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Topics 7 & 17

Topics 7 (smiley laugh 4.5 h) & 17 (laugh 4 h) : Equilibrium

Introduction

Equilibrium is another key concept that runs right through the whole of chemistry. Technically all chemical reactions are reversible and so can reach a state of equilibrium but in practice many do go virtually to completion. For example, the equilibrium constant for the reaction between zinc metal and copper(II) sulfate solution is in the order of 1 x 1037.

At Standard Level, you need to know the difference between dynamic and static equilibrium for both chemical and physical systems and be able to apply Le Chatelier’s principle to a variety of situations. Only homogeneous equilibria is included and although Standard Level students need to know about the reaction quotient and the equilibrium constant, Kc, and how they are expressed for simple reactions you do not need to do any calculations involving Kc.

Even at Higher Level the calculations are quite limited as you are not required to solve quadratic equations and only problems involving homogeneous equilibrium will be set. This either means simplifying expressions such as (a – x) to just a when x is very small compared to a or ignoring these problems altogether. The relationship between the equilibrium constant and entropy and solving problems using ΔG = − RT log K also needs to be known and understood.

Some points to consider

There are a few classic experiments which can be used for this topic but there are no mandatory requirements. Some are just illustrations but others involve determining the value for an equilibrium constant so could be useful to help prepare students for their individual scientific investigation. However it is worth bearing in mind that it may be difficult for students to design a successful practical on this topic partly because it can often take some time before the position of equilibrium is actually reached.

One interesting fact about equilibrium is that examiners seem to find it difficult! During the past few years there have been several mistakes on the IB exam papers and most of them have involved equilibrium so perhaps it is a topic that needs to be taught well! 

One example of these mistakes is Question 7 part (c) on the May 2011 TZ1 Paper 2.


"In an experiment conducted at 25.0 °C, the initial concentration of propanoic acid and methanol were 1.6 mol dm–3 and 2.0 mol dm–3 respectively. Once equilibrium was established, a sample of the mixture was removed and analysed. It was found to contain 0.80 mol dm–3 of compound X. (i) Calculate the concentrations of the other three species present at equilibrium. (ii) State the equilibrium constant expression, Kc, and calculate the equilibrium constant for this reaction at 25.0 °C."


The problem was giving the concentrations of the alcohol and the acid in mol dm-3 which meant that a considerable amount of water was present initially – this fact was ignored in the markscheme. For esterification problems it is more usual to have everything in the liquid phase (not the aqueous phase) and to give just masses or amounts in moles of reactants etc. as the volume remains more or less constant and cancels out when the values are put into the equilibrium expression. Be careful with other equilibrium systems though as the volume may not cancel out so concentrations rather than amounts should be used.

The links on the left give you teaching tips etc. for each of the sub-topics together with questions and answers for each sub-topic.

Once you have finished teaching the whole topic you can give the multiple choice tests on Equilibrium (together with answers):

7. Equilibrium

7 & 17. Equilibrium (1) and 7 & 17. Equilibrium (2)

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