π Understanding Enthalpy of Solution
Enthalpy of solution ($ \Delta H_{solution} $) refers to the change in enthalpy when one mole of a substance dissolves completely in a large excess of solvent. This process involves breaking solute-solute interactions and solvent-solvent interactions, and then forming solute-solvent interactions.
- π§ Breaking Solute-Solute Interactions: Requires energy (endothermic, $ \Delta H > 0 $). This is often the lattice energy for ionic compounds.
- π§ Breaking Solvent-Solvent Interactions: Also requires energy (endothermic, $ \Delta H > 0 $). Think of this as making space for the solute.
- π€ Forming Solute-Solvent Interactions: Releases energy (exothermic, $ \Delta H < 0 $). These are the attractive forces between solute and solvent particles.
π§ͺ Understanding Enthalpy of Hydration
Enthalpy of hydration ($ \Delta H_{hydration} $) is a specific case of solvation where the solvent is water. It represents the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution.
- π¨ Gaseous Ions: Enthalpy of hydration always starts with gaseous ions.
- π§ Interaction with Water: These ions then interact strongly with water molecules, releasing a significant amount of energy (exothermic, $ \Delta H < 0 $).
- β‘ Ion-Dipole Interactions: The negative oxygen end of water molecules surrounds positive ions, and the positive hydrogen end surrounds negative ions.
π Enthalpy of Solution vs. Enthalpy of Hydration: Key Differences
| Feature |
Enthalpy of Solution ($ \Delta H_{solution} $) |
Enthalpy of Hydration ($ \Delta H_{hydration} $) |
| Definition |
Enthalpy change when one mole of a substance dissolves in a solvent. |
Enthalpy change when one mole of gaseous ions dissolves in water. |
| Initial State |
Solid, liquid, or gas dissolving in a solvent. |
Gaseous ions dissolving in water. |
| Solvent |
Any solvent. |
Specifically water. |
| Process Steps |
Breaking solute-solute & solvent-solvent interactions, forming solute-solvent interactions. |
Interaction of gaseous ions with water molecules. |
| Formula |
$ \Delta H_{solution} = \Delta H_{lattice} + \Delta H_{hydration} $ (for ionic compounds) |
Direct hydration of gaseous ions. |
π Key Takeaways
- π― Scope: Enthalpy of hydration is a specific type of enthalpy of solution where the solute is an ionic compound and the solvent is water.
- π‘οΈ Ionic Compounds: For dissolving ionic compounds, enthalpy of solution includes the lattice energy (energy to break apart the crystal lattice) and the enthalpy of hydration.
- β Relationship: $ \Delta H_{solution} $ can be thought of as the sum of the lattice energy (endothermic) and the enthalpy of hydration (exothermic) for ionic compounds: $ \Delta H_{solution} = \Delta H_{lattice} + \Delta H_{hydration} $.