THE EQUILIBRIUM CONSTANT Kc
For an equilibrium reaction of the form...
aA
+
bB
↔
cC
+
dD
then (at constant temperature)
Kc = [C]c x [D]d
[A]a x [B]b
where [ ] denotes the equilibrium concentration in mol dm-3
Kc
is known as the Equilibrium Constant
Kc is affected by a change of temperature
Kc is not affected by
–
–
–
a change in concentration of reactants or products
a change of pressure
adding a catalyst
Kc
In equilibrium calculations we use the figures from the given equation
as our indices.
For the reaction
eg. H 2 (g) + I 2(g) ⇔
Kc =
[HI]2
[H2] [I2]
2HI (g)
= 49
For the reaction
2HI (g) ⇔ H 2 (g) + I 2(g)
Kc =
[H2] [I 2]
[HI]2
=
1
49
Units of Kc
The units of Kc depend on the number of particles on both sides of
the equation.
Eg.
H 2 (g) + I 2 (g)
⇔
Kc = [HI]2
=
mol2 dm-6
[H2] [I2]
mol dm-3 x mol dm -3
Eg.
N 2 (g) + 3H 2 (g)
Kc = [NH3]2 =
[N2] [H2]3
⇔
2HI (g)
= no units
2NH 3 (g)
mol2 dm -6
=
mol dm -3 x mol 3 dm -9
mol -2 dm 6