Summary
The enthalpy change of hydration (ΔH_hyd) is the energy change when gaseous ions dissolve in water to form hydrated ions, usually releasing energy. The enthalpy change of solution (ΔH_sol) is the energy change when a solute dissolves in a solvent, depending on the balance of energy required to break and form intermolecular forces.
- Enthalpy Change of Hydration (ΔH_hyd) — energy change when gaseous ions dissolve in water. Example: Na+ and Cl- ions in NaCl become surrounded by water molecules, releasing energy.
- Enthalpy Change of Solution (ΔH_sol) — energy change when a solute dissolves in a solvent. Example: NaOH dissolving in water releases energy, making ΔH_sol negative.
- Energy Cycle — a diagram showing enthalpy changes in a dissolution process. Example: Involves lattice energy, ΔH_sol, and ΔH_hyd to calculate unknowns using Hess's law.
Exam Tips
Key Definitions to Remember
- Enthalpy Change of Hydration (ΔH_hyd)
- Enthalpy Change of Solution (ΔH_sol)
- Energy Cycle
Common Confusions
- Confusing exothermic and endothermic processes in ΔH_sol
- Misunderstanding the role of ionic charge and radius in ΔH_hyd
Typical Exam Questions
- What is the enthalpy change of hydration? The energy change when gaseous ions dissolve in water to form hydrated ions.
- How does ionic charge affect ΔH_hyd? Higher ionic charge increases the magnitude of ΔH_hyd, making it more negative.
- How do you use an energy cycle to calculate unknown enthalpy changes? Apply Hess's law and algebraic manipulation to relate known and unknown values.
What Examiners Usually Test
- Understanding of energy cycles and Hess's law
- Ability to calculate enthalpy changes using given data
- Effects of ionic charge and radius on enthalpy changes