Enthalpy of Hydration (Oxford AQA International A Level Chemistry)

Enthalpy of Hydration Calculations

Enthalpy of solution

• The standard enthalpy change of solution (ΔH_sol^ꝋ) is the enthalpy change 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 solid is dissolved in sufficient water

  • For example, the enthalpy changes of solution for potassium chloride are described by the following equations:

KCl (s) + aq → KCl (aq)

OR

KCl (s) + aq → K⁺(aq) + Cl^-(aq)

• ΔH_sol^ꝋ can be exothermic (negative) or endothermic (positive)

Enthalpy of hydration

• The standard enthalpy change of hydration (ΔH_hyd^ꝋ) is the enthalpy change when 1 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 described by the following equation:

Mg²⁺(g) + aq → Mg²⁺(aq)

• Hydration enthalpies are the measure of the energy that is released when there is an attraction formed between the ions and water molecules

• Hydration enthalpies are exothermic

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

• Water is a polar molecule with a δ- oxygen (O) atom and δ+ hydrogen (H) atoms which will form ion-dipole attractions with the ions present in the solution

• The oxygen atom in water will be attracted to the positive ions and the hydrogen atoms will be attracted to the negative ions

Hydration of cations and anions

How are enthalpy of solution and hydration enthalpies related?

Enthalpy of solution = - lattice formation enthalpy + hydration enthalpy

OR

Enthalpy of solution = lattice dissociation 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

Calculations

• Questions in this topic typically ask you to calculate the hydration enthalpy of one of the ions, given the lattice enthalpy, enthalpy of solution and hydration enthalpy of the other ion

• This can be done by constructing an appropriate Hess's Law cycle to find the unknown energy value

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

• According to Hess’s law, the enthalpy change for both routes is the same, such that:

ΔH_sol^ꝋ = - ΔH_latt^ꝋ + ΔH_hyd^ꝋ

ΔH_hyd^ꝋ = ΔH_latt^ꝋ + ΔH_sol^ꝋ

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

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