Kp
The equilibrium constant in terms of partial pressures
Kp
Mole fraction
Partial pressures
Partial pressure, pThe contribution of a gas towards the total pressurePartial 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
Sum of partial pressures = Total pressurep(N2) + p(O2) = 128 + 192 = 320 kPa
x(N2) = = 0.4 x(O2) = = 0.6
p(O2) = x(O2)P = 0.6 x 320 = 192 kPa
p(N2) = x(N2)P = 0.4 x 320 = 128 kPa
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What is Kp
Similar to Kc but partial pressures used in place of concentration
Equilibrium: 2SO2(g) + O2(g) 2SO⇌ 3(g)
Units:
Kp = 22 )(SOp
23 )(SOp
)( 2Op
2)(kPa
)()( 2 kPakPa
1kPaKp =
Calculating Kp
Equilibrium: 2SO2(g) + O2(g) 2SO⇌ 3(g)
Partial pressures: SO2(g), 74 kPa; O2(g), 23 kPa; SO3(g), 142 kPa
Kp = 22 )(SOp
23 )(SOp
)( 2Op
Kp =2142
274 23x
x
= 0.160 kPa–1
Heterogeneous equilibria
Equilibrium contains different phases
Equilibrium: CaCO3(s) CaO(s) + CO⇌ 2(g)
Kp expression contains only gaseous species
Kp = p(CO2)
Solid species are omitted (solids have no gas pressure)
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