7. Equilibrium

7. Equilibrium

7.1 Equilibrium

Nature of science:

• Obtaining evidence for scientific theories - isotopic labelling and its use in defining equilibrium.

• Common language across different disciplines - the term dynamic equilibrium is used in other contexts, but not necessarily with the chemistry definition in mind.

Understandings:

• A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.

• The equilibrium law describes how the equilibrium constant (Kc ) can be determined for a particular chemical reaction.

• The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.

• The reaction quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time. Q is the equilibrium expression with non-equilibrium concentrations. The position of the equilibrium changes with changes in concentration, pressure, and temperature.

• A catalyst has no effect on the position of equilibrium or the equilibrium constant.

Applications and skills:

• The characteristics of chemical and physical systems in a state of equilibrium.

• Deduction of the equilibrium constant expression (Kc ) from an equation for a homogeneous reaction.

• Determination of the relationship between different equilibrium constants (Kc ) for the same reaction at the same temperature.

• Application of Le Châtelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant.