Enthalpy of Solution & Hydration (AQA A Level Chemistry): Revision Note

Solution & Hydration

Enthalpy of solution

• The standard enthalpy change of solution () is the enthalpy change that occurs when one mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

• The symbol (aq) is used to show that the substance is dissolved in a large excess of water

• For example, the enthalpy change of solution for potassium chloride can be represented by either of the following equations:

KCl(s) + aq → KCl(aq)

OR

KCl(s) + aq → K⁺(aq) + Cl^-(aq)

• The standard enthalpy change of solution can be exothermic (negative) or endothermic (positive)

Enthalpy of hydration

• The standard enthalpy change of hydration () is the enthalpy change when one mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution

  • For example, the enthalpy change of hydration for magnesium ions is represented by the equation:

Mg²⁺(g) + aq → Mg²⁺(aq)

• Hydration enthalpies measure the energy released when attractions form between ions and water molecules

  • These enthalpy changes are always exothermic

• When an ionic solid dissolves in water, positive and negative ions are formed

• Water is a polar molecule, with a δ⁻ oxygen atom and δ⁺ hydrogen atoms, which form ion–dipole attractions with the ions in solution.

• The oxygen atoms are attracted to positive ions, while the hydrogen atoms are attracted to negative ions

How are the enthalpies of solution and hydration related?

• From the diagram, we can see that the relationship is

Enthalpy of solution = reverse lattice enthalpy* + hydration enthalpy

• The hydration enthalpy is the sum of the hydration enthalpies of each ion

• If there is more than one cation or anion, such as in MgCl₂, then you must multiply by the appropriate coefficient for that ion

Calculating Enthalpy Changes

• Questions on this topic often require you to calculate the hydration enthalpy of one ion, given the lattice enthalpy, the enthalpy of solution, and the hydration enthalpy of the other ion

• This can be done by constructing an appropriate energy cycle and applying Hess’s Law to determine the unknown enthalpy change

• The energy cycle above shows that there are two possible routes from gaseous ions to ions in aqueous solution

  • Route 1 (indirect route): gaseous ions → ionic solid → ions in aqueous solution

  • Route 2 (direct route): gaseous ions → ions in aqueous solution

• According to Hess’s Law, the total enthalpy change for both routes is the same, so:

= +

• Each ion will have its own enthalpy change of hydration, , which will need to be taken into account during calculations

  • The total is found by adding the  values of both anions and cations together