Thermodynamics (A-level only)

Thermodynamics in AQA A-Level Chemistry explores the energy changes that accompany chemical reactions and physical processes. This topic builds on enthalpy changes from AS level, introducing the concept of entropy (S) and the Gibbs free energy change (ΔG) to predict the spontaneity of reactions. You'll learn how to calculate entropy changes for reactions and understand the balance between enthalpy and entropy that determines whether a process is feasible under given conditions.

The second law of thermodynamics states that the total entropy of the universe increases for a spontaneous process. In chemistry, we focus on the system, but we must consider the surroundings to determine spontaneity. The Gibbs free energy equation, ΔG = ΔH – TΔS, combines enthalpy change (ΔH), temperature (T), and entropy change (ΔS) to predict reaction feasibility. A negative ΔG indicates a spontaneous process, while a positive ΔG means non-spontaneous. This topic is crucial for understanding why some reactions occur despite being endothermic, and it has applications in predicting reaction conditions in industrial processes like the Haber process.

Thermodynamics also covers the concept of standard states and standard molar entropies, allowing you to calculate entropy changes for reactions using tabulated data. You'll learn to interpret ΔG values to determine the temperature at which a reaction becomes spontaneous, and how to calculate ΔG for reactions under non-standard conditions using the relationship ΔG = ΔG° + RT ln Q. Mastering thermodynamics is essential for understanding chemical equilibrium and electrochemistry, as it provides the theoretical foundation for predicting reaction direction and extent.