Chemical Equilibrium — AI Study Guide

Mastering Chemical Equilibrium

Chemical equilibrium occurs when the rates of forward and reverse reactions are equal and concentrations no longer change. At equilibrium, a dynamic balance exists — reactions still occur in both directions, but at equal rates. The equilibrium constant (K_eq or K_c) is the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficients, at equilibrium. K > 1 indicates products are favored; K < 1 indicates reactants are favored.

Le Chatelier's principle states that a system at equilibrium will shift to counteract any disturbance. Adding reactants shifts equilibrium toward products. Removing products shifts equilibrium toward products. Increasing pressure (by decreasing volume) shifts equilibrium toward the side with fewer moles of gas. Increasing temperature shifts equilibrium toward the endothermic direction. A catalyst speeds the approach to equilibrium without changing the equilibrium position.

Acid-base equilibria are among the most important applications of equilibrium in chemistry. The acid dissociation constant K_a measures acid strength — the higher K_a (lower pK_a), the stronger the acid. Weak acids partially dissociate in water; strong acids dissociate completely. Buffer solutions (weak acid + conjugate base) resist pH changes through equilibrium shifts. The Henderson-Hasselbalch equation (pH = pK_a + log([A-]/[HA])) relates buffer composition to pH.

Solubility equilibria describe the dissolution of sparingly soluble ionic compounds in water. The solubility product constant (K_sp) is the equilibrium constant for dissolution. The common ion effect — addition of a common ion decreases solubility — follows directly from Le Chatelier's principle. Predicting precipitation requires comparing the reaction quotient Q to K_sp: if Q > K_sp, precipitation occurs; if Q < K_sp, the solution is unsaturated.

Frequently Asked Questions: Chemical Equilibrium

What does the equilibrium constant K tell you?

The equilibrium constant K (at a given temperature) indicates the relative amounts of products and reactants at equilibrium. K >> 1 (large K): equilibrium lies far to the right, products are strongly favored. K << 1 (small K): equilibrium lies far to the left, reactants are strongly favored. K ≈ 1: significant amounts of both products and reactants are present at equilibrium. K changes with temperature but is not affected by concentration changes or the presence of a catalyst.

What is a buffer and how does it work?

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (or weak base and its conjugate acid) in significant concentrations. When acid is added, the conjugate base neutralizes it (A- + H+ → HA). When base is added, the weak acid neutralizes it (HA + OH- → A- + H2O). Buffers work best within ±1 pH unit of the weak acid's pK_a.

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