Progression of ideas
In the study of the effect of concentration and pressure on equilibrium position the need for temperature to remain constant should be emphasised. This leads naturally to the question of what happens if the temperature isn't constant. Simple class experiments or demonstrations can be used to illustrate that temperature does indeed affect the position of equilibrium.
What class experiments or teacher demonstrations could you use to introduce the effect of temperature on equilibrium position?
The change of colour of cobalt complexes can be used as an effective class experiment. The effect of temperature on the NO2/N2O4 equilibrium is an excellent teacher demonstration. The iodine monochloride/iodine trichloride equilibrium is explored in detail in Developing Understanding.
The question still remains of why the equilibrium position has changed. What data might students be given that will enable them to find an answer to this question?
Students could be given data showing the effect of temperature on Kc and/or Kp.
What patterns should students discover as a result of analysing this data?
They should find that Kc and Kp decrease with increasing temperature for an exothermic reaction and increase with increasing temperature for an endothermic reaction.
The use of Le Chatelier's Principle to predict the effect of temperature on equilibrium position can be introduced at this point. Emphasise that this is a useful device and not an explanation.
What prior learning do students need in place before they will be able to understand why increasing the temperature affects the position of equilibrium?
Students need to know that:
(i) The rate of reaction depends on the rate constant of the reaction.
(ii) The rate constant is dependent on temperature.
What new ideas need to be added so that students can explain why raising the temperature changes the position of equilibrium?
(i) Kc depends on the ratio of the rate constants for forward and back reactions.
(ii) Kc depends on the enthalpy change of reaction, ΔH,and temperature.
The next stage is to consider the effect of a catalyst on an equilibrium mixture.
How can you explain to students that a catalyst will increase the rate of a reaction but doesn't affect the position of an equilibrium?
You need to emphasise that the rates of both forward and reverse reactions are increased by the same amount.