Kp: Use Kp with gases. Same as Kc but equilibrium constant in terms of partial pressure instead

of concentration. Products/ reactants. Balancing numbers is the power.

If asked to write Kp expression, no square brackets and remember p… Only include gases, omit

other states.

Units for partial pressure are kPa, Pa (pascals) or atm (atmospheres). Have to be same units for

all in calculation.

Moles of Gas: Under the same temperature and pressure, same volume of different gases, are

same moles.

Mole fraction x(A) = number of moles A/ total number of moles in gas mixture

Partial Pressure: In a gas mixture, the partial pressure of a gas is the contribution the gas makes

towards total pressure.

Partial pressure p(A) = moles fraction A x total pressure

Remember sum of partial pressure equals total pressure.

An Example: An equilibrium mixture at 400C contains 18 mol of N2, 54 mol of H2 and 48 mol of

NH3. Total equilibrium pressure is 200 atm. Find Kp.

Mole fractions- x(N2) = 18/ 120.

Partial Pressure- p(N2) = 18/ 120 x 200 = 80 -> use this value in Kp.

1 mol of carbon dioxide, 3 mole of hydrogen. At equilibrium, 0.86 mol of methanol produced at

pressure of kPa.

– Draw table like do with Kc and use molar ratio to find equilibrium moles of reactants.

– Find total equilibrium moles. Find mole fraction for each reactant and product.

– Multiply by total pressure- 500 kPa to find partial pressure for each reactant and product. Do

mole fraction x total pressure in one calculation – quicker.

– Sub this value into kPa.