The study of equilibria at A level can be divided into qualitative aspects, including the application of Le Chatelier's Pprinciple to deducing the effect of constraints on equilibrium yield and equilibrium position, and quantitaive treatments involving the equilibruium constant. Traditionally this has been reflected in the division of study between first and second year A level.
Qualitative aspects of the topic go little further than GCSE, and no new material beyond GCSE is added, although students are expected to have a deeper appreciation of the idea of compromise conditions in industrial manufacture.
The quantitative treatment includes developing an understanding of the equilibrium law and constant (Kc). Students often find it difficult to rationalise Le Chatelier's principle, and the effect of factors on equilibrium position and yield, with the fact that the only factor that affects the equilibrium constant is temperature. To many students this seems counter-intuitive.
Once the basic ideas of equilibrium are mastered the concept is applied to the much larger topic of acid-base equilibria, including calculation of pH for strong and weak acids and buffers. This area has been assigned to a separate list which also covers equilibria of gaseous sytems and Kp.
The 2015 revision of the A level syllabus has seen Kc moved to the first year of academic study and this is a departure from the previous revision.
As such this list provides resources relating to the first year of A level (or AS) which includes qualitative aspects and the equilibrium constant (Kc).
Second year topics may be found under the list Acid-Base Equilibria which also covers Kp.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or other recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.’
Links and Resources
This comprehensive resource provides much detailed background material on the topics covered in this module. The resource would be useful for teachers when planning lessons, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. An invaluable resource for planning, and for setting example problems to support teaching and learning.
Pages 33 to 37 outline a structured approach to experimentally measuring the equilibrium constant for hydrolysis of an ester leading the student step-by-step through the experimental procedure and subsequent calculations.
This resource provides useful worksheets on dynamic equlibrium, Le Chatelier's principle and industrial applications including the idea of compromise conditions. The material could be useful to reinforce learning either in class or as homework exercises.
This resource describes a simple visual demonstration of an equilibrium system involving dissolved CO2 , and the effect of shifting the equilibrium position.The resource gives both a method using readily available equipment, and details of the equlibrium chemistry involved. This would make a good starter to stimulate discussion of equilibria and the application of Le Chatelier's principle, to help revise and consolidate students' understanding before moving onto a more in depth analysis.
This is a very nice and effective experiment that revises and reinforces the link between equilibria and energetics in a qualitative way. After demonstrating very simply that stretching of a rubber band is an exothermic process, students investigate this qualitative relationship further and relate their observations to Le Chatelier's principle. Teacher notes and a student worksheet are included. This is a good class activity to help revise and consolidate GCSE understanding.
The first part of this activity, aimed at gifted and talented students, explores equilibria in more detail. Students are presented with a number of cards bearing statements which they have to arrange in a sensible sequence in oder to produce a flow chart of ideas in a logical sequence to summarise the theme.
With a group of average ability, this could take the form of a guided whole class activity with teacher input as required.
This is a nice resource which will make students think and question ideas and therefore help to consolidate their learning in this important area.
Although this resource is presented as a demonstration it can very easily be adapted as a class practical allowing students to investigate the properties of an unfamiliar equilibrium system.
The rubric makes suggestions for investigating the temperature dependence of the equilibrium and also the effect of chloride ions. The shifts in equilibrium position can be readily reversed.
An extension is to add silver nitrate to remove chloride ions, causing an initially blue solution to turn pink.
This is another interesting but simple class activity to help consolidate students' understanding of the application of Le Chatelier's principle.
Experiment 11.2 on page 297 (topic 11) gives instructions on experimental measurement of the equilibrium constant for the redox reaction between Ag+ and Fe2+. The mixture at equilibrium is titrated with potassium thiocyanate which enables the equilibrium concentration of silver ions to be established.
The reaction requires setting up in one lesson and titrating in the next as it is slow to establish and leaving to stand overnight is recommended.
Although the reaction is a redox reaction the titration is not a redox titration, and so can be understood by first year A level students.
This is an interesting and novel experiment enabling students to practise a number of skills and to extend their understanding of volumetric analysis (titration) beyond that of acid-base reactions.
This resource outlines the experimental determination of the partition coefficient of an organic diacid between water and butan-1-ol.
Although partition coefficients are not included in the A level specification they provide an alternative, and simple system, in which to apply the principles of the equilibrium law and to measure an equilibrium constant, therefore extending the students repertoire and understanding.
Note that the partition coefficient between water and butan-ol for the dissolved acid, X is given by:
Kc = concentration of X in water/concentration of X in butan-1-ol
The value for the coefficient is not given and is not easily found in the literature. One solution is to provide students with a teacher result or to take the class average.
Whilst the process itself is what is of value here, students invariably want to know the 'right answer' and this could stimulate a discussion of how we decide on this.