Understanding equilibrium: a delicate balance

Any student asked to conduct an internet search for the meaning of the term equilibrium is likely to find a variety of definitions ranging from ‘a state in which opposing forces or influences are balanced’ to ‘a calm state of mind’. Indeed, in school, pupils’ first experience of the term may well be in these contexts. In the science classroom it could be while studying balanced forces. At pre-16 level the concept of chemical equilibrium is likely to be first encountered in the study of the Haber process, which is often given the treatment of ‘learn the reaction conditions and, by the way, the symbol for the equilibrium is ⇌’ At this stage Le Chatelier’s principle (also known as the equilibrium law) may well be introduced and pupils asked to apply it.

The idea of equilibrium being a balancing act soon becomes entrenched. So it is unsurprising that when met at post-16 level teachers need to unpick pupils’ misconceptions in order to provide a concrete understanding of the topic that will allow for the application of a quantitative approach. Much academic research has been conducted to determine exactly what students find difficult as they study this aspect of chemistry and this is reflected in the approach outlined below. Developing an understanding of these key concepts is essential in allowing pupils to make progress in the study of chemical equilibria.

Steve Hacker describes La Chatelier's principle and the key features of dynamic equilibrium, considering how best to teach these to your students.