# Simple Equilibria and Acid-Base Reactions

## Simple Equilibria and Acid-Base Reactions

Simple Equilibria

• Equilibrium in a chemical reaction is the state at which the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of the substances present.

• A reversible reaction is one that can proceed in both the forward and reverse directions.

• The equilibrium constant (Kc) is a measure of the ratio of the concentrations of the products to reactants when a reaction has reached equilibrium, raised to the power of their stoichiometric coefficients.

• Le Chatelier’s Principle states that if a change is imposed on a system at equilibrium, the system will adjust itself to counteract that change. This can involve changes in concentration, temperature, pressure or volume.

• The movement of a reaction towards equilibrium is often graphically represented on reaction concentration-time graphs.

Acid-Base Reactions

• An acid is a substance that donates a hydrogen ion (H+) when in a solution (according to the Brønsted–Lowry definition). They also turn litmus paper red and have pH values less than 7.

• A base is a substance that accepts a hydrogen ion (H+) in a solution. They turn litmus paper blue and have pH values greater than 7.

• In water, the hydrogen ion exists as the hydronium ion (H3O+).

• The strength of an acid or base is defined by its ability to ionise in water. Strong acids and bases fully ionise, while weak acids and bases only partially ionise.

• The pH scale is a measure of the concentration of hydrogen ions in a solution, thereby indicating its acidity or basicity. The pH value can be calculated using the formula, pH = -log[H+].

• Neutralisation reactions occur when an acid and a base react together to form a salt and water. The salt formed depends on the specific acid and base used.

• The process of titration can be used to determine the concentration of an acidic or basic solution. It involves the slow addition of an acid to a base (or vice versa) until the solution is neutral, using an indicator to signal the endpoint.

• Buffers are solutions that resist changes in pH when small amounts of an acid or a base are added, or when the solution is diluted. They are often a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.