Energy changes in solutions CI 4.5 Energy changes in solutions

Why do some ionic substances dissolve in water, whilst others are insoluble? If there is enough energy to separate the ions in the lattice, the substance will be soluble Ions in solid lattice Ions in solution

Lattice enthalpy, HLE The enthalpy change when 1 mole of solid is formed from the separate ions This is always an exothermic process HLE is always negative The energy to break up a lattice = - HLE -HLE is a positive value

Which will give more negative HLE? Small ionic charge Large ionic charge Small ionic radius Large ionic radius    

Where does the energy come from to break up the lattice? Hydration 3+ 3-

Enthalpy of hydration,Hhyd Hhyd is the enthalpy change when a solution of ions is made from 1 mole of gaseous ions Exothermic – because bonds are made Is hydration exothermic or endothermic?

Example of hydration Na+ (g) + aq Na+ (aq) Hhyd = - 406 kJ mol-1

Different ions An ionic compound contains cations and anions Total Hhyd = Hhyd (cation) + Hhyd (anion) When NaCl is dissolved: Total Hhyd = Hhyd (Na+) + Hhyd (Cl-)

Which will give more negative Hhyd? Small ionic charge Large ionic charge Small ionic radius Large ionic radius    

Water is not the only solvent Enthalpy of solvation, Hsolv , is used for other solvents

Enthalpy change of solution This is the enthalpy change when 1 mole of a solute dissolves to form an infinitely dilute solution Hsolution = Hhyd (cation) + Hhyd (anion) - HLE If Hsolution is negative, it is more likely that the substance will dissolve

What decides solubility? Energy is put in to separate the ions in the lattice [ - HLE] Energy is released by hydration of the ions If more energy is released than used up, then Hsolution will be negative and the solute is more likely to dissolve. [Hhyd (cation) + Hhyd (anion)]

Enthalpy cycle for solution Hsolution Ionic lattice + solvent solution - HLE Hhyd (cation) + Hhyd (anion) Gaseous ions + solvent Hsolution = Hhyd (cation) + Hhyd (anion) - HLE

Enthalpy level diagrams Make it easier to compare the sizes of the enthalpy changes

enthalpy -ΔHLE ΔHhyd(cat) ΔHhyd(an) -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) Is ΔHsolution Endothermic or exothermic ? enthalpy Gaseous ions -ΔHLE ΔHhyd(cat) ΔHhyd(an) Solute + solvent ΔHsolution Solution -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) ΔHsolution =

enthalpy ΔHhyd(cat) -ΔHLE ΔHhyd(an) -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) Gaseous ions ΔHhyd(cat) -ΔHLE ΔHhyd(an) Is ΔHsolution exothermic or endothermic? Solution ΔHsolution Solute + solvent -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) ΔHsolution =

Solubility The more negative the value for ΔHsolution the more likely the solute is to dissolve. If ΔHsolution is very large and positive, the solute will not dissolve. If ΔHsolution is small and positive, the solute may dissolve, if there is sufficient increase in entropy.

Over to you! ? Time to try the problems.