Silver and lead halides

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Lesson organisation

These reactions can be demonstrated or investigated as a class practical.

Reactions can be confined to the silver halides as part of an investigation of Group 7 chemistry. Using ammonia to distinguish between the silver halides is more appropriate at an advanced level.

Apparatus and chemicals

Eye protection

Each demonstration or working group requires:
Test-tube rack (see note 1)
Test-tubes, 6
Boiling tubes, 3
Test-tube holder (to fit a boiling tube)
Teat pipettes, 2
Beaker (250 cm³ or 500 cm³)
Bunsen burner

For optional part:
Test-tubes, 3
Teat pipettes, 2
Corks or rubber bungs to fit test-tubes, 3

About half a test-tube of each of the following solutions:
Potassium (or sodium) chloride solution about 0.1 mol dm³ (Low Hazard)
Potassium (or sodium) bromide solution about 0.1 mol dm³ (Low Hazard)
Potassium (or sodium) iodide solution about 0.1 mol dm³ (Low Hazard)

Silver nitrate solution, about 0.05 M (Low Hazard but stains fingers and clothing at this concentration, Danger to the environment*) about 1 cm³
Lead nitrate solution, about 0.1 M (Toxic, Danger to the environment) about 1 cm³

For optional part, in a fume cupboard:
Dilute ammonia solution ~0.1 M (Low Hazard)
Concentrated ammonia solution ~8 M (Corrosive, Danger to the environment),
Teat pipettes
Corks or rubber bungs to fit test-tubes, 3

Technical notes

Potassium (or sodium) chloride solution (Low Hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 51 or 63
Potassium (or sodium) bromide (Low Hazard) Refer to CLEAPSS Hazcard 47B
Potassium (or sodium) iodide solution (Low Hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 55
Silver nitrate solution (Low Hazard but stains fingers and clothing at concentration used, Danger to the environment) Refer to CLEAPSS Hazcard 87 and Recipe card 58
Lead nitrate solution (Toxic, Danger to the environment) Refer to CLEAPSS Hazcard 57A, Recipe card 42
Dilute ammonia solution (Low Hazard) Refer to CLEAPSS Hazcard 6 and Recipe card 4, and L195 Safer chemicals, safer reactions
Concentrated ammonia solution (Corrosive, Danger to the environment) Refer to CLEAPSS Hazcard 6 and Recipe card 4

Place sufficient test-tube racks to accommodate 3 test-tubes per group in a brightly lit part of the laboratory – such as on a window sill, and sufficient to accommodate 3 test-tubes per group in a dark part of the laboratory - such as in an empty cupboard.

Procedure

Silver halides

a Pour about 3 cm³ of each of the halide solutions into separate test-tubes.

b Add a few drops of silver nitrate solution to the test-tube containing potassium chloride solution. A white precipitate of silver chloride forms.

c Add a few drops of silver nitrate solution to potassium bromide solution. A cream or off-white coloured precipitate of silver bromide forms.

d Add a few drops of silver nitrate solution to potassium iodide solution. A yellow precipitate of silver iodide forms.

e Pour half the contents of the three test-tubes into another three labelled test-tubes.

f Place one set of three test-tubes in a cupboard and the other set in bright light, such as on a window sill, and leave for 5–10 mins. In bright light, the silver chloride darkens quickly, the silver bromide more slowly, and the silver iodide is not affected at all. Compare with the solutions kept in the dark.

g–j Optional: use ammonia solution to distinguish between the silver halides.

g Prepare fresh samples of the silver halide precipitates as above, a–d.

h Slowly add an equal volume of dilute ammonia solution to the test-tube containing silver chloride using a teat pipette. Shake well after each addition to mix the contents. The precipitate dissolves, giving a colourless solution.

i Add an equal volume of dilute ammonia solution to the test-tube containing silver bromide. Shake to mix. The precipitate does not dissolve. Now add concentrated ammonia solution to almost fill the test-tube, stopper the tube and invert to mix. Most of the precipitate dissolves.

j Add an excess of concentrated ammonia solution to the test-tube containing silver iodide, stopper and invert to mix. The precipitate does not dissolve.

Lead halides

a Pour about 5 cm³ of the potassium halide solutions into separate boiling tubes.

b Add five drops of lead nitrate (Toxic) solution to the test-tube containing potassium chloride solution. A white precipitate of lead(II) chloride forms.

c Heat the mixture carefully over a gentle flame until it boils. Avoid using a yellow tipped flame as it will make the tube sooty. The precipitate dissolves.

d Place the boiling tube in a beaker of cold water to cool. Fine crystals of lead chloride appear.

e Repeat b–d with potassium bromide solution. A white precipitate of lead(II) bromide forms, which dissolves on heating and recrystallises on cooling.

f Repeat b–d with potassium iodide solution. A yellow precipitate of lead(II) iodide forms which dissolves on heating to give a colourless solution. On cooling, fine shimmering yellow crystals of lead(II) iodide form.

Teaching notes

Any spillages of silver or lead nitrate on the skin should be washed off with plenty of water. Silver nitrate causes black stains on the skin which wear off slowly.

These precipitation reactions can be represented by the following equations,
where X = Cl, Br or I:

KX(aq) (or Na) + AgNO₃(aq) → AgX(s) + KNO₃(aq) (or Na)

2KX(aq) + Pb(NO₃)₂(aq) → PbX₂(s) + 2KNO₃(aq)

or the general ionic equations:

X^-(aq) + Ag ⁺(aq) → AgX(s)

2X^-(aq) + Pb²⁺(aq) → PbX₂(s)

The silver chloride experiment can be modified to produce a photographic paper on which an image can be recorded. Here a simple extension is to filter off the freshly prepared silver chloride precipitate (covering the funnel to exclude light), and then opening the filter paper out onto a white tile and placing it in bright light. The silver chloride darkens quickly. Partly covering the precipitate on the paper will emphasise the effect of light.

The decomposition of silver chloride is an example of a photochemical reaction. Silver metal and chlorine atoms are produced.

The optional experiments involving ammonia to distinguish between the silver halides should be tried beforehand. This is because the result depends on the relative amounts of the precipitate and ammonia. Silver chloride and iodide are not usually a problem - the silver iodide loses some of its yellow colour. But the extent to which the silver bromide dissolves depends on the actual concentration of ammonia in the test-tube.

The recrystallisation of lead iodide is particularly eye-catching, producing a shower of fine yellow crystals.

Health and Safety checked, October 2007