Topic 17.1: Equilibrium – The equilibrium law
The position of equilibrium can be quantified by the equilibrium law. The equilibrium constant for a particular reaction only depends on the
temperature.
• Understanding: Le Châtelier’s principle for changes in concentration can be explained by the equilibrium law.
▪ Le Chatelier’s principle: states that a when there is a change in condition that affects equilibrium, a system in dynamic
equilibrium tend to relieve the change by balancing the forward and reverse reaction to return the system to equilibrium
▪ Law of chemical equilibrium: states that at a given temperature the ratio of the concentration of products (to the power of their
molar coefficient) to the concentration of reactants (to the power of their molar coefficient) is constant (equilibrium constant)
▪ The concentration of reactants and products in a solution will shift until the equilibrium constant (given ratio) is reacted
• Understanding: The position of equilibrium corresponds to a maximum value of entropy and a minimum in the value of the Gibbs
free energy.
▪ Position of equilibrium: equilibrium will be spontaneous in the direction that results in a decrease of Gibbs free energy
• This will correspond to a maximum value of entropy
• Understanding: 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, ∆G = −RT ln K .
• Applications and skills: Relationship between ∆G and the equilibrium constant.
Equilibrium constant Equilibrium position Description Gibbs free energy change
K=1 Neutral Neither is favoured ∆G=0
K>1 On the right Products are favoured ∆G<0 (forward reaction is spontaneous)
K<1 On the left Reactants are favoured ∆G>0 (reverse reaction is spontaneous)
▪ Gibbs free energy and equilibrium constant relationship: ∆G = −RT ln K
• Rearranging this equation so K is the subject will give values to deduce position of equilibrium
• ∆G values can be determined from ∆G = ∆H - T∆S
• Applications and skills: Solution of homogeneous equilibrium problems using the expression for Kc.
▪ Homogenous equilibrium: reaction in which all reactants and products ae in the same phase
▪ Process of determining Kc given initial concentration and change in concentration
• Identify balanced equation for the equilibrium reaction
• Determine change in concentration; must reflect stoichiometric coefficients
• Determine final equilibrium concentration for reactants and products
• Identify Kc value from equilibrium concentrations
• Applications and skills: Calculations using the equation ∆G = −RT ln K.
▪ Process of finding Kc given ∆G (or ∆H and ∆S)
▪ Identify ∆G from ∆G = ∆H - T∆S
▪ Rearrange equation ∆G = −RT ln K to place K as the subject
▪ Derive Kc value
• Nature of science: Employing quantitative reasoning—experimentally determined rate expressions for forward and backward
reactions can be deduced directly from the stoichiometric equations and allow Le Châtelier’s principle to be applied.
• Utilization: The concept of a closed system in dynamic equilibrium can be applied to a range of systems in the biological, environmental
and human sciences.
• Guidance: The expression ∆G = −RT ln K is given in the data booklet in section 1.
• Guidance: Students will not be expected to derive the expression ∆G = −RT ln K.
• Guidance: The use of quadratic equations will not be assessed.
The position of equilibrium can be quantified by the equilibrium law. The equilibrium constant for a particular reaction only depends on the
temperature.
• Understanding: Le Châtelier’s principle for changes in concentration can be explained by the equilibrium law.
▪ Le Chatelier’s principle: states that a when there is a change in condition that affects equilibrium, a system in dynamic
equilibrium tend to relieve the change by balancing the forward and reverse reaction to return the system to equilibrium
▪ Law of chemical equilibrium: states that at a given temperature the ratio of the concentration of products (to the power of their
molar coefficient) to the concentration of reactants (to the power of their molar coefficient) is constant (equilibrium constant)
▪ The concentration of reactants and products in a solution will shift until the equilibrium constant (given ratio) is reacted
• Understanding: The position of equilibrium corresponds to a maximum value of entropy and a minimum in the value of the Gibbs
free energy.
▪ Position of equilibrium: equilibrium will be spontaneous in the direction that results in a decrease of Gibbs free energy
• This will correspond to a maximum value of entropy
• Understanding: 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, ∆G = −RT ln K .
• Applications and skills: Relationship between ∆G and the equilibrium constant.
Equilibrium constant Equilibrium position Description Gibbs free energy change
K=1 Neutral Neither is favoured ∆G=0
K>1 On the right Products are favoured ∆G<0 (forward reaction is spontaneous)
K<1 On the left Reactants are favoured ∆G>0 (reverse reaction is spontaneous)
▪ Gibbs free energy and equilibrium constant relationship: ∆G = −RT ln K
• Rearranging this equation so K is the subject will give values to deduce position of equilibrium
• ∆G values can be determined from ∆G = ∆H - T∆S
• Applications and skills: Solution of homogeneous equilibrium problems using the expression for Kc.
▪ Homogenous equilibrium: reaction in which all reactants and products ae in the same phase
▪ Process of determining Kc given initial concentration and change in concentration
• Identify balanced equation for the equilibrium reaction
• Determine change in concentration; must reflect stoichiometric coefficients
• Determine final equilibrium concentration for reactants and products
• Identify Kc value from equilibrium concentrations
• Applications and skills: Calculations using the equation ∆G = −RT ln K.
▪ Process of finding Kc given ∆G (or ∆H and ∆S)
▪ Identify ∆G from ∆G = ∆H - T∆S
▪ Rearrange equation ∆G = −RT ln K to place K as the subject
▪ Derive Kc value
• Nature of science: Employing quantitative reasoning—experimentally determined rate expressions for forward and backward
reactions can be deduced directly from the stoichiometric equations and allow Le Châtelier’s principle to be applied.
• Utilization: The concept of a closed system in dynamic equilibrium can be applied to a range of systems in the biological, environmental
and human sciences.
• Guidance: The expression ∆G = −RT ln K is given in the data booklet in section 1.
• Guidance: Students will not be expected to derive the expression ∆G = −RT ln K.
• Guidance: The use of quadratic equations will not be assessed.
