Enthalpy of solution

Whenever an ionic compound is dissolved in a solvent there are two steps that need to take place.

These are:

  1. Separate particles of the solute from each other
  2. Combine separated solute and solvent particles to make the solution

The enthalpy change for this reaction can be worked out as:

\Delta H_{solution}=\Delta H_{lattice\, dissociation\, enthalpy}+\Delta H_{hydration}

The lattice dissociation enthalpy is the breaking down of a solid into its gaseous ions, and this is always positive.

NaCl_{(s)}\rightarrow Na^+_{(g)}+Cl^-_{(g)}

When looking at trends down a group the lattice energy would decrease as the radius of the ion would increase. This means that the lattice dissociation enthalpy would become less endothermic.

The hydration enthalpy is the energy released when water surrounds the gaseous ions. The interaction between the water solvent molecule and the ion is bigger the smaller the ion is and therefore it becomes less exothermic as the ions get bigger.

Apart from the enthalpy of solution the solubility of solution is also dependant on the entropy change of the solution.

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