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
-HLE is a positive
This is always an exothermic
process
value
HLE is always negative
The energy to break up a lattice = - HLE
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
Is hydration
exothermic or
endothermic?
Exothermic –
because bonds
are made
Example of hydration
Na+ (g)
Hhyd
+ aq
=
- 406 kJ mol-1
Na+ (aq)
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
[Hhyd (cation) + Hhyd (anion)]
If more energy is released than used up,
then Hsolution will be negative and the
solute is more likely to dissolve.
Enthalpy cycle for solution
Ionic lattice + solvent
Hsolution
- HLE
solution
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
e
Is ΔH
nEndothermic or
t exothermic ? -ΔHLE
h
Solute + solvent
a
l
ΔH
p
y
Gaseous ions
solution
ΔHhyd(cat)
ΔHhyd(an)
solution
ΔHsolution =
-ΔHLE
Solution
+ ΔHhyd(cat)
+ ΔHhyd(an)
e
n
t
h
a
l
p
y
Gaseous ions
ΔHhyd(cat)
-ΔHLE
ΔHhyd(an)
Solution
ΔHsolution
Solute + solvent
ΔHsolution =
-ΔHLE
+ ΔHhyd(cat)
Is ΔHsolution
exothermic or
endothermic?
+ ΔHhyd(an)
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.