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

Which will give more negative  H LE ? 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,  H hyd  H hyd is the enthalpy change when a solution of ions is made from 1 mole of gaseous ions Is hydration exothermic or endothermic? Exothermic – because bonds are made

Example of hydration Na + (g) + aq Na + (aq)  H hyd = kJ mol -1

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

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

Water is not the only solvent Enthalpy of solvation,  H solv, 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  H solution =  H hyd (cation) +  H hyd (anion) -  H LE If  H solution 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 [ -  H LE ] Energy is released by hydration of the ions If more energy is released than used up, then  H solution will be negative and the solute is more likely to dissolve. If more energy is released than used up, then  H solution will be negative and the solute is more likely to dissolve. [  H hyd (cation) +  H hyd (anion) ]

Enthalpy cycle for solution Ionic lattice + solvent solution  H solution Gaseous ions + solvent -  H LE  H hyd (cation) +  H hyd (anion) +  H hyd (anion)  H solution =  H hyd (cation) +  H hyd (anion) -  H LE

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

enthalpyenthalpy Solute + solvent Gaseous ions Solution ΔH solution -ΔH LE ΔH hyd(cat) ΔH hyd(an) Is ΔH solution Endothermic or exothermic ? -ΔH LE + ΔH hyd(cat) + ΔH hyd(an) ΔH solution =

enthalpyenthalpy Solute + solvent Gaseous ions Solution ΔH solution -ΔH LE ΔH hyd(cat) ΔH hyd(an) -ΔH LE + ΔH hyd(cat) + ΔH hyd(an) ΔH solution = Is ΔH solution exothermic or endothermic?

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

Entropy An increase in entropy favours dissolving – even if a little energy is needed. Substances with a small positive Substances with a small positive ΔH solution can still dissolve if there is a favourable entropy change. CaCO3 has a small negative ΔH solution but a large entropy decrease so it is insoluble. Why is there a decrease in entropy?

enthalpyenthalpy Solute + solvent Gaseous ions Solution ΔH solution -ΔH LE ΔH solv(cat) ΔH solv(an) -ΔH LE + ΔH Solv(cat) + ΔH solv(an) ΔH solution = Is ΔH solution exothermic or endothermic?

Over to you! Time to try the problems on page 82: 1256 ?