What's the role of partial pressures in gas phase equilibria?

Partial pressures in gas phase equilibria determine the direction of the reaction and the equilibrium constant.

In more detail, the concept of partial pressures is crucial in understanding gas phase equilibria. This is based on Le Chatelier's principle, which states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In the context of gases, these conditions can be changes in pressure, volume, or the amount of gas.

The partial pressure of a gas is the pressure that the gas would have if it alone occupied the entire volume of the mixture. In a mixture of gases, each gas exerts a pressure independently of the others. This is known as its partial pressure. The total pressure of the gas mixture is the sum of the partial pressures of each individual gas.

In gas phase equilibria, the reaction can move in the direction of the gas with lower partial pressure when the pressure is increased, as this reduces the total pressure. Conversely, if the pressure is decreased, the reaction will move in the direction of the gas with higher partial pressure to increase the total pressure. This is a direct application of Le Chatelier's principle.

The equilibrium constant for a gas phase reaction, known as Kp, is also defined in terms of partial pressures. It is the ratio of the product of the partial pressures of the products to the product of the partial pressures of the reactants, each raised to a power equal to its stoichiometric coefficient in the balanced chemical equation. Changes in partial pressures can therefore shift the equilibrium position and alter the value of Kp.

In summary, partial pressures play a key role in gas phase equilibria. They determine the direction in which a reaction will proceed in response to changes in conditions, and they are used to calculate the equilibrium constant. Understanding the concept of partial pressures is therefore essential for predicting and analysing the behaviour of gases in equilibrium reactions.