What is the Difference Between Gibbs Free Energy and Standard Free Energy?

06/03/2025 • Maryella Faratro

The main difference between Gibbs free energy and standard free energy lies in the conditions under which they are calculated and applied. Here are the key differences:

  1. Conditions: Gibbs free energy (ΔG) is a general expression that combines enthalpy and entropy into a single value, and it depends on the reactants' nature and concentrations. In contrast, standard free energy (ΔG°) is calculated under standard conditions, which are typically 1 bar and 298 K.
  2. Non-zero value: ΔG is not a non-zero value, whereas ΔG° is a non-zero value.
  3. Temperature and pressure dependence: Standard free energy changes (ΔG°) are dependent on temperature (T) and pressure, while non-standard free energy changes (ΔG) can be calculated using the equation ΔG = ΔG° + RT ln Q, where R is the gas constant, T is the temperature in Kelvin, and Q is the reaction quotient.
  4. Application: The standard change in free energy (ΔG°) applies only when the reactants and products are in their standard states. In all other circumstances, the non-standard free energy change (ΔG) must be considered.

In summary, Gibbs free energy (ΔG) is a general expression that depends on the specific conditions and concentrations of reactants, while standard free energy (ΔG°) is a specific value calculated under standard conditions. The non-standard free energy change (ΔG) is used to determine the spontaneity of a reaction under non-standard conditions, taking into account temperature and pressure dependence.

Comparative Table: Gibbs Free Energy vs Standard Free Energy

Gibbs free energy and standard free energy are both thermodynamic quantities that describe the spontaneity of chemical reactions. Here is a table highlighting the differences between them:

Gibbs Free Energy (G) Standard Free Energy (G°)
Depends on experimental conditions Depends on standard conditions (1 atm, 1 M concentrations)
Change in G (ΔG) determines the spontaneity of a reaction at constant temperature and pressure Standard free energy change (ΔG°) is the change in free energy under standard conditions
ΔG = ΔH - TΔS, where ΔH is enthalpy change, T is temperature in Kelvin, and ΔS is entropy change ΔG° = ΔH° - TΔS°, where ΔH° is standard enthalpy change, T is temperature in Kelvin, and ΔS° is standard entropy change
Positive ΔG indicates a non-spontaneous reaction, while a negative ΔG indicates a spontaneous reaction Positive ΔG° indicates a non-spontaneous reaction, while a negative ΔG° indicates a spontaneous reaction

In summary, Gibbs free energy depends on the experimental conditions, while standard free energy is calculated under standard conditions (1 atm pressure and 1 M concentrations). Both quantities can be used to determine the spontaneity of a reaction, with a positive value indicating a non-spontaneous reaction and a negative value indicating a spontaneous reaction.

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