure is that the Gibbs free energy of the system be at the minimum value. The Gibbs free energy is sometimes called the thermodynamic potential at constant pressure in order to indicate its analogy with the potential energy of a mechanical system, which also has a minimum value under equilibrium conditions.

For detailed discussions on the spontaneity of a process on the basis of Gibbs free energy change, check out BYJU’S -The Learning App.