Calculations Involving Equilibrium Constants - SS2 Chemistry Lesson Note

Equilibrium constants (Kc and Kp) can be used in various calculations to determine concentrations, partial pressures, and other quantities related to chemical equilibrium. Here are some important calculations involving equilibrium constants:

Calculating Equilibrium Concentrations or Partial Pressures: Given the initial concentrations or partial pressures of reactants and the equilibrium constant, you can calculate the equilibrium concentrations or partial pressures of reactants and products. Set up an ICE (Initial, Change, Equilibrium) table to determine the changes in concentrations or partial pressures and use the stoichiometry and the equilibrium constant expression to solve for the unknowns.

Determining Reaction Quotient (Q) and Comparing with Equilibrium Constant (K): The reaction quotient (Q) is calculated in the same way as the equilibrium constant, but it is determined using the concentrations or partial pressures at any point during the reaction, not just at equilibrium. By comparing the calculated Q value with the equilibrium constant (K), you can determine if the reaction is at equilibrium or if it will shift to reach equilibrium. If Q = K, the system is at equilibrium.

Predicting the Direction of a Reaction: By comparing the initial concentrations or partial pressures of reactants and products with the equilibrium constant (K), you can determine the direction in which the reaction will proceed to reach equilibrium. If Q < K, the reaction will shift forward (towards the products) to reach equilibrium. If Q > K, the reaction will shift backwards (towards the reactants). If Q = K, the system is already at equilibrium.

Calculating Equilibrium Constants from Known Equilibrium Concentrations or Partial Pressures: If the equilibrium concentrations or partial pressures of reactants and products are known, the equilibrium constant (K) can be calculated by substituting these values into the equilibrium constant expression.

Calculating Equilibrium Constants for Reversible Reactions: If a reaction can be expressed as the sum of two or more individual reactions, the equilibrium constant for the overall reaction can be determined by multiplying the equilibrium constants of the individual reactions. If a reaction is reversed, the reciprocal of the equilibrium constant is used.

Calculating Equilibrium Constants for Reaction Quotients: If the reaction quotient (Q) is known for a reaction, the equilibrium constant (K) can be calculated by determining the concentrations or partial pressures at equilibrium and using the same approach as mentioned earlier.

Using Equilibrium Constants to Calculate Changes in Concentrations or Partial Pressures: Equilibrium constants can be used to determine how changes in conditions, such as temperature or pressure, affect the equilibrium concentrations or partial pressures of reactants and products. By applying Le Chatelier's Principle and considering the effect of the change on the equilibrium constant, you can predict the direction and magnitude of the changes in the concentrations or partial pressures.

Calculations involving equilibrium constants provide a quantitative understanding of chemical equilibrium and allow for predictions of equilibrium concentrations, direction of reactions, and the effect of changes in conditions on the equilibrium position. These calculations are essential for optimising reaction conditions, designing chemical processes, and interpreting experimental data related to equilibrium systems.