IB Chemistry - Equilibrium

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Syllabus ref: 7.1 Syllabus ref: 17.1

Dynamic chemical equilibrium arises when the rate of a forward reaction equals the rate of the reverse reaction. The two reactions proceed at an equal rate so the amounts of products and reactants remain constant. The symbol used for dynamic equilibrium is two half headed arrows pointing in opposite directions.

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.

Employing quantitative reasoning-experimentally determined rate expressions for forward and backward reactions can be deduced directly from the stoichiometric equations and allow Le Chatelier's principle to be applied.


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.

Understandings - HL

Essential idea: The position of equilibrium can be quantified by the equilibrium law. The equilibrium constant for a particular reaction only depends on the temperature

Le Châtelier's principle for changes in concentration can be explained by the equilibrium law

The position of equilibrium corresponds to a maximum value of entropy and a minimum in the value of the Gibbs free energy.

The Gibbs free energy change of a reaction and the equilibrium constant can both be used to measure the position of an equilibrium reaction and are related by the equation, ΔG = -RTlnK.

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.

Applications and skills - HL

Solution of homogeneous equilibrium problems using the expression for Kc.

Relationship between ΔG and the equilibrium constant.

Calculations using the equation ΔG = -RTlnK .

In Chapter 7.2