In the context of Gibbs free energy, "ΔG" refers to the change in Gibbs free energy between the initial and final states of a chemical reaction or process under specific conditions. It takes into account the actual concentrations or pressures of the reactants and products.
On the other hand, "ΔG°" (Delta G naught) represents the standard Gibbs free energy change, which is the change in Gibbs free energy under standard conditions. Standard conditions typically refer to a temperature of 25°C (298 K), a pressure of 1 bar, and concentrations of 1 mole per liter for each species involved in the reaction.
The key differences between ΔG and ΔG° are:
Conditions: ΔG considers the actual conditions of a reaction, including non-standard temperatures, pressures, and concentrations. In contrast, ΔG° specifically represents the change in Gibbs free energy under standard conditions.
Comparability: ΔG values can vary depending on the specific conditions of a reaction. However, ΔG° values are standardized and allow for direct comparison between different reactions.
Equilibrium: ΔG is used to determine the direction and spontaneity of a reaction at any given set of conditions. If ΔG is negative, the reaction is thermodynamically favorable in the forward direction. If ΔG is positive, the reaction is thermodynamically unfavorable in the forward direction. ΔG°, on the other hand, specifically refers to the change in Gibbs free energy at equilibrium under standard conditions. If ΔG° is negative, it indicates that the reaction is thermodynamically favorable at standard conditions.
Calculation: ΔG is calculated using the equation ΔG = ΔG° + RT ln(Q), where Q is the reaction quotient and R is the gas constant. ΔG°, on the other hand, represents the value of ΔG at equilibrium under standard conditions.
In summary, ΔG represents the change in Gibbs free energy under specific conditions, while ΔG° represents the change in Gibbs free energy at equilibrium under standard conditions. ΔG° provides a standardized value for comparing different reactions, while ΔG accounts for the specific conditions of the reaction being considered.