250387 kp presentation

Kp


Kp
The equilibrium constant in terms of
partial pressures


Mole fraction


Partial pressures
Partial pressure, p
The contribution of a gas towards the total pressure
Partial pressure = mole fraction x Total pressure
A gas mixture with a total pressure of 320 kPa contains 2 mol of N2(g) and 3 mol of O2(g).
Mole fractions
Partial pressures

x(N2) =

2
5

= 0.4

p(N ) = x(N2)P
p(N ) + p(O ) = 128 + 192 = 320 kPa
= 0.4 x 320
= 128 kPa

Sum of partial pressures = Total pressure
2
2
2

x(O2) =

3
5

= 0.6

p(O2) = x(O2)P
= 0.6 x 320
= 192 kPa


What is Kp
Similar to Kc but partial pressures used in place of concentration
Equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g)

Kp =

Units:

p(SO3 )2
p(SO2 )2 p(O2 )

Kp =

(kPa )2
( kPa )2 ( kPa )

= kPa

−1


Calculating Kp
Equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g)
Partial pressures:
SO2(g), 74 kPa; O2(g), 23 kPa;

Kp =

SO3(g), 142 kPa

p(SO3 )2
p(SO2 )2 x p(O2 )
2

142
Kp =
74 2 x 23

= 0.160 kPa–1


Heterogeneous equilibria
Equilibrium contains different phases

Equilibrium: CaCO3(s) ⇌ CaO(s) + CO2(g)
Kp expression contains only gaseous species
Kp = p(CO2)
Solid species are omitted
(solids have no gas pressure)