AQA Chemistry A-level - Born-Haber Cycles

AQA Chemistry A-level - Thermodynamics

3.1.8.1 Born-Haber Cycles

• Born-Haber cycles are similar to Hess's Law cycles, allowing calculation of enthalpy changes that can't be measured directly.
• Lattice Enthalpies (ΔH): Used for ionic substances, these represent the energy required to break an ionic lattice into gaseous ions.
• Lattice dissociation enthalpy: The energy needed to separate an ionic lattice into its gaseous ions.
• Lattice formation enthalpy: The energy required to form an ionic lattice from its gaseous ions.
• Atomisation Enthalpy (ΔatH): The energy needed to form one mole of gaseous atoms from the element in its standard state.
• Enthalpy of Electron Affinity (ΔH): The enthalpy change when one mole of electrons is added to a mole of gaseous atoms.

Born-Haber Cycle Example

• The example shows calculating enthalpy of formation (ΔHf°) of NaCl(s), calculating the enthalpy change by following the arrows until the desired state.
• If the cycle reverses the direction of the arrow, reverse the sign of the enthalpy change.

Enthalpies of Solution and Hydration

• Enthalpies of Solution (ΔsolH): The enthalpy change when one mole of an ionic solid dissolves in water to infinite dilution.
• Enthalpies of Hydration (ΔhydH): The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions.

Perfect Ionic Model

• The perfect ionic model assumes ions are perfectly spherical and show no covalent character.
• Hydration enthalpy is affected by the size and charge of the ions. Covalent character occurs if the ions have varying sizes or charges leading to unequal distributions of charges.

3.1.8.2 Gibbs Free-Energy and Entropy

• Entropy (ΔS): A measure of disorder; increasing temperature leads to more disorder.

• Entropy increases from solids (ordered) to liquids (less ordered) to gases (disordered).

• The entropy of vaporisation exceeds the entropy of fusion because gaseous state is more disordered than liquid.

• Gibbs Free Energy (ΔG): ΔG = ΔH - TΔS (at constant temperature and pressure), this measures if a reaction or process will occur spontaneously.

• Negative ΔG indicates spontaneous reactions.

• A graph of ΔG against temperature will be a straight line with a negative gradient for spontaneous reactions.

• If ΔH and ΔS are both positive or negative, a reaction feasibility depends on the temperature.

This quiz focuses on the Born-Haber cycles within the AQA Chemistry A-level curriculum. It covers key concepts such as lattice enthalpies, atomisation enthalpy, and the enthalpy of electron affinity, providing important insights into the enthalpy changes in ionic compounds. Test your understanding by applying these concepts to example problems.