Lattice Enthalpy
- Standard lattice enthalpy of formation, ΔLEH⦵ is the enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions.
- Lattice enthalpy gives an indication of the strength of the ionic bonds
- Greater ionic charge and smaller ionic radius result in stronger bonding. More energy is stored in stronger bonds therefore lattice enthalpy values are more exothermic
- Lattice enthalpy cannot be measured directly, so other measurements need to be known
- Standard Enthalpy of Formation, ΔfH⦵ is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states, under standard conditions
- Standard Enthalpy of Atomisation, ΔaH⦵ is the enthalpy change when one mole of gaseous atoms are formed from an element in its standard state.
- First ionisation energy, ΔI 1H⦵ is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
- First electron affinity, ΔEA 1H⦵ is the enthalpy change when one mole of gaseous 1- ions are formed from one mole of gaseous atoms
- Second electron affinity, ΔEA 2H⦵ is the enthalpy change when one mole of gaseous ions are formed from one mole of gaseous 1- ions
Born-Haber Cycles
- Lattice enthalpy cannot be measured directly, so Hess’s law is used (the total enthalpy change is independent of the route taken).
- A Born-Haber cycle is used to calculate lattice enthalpy e.g.
- Calculations involving group 2 elements require extra steps compared to sodium chloride:
- There are two moles of chlorine ions in each mole of MgCl2, so the atomisation enthalpy of chlorine needs to be doubled
- Group 2 elements form 2+ ions, so the second ionisation energy of sodium must also be included
- The first electron affinity of chlorine also needs to be doubled as two moles of Cl- ions are being formed
- The Born-Haber cycle can also be used to calculate one of the other enthalpy changes in the same way e.g.
Dissolving Ionic Compounds
- Dissolving an ionic compound has two steps
- The bonds between the ions in the lattice break (endothermic)
- Bonds between the ions and water is made (hydration- is exothermic)
- The bonds between the ions in the lattice break (endothermic)
- Standard Enthalpy of Solution, ΔsolH⦵ is the enthalpy change when one mole of solute dissolves completely in sufficient solvent under standard conditions to form a solution in which the molecules or ions are far enough apart not to interact with each other
- The hydration of ions requires the interactions between the solvent and the solute to be of similar strength to the interactions between the positively and negatively charged ions in the lattice for the ions to dissolve
- Ions dissolve well in polar solvents, like water, because of the favourable electrostatic interactions between the oppositely charged ions in the solvent and the ions
- Standard enthalpy of hydration, ΔhydH is the enthalpy change when one mole of aqueous ions is formed from gaseous ions under standard conditions.
- The enthalpy of solution and the enthalpy of hydration provides another path to calculate the lattice enthalpy of dissociation / formation
