Gibbs Energy Change and Equilibrium Constant
Recall and apply the relationship between standard Gibbs energy change (ΔG°) and the equilibrium constant (K) for a reversible reaction.
What is Gibbs Energy Change and Equilibrium Constant?
The change in Gibbs free energy that accompanies a chemical reaction when all reactants and products are in their standard states (usually 1 atm pressure for gases, 1 M concentration for solutions, and a specified temperature, typically 298 K).
Key formula / rule: Standard Gibbs Energy Change and Equilibrium Constant
Key points
- Understand the relationship between standard Gibbs energy change and the equilibrium constant.
- Calculate ΔG° from K or K from ΔG°.
- Predict the spontaneity and extent of a reaction based on ΔG° and K.
- Apply the equation ΔG° = -RT ln K to solve problems.
Common exam trap
Confusing ΔG° with ΔG (Gibbs energy change under non-standard conditions).
Definitions
- Term
Standard Gibbs Energy Change (ΔG°)
- Meaning
The change in Gibbs free energy that accompanies a chemical reaction when all reactants and products are in their standard states (usually 1 atm pressure for gases, 1 M concentration for solutions, and a specified temperature, typically 298 K).
- Term
Equilibrium Constant (K)
- Meaning
A ratio of the product concentrations to the reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. It indicates the relative amounts of products and reactants present at equilibrium.
Learning objectives
Understand the relationship between standard Gibbs energy change and the equilibrium constant.
Calculate ΔG° from K or K from ΔG°.
Predict the spontaneity and extent of a reaction based on ΔG° and K.
Apply the equation ΔG° = -RT ln K to solve problems.
Formulae
- Name
Standard Gibbs Energy Change and Equilibrium Constant
- Note
Where ΔG° is the standard Gibbs energy change, R is the ideal gas constant (8.314 J K⁻¹ mol⁻¹), T is the absolute temperature in Kelvin, and K is the equilibrium constant.
- Expression
ΔG° = -RT ln K
- Name
Alternative form using log base 10
- Note
This form is useful when dealing with log₁₀ values.
- Expression
ΔG° = -2.303 RT log₁₀ K
Prerequisites
Thermodynamics (Gibbs Free Energy)
Chemical Equilibrium (Equilibrium Constant)
Logarithms
Common mistakes
Confusing ΔG° with ΔG (Gibbs energy change under non-standard conditions).
Incorrectly using the sign of ΔG° to determine spontaneity without considering K.
Using incorrect units for R or T.
Forgetting the negative sign in the equation ΔG° = -RT ln K.
Mistaking K for the rate constant.
Keywords
Gibbs Energy
Standard Gibbs Energy Change
Equilibrium Constant
Spontaneity
Thermodynamics
Chemical Equilibrium
Nernst Equation
Practice preview
At a given temperature, if ΔG° = 0 for a reversible reaction, what is the value of the equilibrium constant K?…
easy
Calculate the standard Gibbs energy change (ΔG°) for a reaction at 298 K if its equilibrium constant (K) is 10^5. (R = 8.314 J K⁻¹ mol⁻¹)…
hard
If the equilibrium constant (K) for a reaction is very large (K >> 1), what can be inferred about the standard Gibbs energy change (ΔG°)?…
medium