1. Predicting solubility from the enthalpy and entropy of solution
A2 Unit 4.4 How far? Entropy
p48-49

2. Dissolving and enthalpy changes
Warning: Hess’s law is back, please remain calm!

3. Dissolving and enthalpy changes
There are three key enthalpy changes involved when a substance dissolves:
Enthalpy change of solution ∆Hsol
Lattice enthalpy ∆Hlatt
Enthalpy change of hydration ∆Hhyd

4. Lattice energy Explain it:
Standard lattice enthalpy (∆Hθlatt)is the energy change when 1 mole of a solid ionic compound is formed from gaseous ions, under standard conditions
How is lattice dissociation enthalpy related? What would be the difference between the two?
Explain it:
when the ionic radii increases (cation and anion), the lattice energy decreases
the bigger the charge on the ion, the higher the lattice enthalpy

5. Enthalpy of hydration Explain it:
The energy released when 1 mole of gaseous ions dissolve in water under standard condition to produce a 1.0 mol dm-3 solution.
Mg2+(g) → Mg2+(aq)
Always -ve
Explain it:
The smaller and more highly charged the ion, the more negative the hydration enthalpy
What interactions take place between the ions and water molecule?

6. Enthalpy of solution
The enthalpy change when 1 mole of a solute is dissolved in a sufficient volume of solvent that no further enthalpy change occurs on dilution.
NaCl(s) → Na+(aq) + Cl-(aq)
This can’t be measured directly – you need an ENTHALPY CYCLE
Difference between the energy needed to separate the ions from the lattice and the energy given out when the ions are hydrated

9. Practical 4.5
Draw a Hess cycle linking enthalpy of solution, the hydration enthalpies and the lattice enthalpy for calcium chloride.

10. Questions State two assumptions made during this experiment
State the main source of error during the experiment
Explain why hydration enthalpies are negative values
State and explain the relationship between enthalpy of solution values and the solubility of a salt
Apart from lattice and hydration enthalpies what other factor is important in determining the solubility of a salt in water?

11. Apply it
Pg 49 Q1 & Q2

12. Predicting solubility
We can use ∆G = ∆H - TΔS to predict whether an ionic compound will dissolve under standard conditions
e.g. does silver bromide dissolve in water at 298K? ΔSsys = J mol-1 K-1 , and ∆H = kJ mol-1
and does sodium bromide dissolve in water at 298K?
ΔSsys = J mol-1 K-1 , and ∆H = -0.6 kJ mol-1

13. Learning objectives
define the term enthalpy of hydration of an ion and use it and lattice energy to calculate the enthalpy of solution of an ionic compound
understand the factors that affect the values of enthalpy of hydration and the lattice energy of an ionic compound
use entropy and enthalpy of solution values to predict the solubility of ionic compounds

16. Jan 2010 Q25