Chemical Equilibria and Reaction Quotients

Chemical reactions can be reversible, meaning reactants form products and products revert to reactants. When the rates of the forward and reverse reactions are equal, the system is at dynamic equilibrium. This differs from unidirectional reactions where reactants fully convert to products. Calculating concentrations at equilibrium requires a specific mathematical approach.

To determine equilibrium concentrations, chemists use an 'ICE box' (Initial, Change, Equilibrium). For a given reaction and initial amounts, the 'Initial' row lists starting concentrations. The 'Change' row represents the shift towards equilibrium using 'x' for unknown changes, noting depletion of reactants (negative x) and formation of products (positive x), scaled by stoichiometric coefficients. The 'Equilibrium' row sums initial and change values. An example demonstrates how to set up and solve an ICE box when one equilibrium concentration is known.

Every equilibrium has an equilibrium constant, Kc, calculated by the ratio of product concentrations (raised to their stoichiometric powers) to reactant concentrations (raised to their stoichiometric powers). Kc indicates whether products (Kc > 1) or reactants (Kc < 1) are favored at equilibrium. Only gases and aqueous species are included in the expression, not solids or pure liquids, as their concentrations are considered constant.

The reaction quotient, Q, is used to predict the direction a non-equilibrium mixture will shift. By plugging non-equilibrium concentrations into the Kc expression, Q is calculated. If Kc > Q, the reaction shifts right (towards products). If Kc < Q, it shifts left (towards reactants). If Kc = Q, the system is already at equilibrium.

A more challenging ICE box example is presented where initial concentrations and Kc are given. The setup involves careful consideration of stoichiometric coefficients for the 'Change' row (e.g., -2x for a reactant with a coefficient of 2). The equilibrium expressions are then plugged into the Kc equation, and algebraic methods, possibly including the quadratic equation, are used to solve for x and subsequently determine all equilibrium concentrations.