Equilibrium Constant (Kc/Kp) Calculator

The Balance of Reactions: A Guide to the Equilibrium Constant

Many chemical reactions are reversible, meaning they can proceed in both the forward direction (reactants to products) and the reverse direction (products to reactants). Initially, a reaction mixture may contain only reactants, but as the reaction proceeds, products are formed, and these products can start reacting to re-form the original reactants. Eventually, the reaction reaches a state of dynamic equilibrium, where the rate of the forward reaction is exactly equal to the rate of the reverse reaction. At this point, the concentrations of reactants and products no longer change; they have reached a stable balance.

The equilibrium constant (K) is a value that expresses the relationship between the concentrations of products and reactants when a reaction is at equilibrium. It provides a crucial measure of the extent to which a reaction will proceed. This calculator is designed to help you determine the equilibrium constant for a reaction by providing the equilibrium concentrations or pressures of the substances involved. It's a fundamental tool for chemistry students and professionals for predicting the composition of a mixture at equilibrium.

The Equilibrium Constant Expression (Kc and Kp)

For a general reversible reaction: aA + bB ⇌ cC + dD

The equilibrium constant expression is written as the ratio of the concentrations of the products to the concentrations of the reactants, with each concentration raised to the power of its stoichiometric coefficient.

• For concentrations (Kc): Kc = ([C]ᶜ[D]ᵈ) / ([A]ᵃ[B]ᵇ) where [X] denotes the molar concentration of substance X.
• For partial pressures (Kp): For gas-phase reactions, the constant can be expressed in terms of the partial pressures of the gases: Kp = (P_Cᶜ * P_Dᵈ) / (P_Aᵃ * P_Bᵇ).

Interpreting the Value of K

The magnitude of the equilibrium constant tells you about the composition of the equilibrium mixture:

• If K >> 1 (much greater than 1), it indicates that at equilibrium, the mixture contains mostly products. The reaction is said to 'favor the products' or 'lie to the right'.
• If K << 1 (much less than 1), it indicates that the equilibrium mixture contains mostly reactants. The reaction does not proceed very far in the forward direction. It is said to 'favor the reactants' or 'lie to the left'.
• If K ≈ 1, it indicates that the equilibrium mixture contains significant amounts of both reactants and products.