Thermodynamics (A-level only) revision notes

Thermodynamics in Chemistry

Lattice Enthalpy

• Lattice enthalpy is the energy required to separate one mole of an ionic solid into its gaseous ions. It can be calculated using Born-Haber cycles, which relate various enthalpy changes involved in the formation of ionic compounds.
• To construct a Born-Haber cycle, identify the enthalpy changes: formation, ionization, electron affinity, and sublimation. Use Hess's law to relate these changes to calculate lattice enthalpy.

Comparing Lattice Enthalpies

• Experimental lattice enthalpies can be compared to theoretical values calculated using ionic radii and charge. Discrepancies may indicate covalent character in ionic compounds, as greater covalent character leads to stronger interactions than predicted by purely ionic models.

Entropy

• Entropy (S) is a measure of the disorder or dispersal of energy in a system. Higher entropy indicates greater disorder. The change in entropy (ΔS) can be calculated from standard entropy values.
• Entropy changes are crucial in predicting the spontaneity of reactions. A positive ΔS indicates an increase in disorder, which favors spontaneity.

Gibbs Free Energy

• The Gibbs free energy change (ΔG) is calculated using the equation: ΔG = ΔH - TΔS. A negative ΔG indicates a feasible reaction under constant temperature and pressure.
• However, a negative ΔG does not guarantee that a reaction will occur at a measurable rate, as kinetic factors also play a significant role.