1.8 Thermodynamics (A2)
The further study of thermodynamics builds on the Energetics section and is important in understanding the stability of compounds and why chemical reactions occur. Enthalpy change is linked with entropy change enabling the free-energy change to be calculated.
Gibbs free-energy change ΔG and entropy change ΔS - Definitions ⦵ Standard pressure (100kPa), usually with a stated temperature (e.g. 298K) ΔG⦵ Gibbs free-energy change, measured in kJ mol−1 ΔH⦵ Enthalpy change, measured in kJ mol−1 T Temperature measured in Kelvin ΔS⦵ Entropy change, measured in J K−1 mol−1 The feasibility of a physical or a chemical change depends on the balance between the thermodynamic quantities of enthalpy Born-Haber cycles - KEY CONCEPTS 1. Enthalpy is heat change of a system under conditions of constant pressure. ΔH⦵ (298 K) represents the standard enthalpy change of a system at 100kPa and at the stated temperature (i.e. 298 K) per mole of the specified substance. The standard state of a substance is the form of the pure substance at 100kPa at the stated temperature (normally quoted as 298 K).