Hydrogen and iodine vapour react to form hydrogen iodide. In a closed vessel a dynamic equilibrium is formed.

What would the equation for this equilibrium look like?

H₂(g) + I₂(g) _∏ 2HI(g)

This equilibrium can be investigated by keeping mixtures of hydrogen and iodine OR hydrogen iodide in a sealed glass vessel at a fixed temperature until equilibrium has been reached and then analysing the contents of the vessel.

[INSERT CI_01_ima]

Some experimental data are shown in the table below.

[INSERT CI_02_ima]

Data from Nuffield Advanced Chemistry Book II revised edition (1984, 1991 impression)

Which of the rows of data (A to F) could have been obtained from experiments starting from hydrogen iodide only? How do you know?

The data in rows D, E and F could have been produced starting from hydrogen iodide alone because the concentrations of hydrogen and iodine are equal at equilibrium.

Work out values for the relationship  at equilibrium using some of the data sets A to F. What do you notice about the values you have calculated?

The values are different.

Work out values for the relationship  at equilibrium for some of the data sets A to F. What do you notice about the values you have calculated?

The values are constant.

This is one example of the Equilibrium Law. We will look at this in more detail on the next page.