The Equilibrium Constant Kp (OCR A level chemistry)

Sum of mole fractions in a gas mixture

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What is Kp used for?

Equilibria involving gases are usually expressed in terms of Kp, which is the equilibrium constant in terms of partial pressures. (For gases, it is easier to measure pressure than concentration).

Kp for the equilibrium H2(g) + I2(g) <=> 2HI (g)

Kp= p(HI)^2/p(H2) x p(I2)

Kp states

Only includes gases because only gases have partial pressures. Any other species ignored.

Partial pressure, p

The contribution that the gas makes towards the total pressure P. All partial pressures add together to make the total pressure.

Relationship between concentration and pressure

They are proportional to one another and Kp has a direct relationship to Kc.

Mole fraction

Under the same conditions of temperature and pressure, the same volume of different gases contains the same number of moles and gas molecules. The mole fraction of a gas is the same as its proportion by volume to the total volume of gases in a gas mixture.

Writing Kp

Written in a similar way to Kc, but with partial pressures replacing concentration.

Suitable units for partial pressures

kPa, Pa, atm. The same unit must be used for all gases.

Calculate partial pressure p(A)

mole fraction of A x total pressure P (P^A= X^A x P)

Calculate mole fraction x(A)

number of moles of A/total number of moles in gas mixture