Equilibrium Constants: Kp

For gaseous equilibria, it is often more convenient to work with partial pressures rather than concentrations. The equilibrium constant expressed in terms of partial pressures is called Kp. At A-level, Kp is used exclusively for reactions where all species are gases.

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Partial Pressures

In a mixture of gases, each gas contributes to the total pressure. The partial pressure of a gas is the pressure it would exert if it alone occupied the entire container at the same temperature.

Dalton's Law: The total pressure equals the sum of all partial pressures:

P_total = p_A + p_B + p_C + ...

The partial pressure of a gas is related to its mole fraction (x):

p_A = x_A × P_total

Where the mole fraction of A is:

x_A = moles of A / total moles of gas

The mole fractions of all gases in the mixture must sum to 1.

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Worked Example: Calculating Partial Pressures

A mixture at equilibrium contains 2.0 mol N₂, 6.0 mol H₂, and 4.0 mol NH₃ at a total pressure of 200 atm.

Total moles = 2.0 + 6.0 + 4.0 = 12.0 mol