Le Chatelier’s Principle: the effect of concentration on equilibrium
Solid bismuth(III) oxychloride (bismuth(III) oxide chloride) and bismuth(III) chloride (bismuth trichloride) in solution can exist in equilibrium:
BiOCl(s) + 2HCl(aq) ⇌ BiCl3(aq) + H2O(l)
Bismuth(III) oxychloride is dissolved in concentrated hydrochloric acid to give a clear solution of bismuth(III) chloride. Addition of water re-forms the bismuth(III) oxychloride as a white precipitate. Subsequent addition of hydrochloric acid increases the chloride ion concentration and re-dissolves the precipitate. These reversible changes can be repeated several times in sequence.
This demonstration involves an unfamiliar reaction, but bismuth compounds do have everyday uses in medicines for indigestion and diarrhoea, and in cosmetics. The reaction provides a visible demonstration of an easily reversible reaction. A detailed equation for the reaction may not even be necessary for use with all groups.
Alternately decreasing and increasing the hydrochloric acid concentration causes the equilibrium to shift in the direction predicted by Le Chatelier’s principle.
A dark background will make the changes more easily visible.
Time required is about 5 min if the solution is prepared beforehand and beakers and measuring cylinders are lined up on the bench.
The quantities given are for one demonstration:
Bismuth(III) oxychloride (OXIDISING, IRRITANT), 2.6 g
Concentrated hydrochloric acid (CORROSIVE), 83 cm3
Refer to Health & Safety and Technical notes section below for additional information.
Eye protection - safety goggles
Beakers (25, 50, 100, 250, 600 and 2000 cm3), 1 of each (Note 1)
Measuring cylinders (10 cm3), 4 & (50 or 100 cm3), 2 (Note 2)
Top pan balance, accurate to +/- 0.1 g
Health & Safety and Technical notes
Bismuth(III) oxychloride, BiOCl(s) (OXIDISING, IRRITANT) - see CLEAPSS Hazcard.
Bismuth(III) chloride, BiCl3(s) (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Concentrated hydrochloric acid, HCl(aq) (CORROSIVE) - see CLEAPSS Hazcard.
1 These sizes are not critical, the size larger than that specified could easily be substituted in each case.
2 It is possible to manage with just one cylinder of each size by measuring the acid out into suitably sized beakers.
Before the demonstration
a Weigh out 2.6 g (0.01 mol) of bismuth(III) oxychloride. Dissolve the bismuth(III) oxychloride in 3 cm3 of concentrated hydrochloric acid in a boiling tube or small beaker. This will give a clear solution of bismuth(III) chloride.
b Line up on the demonstration bench six beakers of the sizes shown containing water as shown in the table. Beside each beaker place a measuring cylinder (or other suitable container such as a small beaker) containing the volume of concentrated hydrochloric acid shown in the table.
|Beaker number||Size of beaker/cm3||Volume of water in beaker/cm3||Volume of conc. HCI next to beaker/cm3|
c Pour the bismuth(III) chloride solution into the water in beaker 1. A white precipitate of bismuth(III) oxychloride will appear immediately as the equilibrium is displaced to the left by the increased concentration of water and reduction of the acid concentration.
e Now add the 2 cm3 of hydrochloric acid to beaker 1 and stir. The precipitate re-dissolves as bismuth(III) chloride, as the increased concentration of acid moves the equilibrium to the right.
f Now pour the contents of Beaker 1 into Beaker 2. The precipitate re-appears and can be dissolved again on addition of the pre-measured volume of hydrochloric acid - with stirring.
g Pour the contents of Beaker 2 into Beaker 3, followed by addition of acid and so on. Precipitates continue to appear and re-dissolve as predicted by Le Chatelier’s principle although the precipitate takes noticeably longer to re-dissolve.
The experiment can be started from a solution of bismuth(III) chloride in water if bismuth(III) oxychloride is not available.
The demonstration can be carried out more quickly and qualitatively as a test-tube reaction, starting with a little bismuth(III) chloride dissolved in about 1 cm3 of concentrated hydrochloric acid. Adding water to a few drops of this solution produces a white precipitate. This can be reversed by adding more concentrated acid. By trial and error suitable volumes can be found that will enable the cycle to be reversed several times.
Point out that the reaction to re-establish equilibrium gets slower as the solutions get more dilute.
Health and safety checked February 2008
Page last updated on 23 December 2011