Gibbs Free Energy and Reaction Spontaneity
Definition
Gibbs free energy
Gibbs free energy ($\Delta G$) is a thermodynamic property that determines whether a chemical reaction is spontaneous under constant pressure and temperature.
- A reaction is spontaneous if it proceeds toward completion or equilibrium without requiring external energy.
- The relationship between Gibbs free energy and other thermodynamic properties is expressed by the equation: $$\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$$ where:
- $\Delta G^\circ$: Standard Gibbs free energy change ($kJ \, mol^{-1}$)
- $\Delta H^\circ$: Standard enthalpy change ($kJ \, mol^{-1}$)
- $T$: Temperature in Kelvin (K)
- $\Delta S^\circ$: Standard entropy change ($J \, K^{-1} mol^{-1}$)
Let’s break this down:
- Enthalpy ($\Delta H^\circ$):
- Represents the heat energy absorbed or released during a reaction.
- Exothermic reactions ($\Delta H^\circ < 0$) release heat, while endothermic reactions ($\Delta H^\circ > 0$) absorb heat.
- Entropy ($\Delta S^\circ$):
- Measures the disorder or randomness in a system.
- Reactions that increase disorder ($\Delta S^\circ > 0$) are more likely to be spontaneous.
- Temperature ($T$):
- Higher temperatures amplify the influence of entropy on Gibbs free energy.
- Temperature must always be in Kelvin.
- To convert from °C to K, add 273.15.
When Is a Reaction Spontaneous?
The sign of $\Delta G$ determines whether a reaction is spontaneous:
- $\Delta G< 0$: The reaction is spontaneous.
- $\Delta G = 0$: The reaction is at equilibrium.
