Equilibria
The study of equilibria at A level can be divided into qualitative aspects, including the application of Le Chatelier's principle to deducing the effect of constraints on equilibrium yield and equilibrium position, and quantitaive treatments involving the equilibrium 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.
This list provides resources relating to the first year of A level (or AS) which includes qualitative aspects and the equilibrium constant (Kc). This leads to the teaching of equilibria in gases and the equilibrium constant Kp, normally taught in the second year of an A level course.
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.’
Unit P2: Equilibrium I: Principles *suitable for home teaching*
This is a really comprehensive study guide that covers the principles of equilibrium including equilibrium states, applying Le Chatelier's principle and Kp. The theory for each topic is explained and followed by a series of exercises and related experiments.
Nuffield Advanced Chemistry (Third Edition): Students’ Book
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.
