p means the partial pressure of that gas Kp = equilibrium constant Only include gases in the Kp expression. Ignore solids, liquids, and aqueous substances. Working out the unit of Kp Put the unit of pressure(kPa) into the Kp equation kPa2 kPa kPa3 Unit = 1 kPa2 Cancel out units Unit = Unit = kPa-2 However, if the equation is written the other way round, the value of Kp will be the inverse of above and the units will be kPa2 . It is important therefore to write an equation when quoting values of Kp.1 mole of N2 and 3 moles of H2 are added together and the mixture is allowed to reach equilibrium. At equilibrium 20% of the N2 has reacted. If the total pressure is 2kPa what is the value of Kp? N2 (g) + 3H2 (g ) 2 NH3 (g) Example For the following equilibrium N2 H2 NH3 Initial moles 1.0 3.0 0 Equilibrium moles Work out the moles at equilibrium for the reactants and products 20% of the nitrogen had reacted = 0.2 x1.0 = 0.2 moles reacted. Using the balanced equation 3 x 0.2 moles of H2 must have reacted and 2x 0.2 moles of NH3 must have formed moles of reactant at equilibrium = initial moles – moles reacted moles of nitrogen at equilibrium = 1.0 – 0.2 = 0.8 moles of hydrogen at equilibrium =3.0 – 0.20 x3 = 2.40 N2 H2 NH3 Initial moles 1.0 3.0 0 Equilibrium moles 0.80 2.40 0.40 Mole fractions 0.8/3.6 =0.222 2.40/3.6 =0.667 0.40/3.6 =0.111 Partial pressure 0.222 x2 = 0.444 0.667 x2 =1.33 0.111 x2 = 0.222 = 0.2222 0.444×1.333 Kp Finally put concentrations into Kp expression moles of product at equilibrium = initial moles + moles formed moles of ammonia at equilibrium = 0 + (0.2 x 2) = 0.4 p 2 NH3 (g) pN2 (g) p 3H2 (g) Kp= = 0.0469 kPa-2 CaCO3 (s) CaO (s) + CO2 (g) Kp expressions only contain gaseous substances. Any substance with another state is left out Heterogeneous equilibria for Kp Kp =p CO2 Unit
3.1.10 Equilibrium constant K p for homogeneous systems (A-level only)
The equilibrium constant Kp is deduced from the equation for a reversible reaction occurring in the gas phase.
Kp is the equilibrium constant calculated from partial pressures for a system at constant temperature.
Students should be able to:
• construct an expression for K p for a homogeneous system in equilibrium
• perform calculations involving K p