Gibbs free energy change, ΔG

Summary and Exam Tips for Gibbs free energy change, ΔG

Gibbs free energy change, $\Delta G$, is a subtopic of Chemical Energetics in A-Level Physical Chemistry, which falls under the subject Chemistry in the Cambridge International A Levels curriculum. The sustainability of a chemical reaction is influenced by both enthalpy ($\Delta H$) and entropy ($\Delta S$) changes, not just the overall entropy change. The Gibbs free energy ($G$) equation, $\Delta G = \Delta H - T\Delta S$, accounts for these changes and helps determine reaction feasibility. A negative $\Delta G$ indicates a spontaneous reaction, while a positive $\Delta G$ suggests a nonspontaneous reaction. If $\Delta G = 0$, the reaction is at equilibrium. Temperature significantly impacts reaction spontaneity, with guidelines suggesting that increased temperature favors reactions where both $\Delta H$ and $\Delta S$ are positive, and hinders those where both are negative. The equilibrium constant ($K$) is also temperature-dependent, generally favoring endothermic reactions (positive $\Delta H$) with increased temperature. Calculations involving standard free energies of formation ($\Delta G^\circ_f$) can predict reaction spontaneity, as demonstrated in example problems.

Exam Tips

• Understand the Gibbs Equation: Familiarize yourself with $\Delta G = \Delta H - T\Delta S$ and practice rearranging it to solve for different variables like temperature.
• Analyze Reaction Feasibility: Remember that a negative $\Delta G$ indicates spontaneity. Use this to quickly assess reaction feasibility in exam questions.
• Temperature Effects: Be prepared to discuss how temperature changes affect reaction spontaneity, especially in relation to the signs of $\Delta H$ and $\Delta S$.
• Equilibrium Constant: Know how temperature influences the equilibrium constant $K$, particularly in endothermic and exothermic reactions.
• Practice Calculations: Work through example problems to become comfortable with calculating $\Delta G$ using standard free energies of formation.