1. Energy changes in solutions
CI 4.5
Energy changes in solutions

2. 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

3. 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

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

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

6. 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?

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

8. 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-)

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

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

11. 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

12. 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)]

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

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

15. 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 =

16. 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 =

17. 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.

18. Over to you! ?
Time to try the problems.