Introduction to Topic 7 and Topic 17

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 examples of homogeneous equilibria are included and although you need to know about the reaction quotient, Q and the equilibrium constant, K_c, and how they are expressed for simple reactions, you do not need to do any calculations involving K_c.

Even at Higher Level where calculations are required they are quite limited as you are not rexpected 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.