ELECTROCHEMISTRY - Wits Structural Chemistry

Measuring the conductivity of tap water:
κ
=
c
Λ
Λ
m = molar conductivity (S cm 2 mol -1 )
m c = molarity
If κ ∝ c, the molar conductivity should be
independent of the concentration of an
electrolyte. Is this the case
Parallel
plates
293.3 µS / cm
1 S = 1 Siemen
= 1 Ω -1
NO!
Molar conductivity varies with
concentration because:
- no. of ions in solution may not be
proportional to concentration (especially
for weak electrolytes)
- ions interact strongly with one another
reducing effective charge
NB: Don’t confuse conductivity (κ) with conductance (G)
Strong electrolyte – Λ m
decreases slightly as
conc increases.
Weak electrolyte –
normal Λ m at low conc,
but Λ m decreases sharply
as conc increases.
1
G =
R
Kohlrausch’s law:
At low concentrations, molar conductivities vary with the square root of conc.
Λ = Λ − K c
m
Λ o m
= ν
o
m
+
λ
+
+ ν
−λ
−
STRONG ELECTROLYTES
Λ m o = limiting molar conductivity
(ions are infinitely far apart no interaction)
K → related to stoichiometry of electrolyte
Kohlrausch’s law of independent migration of ions:
Λ m o → expressed as sum of contributions from its individual ions.
λ = limiting molar conductivity of ions
ν = no. of ions per formula unit of electrolyte
H +
34.96 OH -
19.91
Na + 5.01 Cl -
7.63
K + 7.35 Br -
7.81
Zn 2+ 10.56 SO 2-
4 16.00
e.g. for MgCl 2 : ν + = 1 ν − = 2
Limiting ionic conductivies in water at 298 K, λ /mS m 2 mol -1
Weak electrolytes do NOT dissociate completely,
conductivity of weak electrolytes depends on the degree of ionization (α)
e.g. HA + H 2 O A - + H 3 O +
+ −
[H3O
][A ]
+
K
[ H3O
] = αc
a =
[HA]
−
[ A ] = αc
2
α c
Ka
=
[ HA] = ( 1−
α)c
1 − α
At infinite dilution the electrolyte is fully
ionised :
Λ o m
= ν
+
λ
+
+ ν
−λ
−
At very low conc’s:
Ostwald’s
dilution
law
WEAK ELECTROLYTES
Λ = α
o
m
Λ m
1 1 Λ
mc
= +
o
Λ
m Λ
m K
( Λ ) o 2
1 1 1
= + Λ
mc
o
o 2
Λ
m Λ
m K
a
( Λ
m
)
y c m x
a
m
(c = formal conc of HA)
Example 6
Calculate the degree of ionization and the acid dissociation constant at 298 K for a
0.010 M acetic acid solution that has a resistance of 2220 Ω. The resistance of a 0.100
M potassium chloride solution was also found to be 28.44 Ω.
Λ m (0.1 M KCl) = 129 S cm 2 mol -1
λ o (H + ) = 349.6 S cm 2 mol -1
λ o (CH 3 COO - ) = 40.9 S cm 2 mol -1
Example 7
The molar conductivity of a strong electrolyte in water at 25 °C was found to be 109.9
S cm 2 mol -1 for a concentration of 6.2 × 10 -3 mol L -1 and 106.1 S cm 2 mol -1 for a
concentration of 1.5 × 10 -3 mol L -1 .
Estimate the limiting molar conductivity of the electrolyte.
5