Overview
This lecture covers key thermodynamic definitions, concepts of lattice enthalpy and the Born-Haber cycle, trends in lattice energies, entropy, Gibbs free energy, and the link between thermodynamics and chemical feasibility, including enthalpies of solution.
Key Thermodynamic Definitions
- Enthalpy of atomisation: energy change to form 1 mole of gaseous atoms from an element in its standard state.
- Bond dissociation enthalpy: energy change to break 1 mole of covalent bonds into gaseous atoms/free radicals.
- First ionisation enthalpy: energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms to form +1 ions.
- Second ionisation enthalpy: energy to remove 1 mole of electrons from 1 mole of gaseous +1 ions to form +2 ions.
- First electron affinity: energy change when 1 mole of gaseous atoms gains electrons to form 1- ions (usually exothermic).
- Second electron affinity: energy change when 1- ions gain another electron per ion to form 2- ions (usually endothermic).
- Lattice enthalpy of formation: energy change when 1 mole of ionic solid forms from gaseous ions.
- Lattice enthalpy of dissociation: energy change when 1 mole of ionic solid separates into gaseous ions.
- Enthalpy of hydration: energy change when 1 mole of gaseous ions dissolves in water to form aqueous ions (always exothermic).
- Enthalpy of solution: energy change when 1 mole of ionic solid dissolves in water to give well-separated ions.
Born-Haber Cycles and Lattice Energy
- Lattice enthalpy cannot be measured directly but is calculated using the Born-Haber cycle and Hess’s law.
- For sodium chloride: fH = atH(Na) + IE(Na) + atH(Cl) + eaH(Cl) + LattH.
- Be aware of sign conventions: lattice formation is negative, lattice dissociation is positive.
- The strength of lattice enthalpy depends on ion size (smaller = stronger) and charge (higher charge = stronger).
Covalent Character and Theoretical vs. Experimental Lattice Energies
- Perfect ionic model assumes ions are purely ionic and spherical; theoretical and experimental values match.
- Covalent character increases when positive ions are small/high charge or negative ions are large/high charge.
- Experimental (Born-Haber) lattice enthalpy is higher when covalent character is present, indicating a stronger real lattice.
Entropy and Gibbs Free Energy
- Entropy (S) measures disorder; higher when there are more ways to arrange the particles or energy.
- Solids < liquids < gases in entropy; mixtures > pure substances.
- Entropy change (ΔS) = S(products) – S(reactants).
- Gibbs free energy (ΔG) combines enthalpy and entropy: ΔG = ΔH - TΔS.
- A reaction is feasible when ΔG < 0; phase changes occur when ΔG = 0.
- Feasibility can depend on temperature, especially when entropy changes are significant.
Enthalpy of Solution and Solubility
- Enthalpy of solution combines lattice dissociation (endothermic) and hydration enthalpy (exothermic).
- ΔHsolution = lattice dissociation enthalpy + sum of hydration enthalpies.
- If ΔHsolution is exothermic, substance is usually soluble; large lattice enthalpy relative to hydration can make a salt insoluble.
- Even if ΔHsolution is endothermic, dissolution can occur if the entropy change is high enough (ΔG negative at higher T).
Key Terms & Definitions
- Enthalpy of atomisation — energy needed to produce gaseous atoms from the standard state.
- Bond dissociation enthalpy — energy to break a covalent bond into atoms or radicals.
- Ionisation enthalpy — energy to remove electrons from gaseous atoms/ions.
- Electron affinity — energy change when an atom/ion gains an electron.
- Lattice enthalpy — energy change when an ionic lattice forms from or dissociates into gaseous ions.
- Enthalpy of hydration — energy released when gaseous ions dissolve in water.
- Enthalpy of solution — energy change when an ionic solid dissolves in water.
- Entropy (S) — measure of disorder; higher values mean more possible arrangements.
- Gibbs free energy (ΔG) — criterion for spontaneity: ΔG = ΔH - TΔS.
Action Items / Next Steps
- Review the definitions and trends for all key thermodynamic terms.
- Practice constructing Born-Haber cycles for different ionic compounds.
- Complete any provided example problems on ΔS, ΔG, and solubility.
- Study the relationship between lattice energy, hydration, and solubility predictions.