Equilibrium constant Kp for homogeneous systems (A-level only)

The equilibrium constant Kp is a fundamental concept in A-level Chemistry that extends the idea of equilibrium to gaseous reactions. Unlike Kc, which uses concentrations, Kp is expressed in terms of the partial pressures of the gases involved. This allows chemists to predict the position of equilibrium and the yield of products under different conditions of pressure and temperature. Understanding Kp is crucial for industrial processes like the Haber process, where manipulating pressure can maximise ammonia production.

Kp is defined for homogeneous gaseous systems, meaning all reactants and products are in the gas phase. The expression for Kp is derived from the balanced chemical equation, with each partial pressure raised to the power of its stoichiometric coefficient. A key point is that Kp is dimensionless when the sum of coefficients on both sides is equal; otherwise, it has units of pressure. The value of Kp changes only with temperature, not with pressure or the presence of a catalyst, making it a powerful tool for predicting equilibrium behaviour.

This topic builds on earlier work on dynamic equilibrium and Le Chatelier's principle. By mastering Kp, students can quantitatively analyse how changes in pressure affect the equilibrium position, which is essential for understanding real-world chemical manufacturing. It also lays the groundwork for more advanced topics like Gibbs free energy and the relationship between Kp and ΔG.