EQUILIBRIUM EXPRESSION

• At equilibrium, the reaction quotient becomes the equilibrium constant,

• It is important to remember that at equilibrium the concentrations of the chemical species are constant, not necessarily equal.

Le Chatelier’s principle

• Le Chatelier discovered that if a chemical system at equilibrium is stressed (disturbed) it will reestablish equilibrium by shifting the reactions involved. This means that the amounts of the reactants and products will change, but the final ratio will remain the same.

• The equilibrium may be stressed in a number of ways: changes in concentration, pressure, and temperature.

Changes in concentration

• If the concentration of a chemical species is decreased, the equilibrium will shift to produce more of it. In doing so, the concentration of chemical species on the other side of the reaction arrows will be decreased.

Change in pressure

• Changes in pressure are significant only if gases are involved.

• The pressure may be changed by changing the volume of the container or by changing the concentration of a gaseous species.

Change in temperature

• Changing the temperature changes the value of the equilibrium constant. It also changes the amount of heat in the system and can be treated as a concentration effect.

Acid-Base equilibrium

• Strong acids completely dissociate in water;

• weak acids only partially dissociate.

Titration Equilibria

• An acid-base titration is a laboratory procedure commonly used to determine the concentration of an unknown solution.

• A base solution of known concentration is added to an acid solution of unknown concentration (or vice versa) until an acid–base indicator visually signals that the endpoint of the titration has been reached.

• The equivalence point is the point at which a stoichiometric amount of the base has been added to the acid.

Solubility Equilibria

• The equilibrium constant expression for systems of slightly soluble salts is called the solubility product constant, Ksp. It is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation.

Titration Equilibria

• An acid–base titration is a laboratory procedure commonly used to determine the concentration of an unknown solution.

• A base solution of known concentration is added to an acid solution of unknown concentration (or vice versa) until an acid–base indicator visually signals that the endpoint of the titration has been reached.

• The equivalence point is the point at which a stoichiometric amount of the base has been added to the acid.