Solubility
Solubility – the maximum amount of a solute that
can be dissolved in a given quantity of solvent at a
specific temperature
E.g., 36.0 g of NaCl dissolves in 100 ml H2O at
293 K
Saturated Solution - a dynamic equilibrium exists
NaCl(s)
Na+(aq) + Cl-(aq)
Unsaturated solution - contains less solute than it
has the capacity to dissolve
E.g., 30.0 g of NaCl in 100 ml H2O at 293 K
Supersaturated solution - contains more solute than
is present in a saturated solution
E.g., Sodium thiosulphate (Na2S2O3) in H2O
231.0 g in 100 ml at 373 K
50 g in 100 ml at 293 K
Crystallisation is expected on cooling but if
allowed to cool very slowly then 231 g is
maintained in solution at 293 K - not in
equilibrium
Temperature and Solubility
• Solubility of ionic compounds usually increases
with temperature
ΔHsoln usually positive
solid + solvent + heat
solution
• Exceptions are sulphates and hydroxides of
metals carrying 2+ or 3+ charges (e.g., Ca(OH)2
and Na2SO4)
ΔHsoln is negative and solubility decreases with T
• Variation in solid solubility is the basis of
fractional crystallisation – allows the separation
of mixture into pure components
• Solubility of gases in water always decreases
with increasing T. ΔHsoln is always negative (no
solute-solute forces)
gas + solvent
solution + heat
Thermal Pollution - threat to aquatic life caused by
reduced solubility of O2 in water in warm waters.
Pressure and Solubility
Pressure has little effect on solubility of liquids or
solids but has a great effect on solubility of gases
Henry’s Law:
Solubility of a gas in a liquid is proportional to the
pressure of the gas above the solution
S = kHP
S = solubility (m/V)
P = pressure
kH = Henry’s Law constant
kH depends on temperature.
At constant temperature:
S1 S 2
=
P1 P2
The following equilibrium exists:
gas + solvent
solution + heat
Gas solubility in liquids is biologically important
E.g., O2 in blood used for metabolism
Water reacts with some acids to aid solution
CO2 (g)
CO2 (aq)
CO2 (aq) +H2O
H2CO3 (aq)
Colligative Properties
Colligative properties – properties of solutions that
depend on solute concentration and not on its
chemical identity
Vapour Pressure Lowering:
Liquids with a measurable vapour pressure are
volatile
E.g., V.P. of water at 293 K = 17.54 mmHg
BUT:
V.P. of an aqueous solution containing 0.01 mole
fraction of ethylene glycol (CH2OHCH2OH) =
17.36 mmHg
So, the vapour pressure of a volatile solvent can be
lowered by addition of a non-volatile solute
For 0.02 mole fraction solution, V.P.= 17.18 mmHg
Vapour pressure lowering depends on the
concentration of solute – it is a colligative property
Raoult’s Law
For a solvent A:
°
PA = xA P A
PA = partial pressure of solvent over solution
P°A = vapour pressure of pure solvent
xA = mole fraction of solvent in solution
P
In solution, xA < 1 and V.P. is lowered by ΔP
ΔP = P°A – PA
P
Using Raoult’s Law
ΔP = P°A – PAxA = P°A (1 – xA)
P
P
For a two component system A and B:
xA + xB = 1
B
ΔP = PA0 x B
Thus, vapour pressure lowering (ΔP) depends on
solute concentration, not on chemical nature.
Raoult’s Law holds best for dilute solutions
E.g., An aqueous solution containing 0.01 mole
fraction of any non-volatile solute will lower
the vapour pressure at 293 K by 0.18 mmHg.
Ideal Solutions
Ideal solution – a solution in which the components
obey Raoult’s Law for all mole fractions.
(Only occurs for chemically similar substances with
virtually identical intermolecular forces, ΔHsoln ≈ 0)
E.g., Heptane (C7H16) and octane (C8H18)
E.g., Benzene (C6H6) and methylbenzene (C7H8)
Both components are volatile:
PB = P x B
0
B
PM = PM0 x M
From Dalton’s Law
P = PB + PM = PB0 x B + PM0 x M
At 293 K: PB = 75 mmHg and PM = 22 mmHg
For an equimolar benzene / methylbenzene mixture:
(xB = xM = 0.5)
P = 75×0.5 + 22×0.5 = 48.5 mmHg
B
B
For benzene: PB = 75×0.5 = 37.5 mmHg
xB (vapour) = 37.5 / 48.5 = 0.77
B
B
Vapour over a solution is richer in the more volatile
component.
Mole Fraction-Pressure diagrams:
The benzene (B) / methylbenzene (M) mixture:
0
PB
Pressure
P
PB
0
PM
Benzene is
more volatile
PM
0
0.5
xB
1.0
B
Composition of vapour can be determined
xB (vapour) = PB / P
B
B
Fractional Distillation:
Separation of liquid components of a solution based
on their different volatility (boiling points)
Non - Ideal Solutions
A solution in which the components do not obey
Raoult’s Law – occurs for substances whose
intermolecular forces are not similar, ΔHsoln ≠ 0
E.g.,
Ethanol (CH3CH2OH) and water
Pressure
0
0.5
xsolute
1.0
Ideal Solution (ΔHsoln = 0)
Positive Deviation (ΔHsoln = +ve)
Negative Deviation (ΔHsoln = -ve)