Something to think about

Once you have understood the concept of dynamic equilibrium (as opposed to static equilibrium) it is worth challenging yourself to think how it could be shown experimentally that either chemical or physical equilibrium is dynamic and not static. There are several ways in which it could be done but one of the most obvious is to use a radioactive isotope. Although the syllabus (sub-topic 2.1 The nuclear atom) does not require you to know specific isotopes, the dangers posed by 131-iodine in nuclear accidents are a good example of utilization and other specific isotopes are mentioned in several of the options.

Imagine pouring a saturated solution of potassium iodide (made with normal non-radioactive iodide ions) onto some solid potassium iodide containing 131-iodide ions. The mixture is then stirred and left for several hours. If the equilibrium was static then once the solid had been filtered from the solution the solution should not show any radioactivity as none of the iodide ions would have exchanged. However, if the iodide ions in solution are in dynamic equilibrium with the iodide ions in the solid then over time the solution will become increasingly more radioactive as more and more 131-iodide ions are incorporated into the solution.

This explains why normal iodine is kept as a safeguard at nuclear reactors. If there is an explosion or melt-down one of the radioactive products is 131-iodine. If it escapes, which did happen for real at the Fukushima nuclear reactor in Japan in 2011 (see above), then flooding the body with normal iodine helps to prevent, or at least minimise, 131-iodine being incorporated into humans via the hormone thyroxine.

You should be able to write equilibrium expressions and have some idea about what the size of the equilibrium constant tells you about the position of equilibrium. It is therefore reasonable to ask whether equilibrium constants have units. Consider the Contact process.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

You should be able to deduce that:

Since you have been told that [ ] means 'concentration', then in terms of units K_c is equal to concentration squared divided by concentration cubed which simplifies to one over concentration, or concentration⁻¹. The units of the equilibrium constant, K_c, will therefore be mol⁻¹ dm³. Using similar logic the units for the equilibrium constant for the Haber process will be concentration⁻², or mol⁻² dm⁶. In fact until 2007 the units of equilibrium constants was a specific assessment statement on the IB Chemistry Diploma programme and questions involving the units were often asked in the exams. The problem is that equilibrium constants do not in fact have units! The reasons is that the square brackets, [ ], actually mean activity rather than concentration and activity has no units. For this reason the values given for all equilibrium constants in examinations (and for K_w in the IB Data booklet) on the current syllabus do not have units.¹

This can cause difficulties when comparing values of K_c for different reactions but not really for the IB, except for Higher Level students who are studying the environmental impact of heavy metals in Option A. The solubility product, K_sp, for silver chloride, AgCl, is 1.8 x 10^-10 whereas the solubility product for silver chromate, Ag₂CrO₄, is 1.3 x 10^-12. Although both are very 'insoluble' it would appear that silver chloride is more soluble as it has a higher K_sp value. However the concentration of silver ions in solution will be greater for silver chromate as the expression is [Ag⁺(aq)]²[CrO₄^2-(aq)], i.e. concentration cubed, compared to the concentration squared expression, [Ag⁺(aq)][Cl^-(aq)] for silver chloride.

Footnote

¹ Actually in the current version of the IB data booklet (4th. edition dated JIB Docs (2) Teamary 2017) in Section 2 the ionic product for water, K_w is given units whereas the solubility product constants, K_sp given in Section 32 do not have units!