Overview
This lecture covers key concepts in chemical energetics, focusing on enthalpy changes, lattice energy, the Born-Haber cycle, factors affecting solubility, ion polarization, and the stability of group 2 compounds.
Enthalpy Changes
- Enthalpy change of atomisation is the enthalpy change when 1 mole of gaseous atoms forms from its elements under standard conditions.
- First electron affinity is exothermic; further electron affinities are endothermic due to electron-electron repulsion.
Lattice Energy
- Lattice energy is the enthalpy change when 1 mole of an ionic solid forms from its gaseous ions under standard conditions.
- The stronger (more negative) the lattice energy, the stronger the ionic bonding and the more stable the compound.
- Similar theoretical and experimental lattice energies indicate pure ionic bonding.
Factors Affecting Lattice Energy
- Lattice energy increases with greater ion charge and smaller ion radius, due to higher charge density and stronger electrostatic attraction.
Born-Haber Cycle
- Born-Haber cycles use enthalpy changes (formation, atomisation, electron affinity, lattice) to calculate lattice energy.
- Formation enthalpy under standard conditions for an element is zero.
Dissolving Salts
- Enthalpy change of solution is the enthalpy change when 1 mole of an ionic solid dissolves in water to form a dilute solution.
- Enthalpy of hydration refers to dissolving gaseous ions in water.
- Factors affecting hydration enthalpy are similar to those affecting lattice energy.
- More negative enthalpy of solution means higher solubility.
Ion Polarisation
- Ion polarisation is the distortion of an anion's electron cloud by a cation.
- Polarising power increases with higher cation charge density and decreases with larger cation size; anion polarizability increases with charge and size.
Thermal Stability of Group 2 Compounds
- Down group 2, polarising power of cations decreases, leading to increased stability of carbonates and nitrates.
- Order of stability: Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺.
Solubility of Group 2 Hydroxides and Sulphates
- Hydroxide solubility increases down the group due to more exothermic solution enthalpy.
- Sulphate solubility decreases down the group as hydration enthalpy drops faster than lattice energy.
Key Terms & Definitions
- Enthalpy Change of Atomisation — Energy change when 1 mole of gaseous atoms forms from elements.
- Lattice Energy — Energy change when an ionic solid forms from gaseous ions.
- Electron Affinity — Energy change when an atom gains 1 mole of electrons to become an anion.
- Born-Haber Cycle — Thermochemical cycle for ionic lattice formation.
- Enthalpy of Solution — Energy change when solid ionic compound dissolves in water.
- Enthalpy of Hydration — Energy change when gaseous ions become hydrated ions.
- Ion Polarisation — Distortion of anion by cation’s electric field.
- Polarising Power — Cation’s capability to distort an anion.
Action Items / Next Steps
- Review Born-Haber cycle diagrams and practice constructing them for different ionic compounds.
- Memorize trends in stability and solubility for group 2 hydroxides and sulphates.