Exothermic metal displacement reactions

Read our standard health & safety guidance

Lesson organisation

The finely-divided metals should be distributed on labelled plastic weighing dishes or watch glasses to avoid cross-contamination and wastage.

Apparatus and chemicals

Eye protection

Each working group requires:

Expanded polystyrene cups standing in glass beakers for stability, 4 (see note 1)
Measuring cylinder (25 cm³)
Thermometer, -10 °C to +110 °C or similar
Large bowl or bucket for collecting the residues from experiments. Do not let students put residues into sinks. (see note 2)

Copper(II) sulfate solution, 1.0 mol dm^-3 (Harmful at this concentration, Danger to the environment) 100 cm³
Powdered or finely–divided metals magnesium (Highly flammable), zinc (Highly flammable), iron (Highly flammable), tin (Low hazard). Two spatula measures of each. See note 3.

Technical notes

Copper(II) sulfate solution (Harmful, Danger to the environment) Refer to CLEAPSS Hazcard 27C and Recipe card 19
Magnesium (Highly flammable) Refer to CLEAPSS Hazcard 59A
Zinc (Highly flammable) Refer to CLEAPSS Hazcard 107
Iron powder (Highly flammable) Refer to CLEAPSS Hazcard 55A
Iron filings (Low hazard) Refer to CLEAPSS Hazcard 55A
Tin (*Low hazard) Refer to CLEAPSS Hazcard 102
Propanone (Highly flammable, Irritant) Refer to CLEAPSS Hazcard 85

1 A desirable, but not essential, addition are lids for the polystyrene cups. A lid can be made by cutting a suitably-sized piece from a polystyrene ceiling tile and making a hole for the thermometer.

2 Provide a bowl or bucket for discarding the residues. Metal residues in sinks are almost impossible to remove. Iron particles rust and cause unsightly stains.

3 Iron filings tend to be greasy and should be degreased using propanone (Highly flammable, Irritant) and dried before being used. Carry out degreasing in a fume cupboard.

Procedure

HEALTH & SAFETY: Wear eye protection

a Measure 20 cm³ of the copper(II) sulfate solution into a polystyrene cup.

b Put the cup into a beaker so that it does not fall over.

c Measure and record the temperature of the solution.

d Add the first of the powdered metals and stir the mixture with the thermometer.
e Observe the temperature over the next few minutes until a maximum temperature is reached.

f Record the temperature rise.

g Repeat the procedure with fresh polystyrene cups using each of the other metals.

Teaching notes

The temperature rises should be approximately:

Magnesium 39°C
Zinc 49°C
Iron 32°C
Tin 22°C

The results are approximately in line with the reactivity series of the metals. The ‘wrong’ order of magnesium and zinc might be due to oxidation on the surface of the magnesium being more extensive than for zinc.

Health & Safety checked, February 2008