Overview
This lecture covers the core principles of AQA A-level thermodynamics, focusing on Born-Haber cycles, key enthalpy and entropy definitions, lattice enthalpy calculations, entropy and feasibility using Gibbs free energy, and enthalpy change of solution.
Fundamental Definitions & Concepts
- Enthalpy change of formation: Enthalpy change when one mole of a compound forms from elements in standard states under standard conditions.
- Lattice enthalpy of formation: Enthalpy change when one mole of a solid ionic compound forms from gaseous ions.
- Lattice enthalpy of dissociation: Enthalpy change when one mole of a solid ionic compound dissociates into gaseous ions.
- Enthalpy change of atomization: Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
- First ionization energy: Enthalpy change when one mole of gaseous 1+ ions is made from gaseous atoms.
- Electron affinity: Enthalpy change when one mole of gaseous 1− ions is formed from gaseous atoms.
- Second ionization/affinity: Removal/addition of a second electron; may be endothermic if adding to a negative ion.
The Born-Haber Cycle
- Born-Haber cycles calculate lattice enthalpies since direct measurement is not possible.
- The cycle shows two routes to form an ionic compound: directly from elements or via formation of gaseous ions.
- Each step corresponds to a defined enthalpy change (formation, atomization, ionization, electron affinity, lattice enthalpy).
- To use the cycle: follow arrows, keeping or reversing signs based on direction; sum enthalpy changes for calculation.
- Always check if results make physical sense (e.g., lattice formation should be exothermic).
Theoretical vs. Experimental Lattice Enthalpy
- Theoretical lattice enthalpy assumes perfect ionic models with spherical ions and even charge distribution.
- Experimental values may differ due to covalent character (polarization of the negative ion by the positive ion).
- Larger distortions and greater polarization lead to bigger differences, indicating more covalent character.
Enthalpy Change of Solution
- Enthalpy change of solution: Enthalpy change when one mole of an ionic substance dissolves in enough solvent to prevent further enthalpy change on dilution.
- Process involves breaking ionic bonds (endothermic, lattice dissociation) and hydrating ions (exothermic, enthalpy of hydration).
- Most ionic compounds dissolve exothermically if hydration enthalpy > lattice dissociation energy.
- Calculated using a Hess's cycle for dissolution.
Entropy and Gibbs Free Energy
- Entropy is a measure of disorder; higher in gases, increases with more particles or greater randomness.
- Positive entropy (ΔS) and negative enthalpy (ΔH) make reactions more feasible.
- Gibbs free energy: ΔG = ΔH - TΔS determines spontaneity; reaction is feasible if ΔG < 0 or ΔG = 0.
- Feasibility depends on both ΔH and ΔS, and temperature can shift whether a process is feasible.
Calculations and Application
- Entropy change: ΔS = S(products) - S(reactants), units J K⁻¹ mol⁻¹.
- Standard entropy values are under standard conditions: 1 mole, 100 kPa, 298 K.
- To find temperature where reaction becomes feasible: set ΔG = 0, so T = ΔH / ΔS (convert units).
Key Terms & Definitions
- Enthalpy (ΔH) — Heat change at constant pressure, usually in kJ/mol.
- Lattice Enthalpy — Energy released/required when ionic solid forms/dissociates from gaseous ions.
- Born-Haber Cycle — Thermochemical cycle for calculating lattice enthalpy.
- Entropy (ΔS) — Measure of disorder, units J K⁻¹ mol⁻¹.
- Gibbs Free Energy (ΔG) — Determines reaction feasibility, ΔG = ΔH - TΔS.
- Enthalpy of Solution — Energy change when a substance dissolves.
Action Items / Next Steps
- Memorize key definitions and practice integrating them into Born-Haber cycles.
- Practice Born-Haber and solution enthalpy calculations using given data.
- Complete homework problems involving entropy changes and Gibbs free energy calculations.
- Review the impact of temperature and entropy on reaction feasibility.