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Rate and equilibria: a source of confusion

Rate and equilibria are two concepts in chemistry that often are taught at similar times, and as they have some common factors (such as temperature), students can easily confuse the two.

The Rate of a reaction is how quickly a reaction proceeds.

Equilibria describes the relative proportions of chemicals that exist together (products and reactants - depending on how this is interpreted).

Some reactions go fully to completion, and they are written in the form:

\[A + B \rightarrow AB\]

However, some do not go fully to completion, and they form a dynamic equilibria:

\[A + B \rightleftharpoons AB\]

In a reaction where the products and reactants are all in the gas phase, the position of this equilibrium can be effected by pressure and temperature.

A reaction might have a fast rate, but the position of equilibria is closer to \(A + B\) than to \(AB\). Although the reaction happens quickly, there may be very little product. Students are also confused by equilibria, as it is dynamic (the reaction doesn’t stop, but reaches a point where the rate of \(A + B\) reacting to \(AB\), is the same as the rate of \(AB\) reacting to \(A + B\)).

One model to think about is walking up an escalator that is moving down! You end up in what appears to be a stationary position, but the escalator is still moving down, while you are moving up.

Also, a reaction that proceeds to an equilibrium (often called a reversible reaction) can start with the products and end up with some reagents, so we could say that in our fictitious example:

\[AB \rightleftharpoons A + B\]

Students can be quite confused by this and in the 14-16 age range, equilibria discussions are usually confined to specific industrial processes, such as the production of ammonia or sulfuric acid. Most of the reactions we carry out in the lab with our students can be considered reactions that go to completion. However, it is worth being specific with the words rate and equilibria, to avoid confusion for students.

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This article is from the free online course:

Teaching Practical Science: Chemistry

National STEM Learning Centre