TOPIC: Solubility Equilibria

Demo-031

Lead Chloride Solubility Experiments

Lead chloride has a convenient Ksp for studying precipitation, temperature effects on solubility, and the common ion effect. This demonstration can be used to illustrate all these phenomena.

MATERIALS

1 liter of 0.2 M lead nitrate solution
1 liter of 0.2 M sodium chloride solution
Two 250 mL graduated cylinders
1 liter of distilled water
A saturated solution of lead chloride made by putting 10-20 g of lead chloride in a 1 liter bottle and filling with distilled water.
Other solutions containing chloride ion at 1-2 M conc. These could be other alkali chlorides, hydrochloric acid, etc.
Several large (demo-size) test tubes and wooden test tube stand
2 liter beaker to collect waste solutions

PRESENTATION

This demonstration should accompany lectures on solubility equilibria. The concept of a solubility product is very useful in understanding these experiments.

Precipitation when Ksp is exceeded. Explain to the class that you are about to mix 100 mL of 0.2 M lead nitrate solution with 100 mL of 0.2 M sodium chloride solution. Ask your students what the concentrations of lead and chloride ions will be in the resulting mixture. Most will volunteer 0.2 M without thinking about it. This is wrong because to volume has doubled. The concentrations are now 0.1 M for both ions. These concentrations will give an ion product greater than Ksp and precipitation should occur.

Perform the experiment. A precipitate of lead chloride should form.

What happens when Q < K? Do a tenfold dilution of the lead nitrate and sodium chloride solutions by pouring 10 mL of each into a graduated cylinder and diluting to 100 mL with distilled water. Redo the discussion of the concentrations after the solutions are mixed. Both should then be 0.01 M, and the ion product will be less than Ksp.

Perform the experiment. Nothing will happen.

Common ion effect (le Chatelier principle). Pour 100 mL of saturated lead chloride solution into a test tube and ask the class what will happen if the concentration of chloride ion were to be increased by adding some chloride salt or some hydrochloric acid. Show that when the concentration of chloride ion is increased, the ion product again exceeds Ksp and precipitation will occur.

Perform the experiment. A precipitate of lead chloride should form.

DISCUSSION

The equilibrium constant Ksp for the solubility of lead chloride in water

PbCl2(s) + H2O(l) Pb2+(aq) + 2 Cl-(aq)

is around 2.4 ´ 10-4. This means that in a true saturated solution the product of the ion concentrations equals this number:

[Pb2+(aq)][Cl-(aq)]2 = Ksp = 2.4 ´ 10-4

Note that there are no restrictions on the magnitudes of thes concentrations other than the fact that their product must equal 2.4 ´ 10-4.

Ksp Exceeded. When the product exceeds Ksp the solution is supersaturated and precipitation occurs until the concentrations become small enough that their product does equal Ksp. If the product is less than Ksp, the saolution is unsaturated and nothing happens. The solution still has the capacity to dissolve more stuff.

If more of either ion is added to a saturated solution so that the concentration product now exceeds Ksp, precipitation again occurs until the concentration product again reaches the Ksp magnitude. This is sometimes called the common ion effect, but it is just an equilibrium shift phenomenon governed by the le Chatelier principle.

HAZARDS

Just don’t drink any of the lead solutions.

 

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