Relationship between free energy (G), enthalpy (H), and entropy (S)

Relationship between free energy (G), enthalpy (H), and entropy (S)

The relationship between free energy (G), enthalpy (H), and entropy (S) is described by the Gibbs free energy equation:

Δ G = Δ H – T Δ S

Let’s break down the relationships between these thermodynamic parameters:

1. Gibbs Free Energy (G):

Represents the maximum reversible work that a system can perform at constant temperature and pressure.

A negative Δ G indicates a spontaneous process (favorable for the process to occur), while a positive Δ G indicates a non-spontaneous process.

2. Enthalpy (H):

Represents the total heat content of the system.

A positive Δ H implies an endothermic process (heat is absorbed), while a negative Δ H implies an exothermic process (heat is released).

Enthalpy contributes to the direction of a process, and if Δ H is negative (exothermic), it favors spontaneity.

3. Entropy (S):

Represents the measure of disorder or randomness in a system.

A positive Δ S implies an increase in disorder, while a negative Δ S implies a decrease in disorder.

Entropy contributes to the spontaneity of a process, and if Δ S is positive, it favors spontaneity.

4. Temperature (T):

The absolute temperature in Kelvin.

The term -T Δ S reflects the temperature-dependent contribution to spontaneity.

Relationships

1. Spontaneity:

If Δ G < 0, the process is spontaneous, and both enthalpy and entropy contribute favorably to the spontaneity.

If Δ G > 0, the process is non-spontaneous, and both enthalpy and entropy contribute unfavorably to spontaneity.

2. Temperature Influence:

The term -T Δ S reflects the contribution of temperature to spontaneity.

At higher temperatures, the contribution of -T Δ S  becomes more significant, making processes with positive Δ H  and positive Δ S more likely to be spontaneous.

3. Enthalpy-Entropy Compensation:

Changes in enthalpy and entropy often interconnect. An exothermic process (negative Δ H) might involve a decrease in entropy (negative Δ S), and vice versa.

4. Equilibrium:

At equilibrium, Δ G = 0.

Equilibrium is achieved when the contributions of enthalpy and entropy are balanced.

In summary, the Gibbs free energy equation provides a comprehensive understanding of the thermodynamics of a process by considering both the heat content (enthalpy) and the degree of disorder (entropy) while accounting for the influence of temperature. It is a powerful tool for predicting the spontaneity and direction of chemical and physical transformations.

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