Calculating the Equilibrium Constant

Substitute the equilibrium concentrations of all reactants and products directly into the equilibrium constant expression, Kc. For example, a gaseous mixture at equilibrium containing 0.10 M sulfur dioxide, 0.15 M oxygen, and 10.88 M sulfur trioxide yields Kc = 7.9 × 10⁴ when substituted into the expression. This direct method requires knowing all equilibrium concentrations.

An ICE table organizes Initial, Change, and Equilibrium concentrations for a reaction. It systematically tracks how concentrations change as a reaction reaches equilibrium, allowing you to calculate unknown equilibrium concentrations from initial concentrations and stoichiometry. This structured approach is essential when only initial concentrations and one equilibrium value are provided.

Use the reaction coefficients to relate concentration changes across all species. If 2x moles of ammonia form at equilibrium, then x moles of nitrogen are consumed and 3x moles of hydrogen are consumed based on the stoichiometry. Subtract these changes from initial concentrations to find equilibrium values, which can then be substituted into the Kc expression.

You need the initial concentrations of all reactants and products, plus the equilibrium concentration of at least one compound. Using reaction stoichiometry and an ICE table, you can derive the remaining equilibrium concentrations. For instance, knowing initial nitrogen and hydrogen concentrations and equilibrium ammonia concentration allows calculation of all equilibrium values needed for Kc.

The variable x represents the extent of reaction. When multiplied by each species' stoichiometric coefficient, it gives the concentration change for that species. For the ammonia synthesis reaction, if 2x = 0.020 M ammonia produced, then x = 0.010 M, meaning 0.010 M nitrogen consumed and 0.030 M hydrogen consumed based on their coefficients.

Yes, Kp for gaseous reactions can be calculated using an ICE table and the equilibrium expression written with partial pressures instead of concentrations. The same systematic approach of organizing initial, change, and equilibrium values applies, but you substitute partial pressure values into the Kp expression rather than molar concentrations.

The magnitude of Kc indicates the extent of reaction at equilibrium. A large Kc, such as 7.9 × 10⁴, indicates the reaction strongly favors products at equilibrium. A small Kc, such as 0.11, indicates the reaction favors reactants. The value is temperature-dependent and characteristic of each reaction, allowing prediction of equilibrium composition.