Exothermic metal displacement reactions

In this practical, students add powdered or finely-divided metals to a copper(II) sulfate solution and measure the temperature rises. The experiment reinforces ideas about energy changes during reactions, the reactivity series of the metals and the chemical behaviour of metals.

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

Equipment

Apparatus

• Eye protection
• Expanded polystyrene cups standing in glass beakers for stability, x4 (see note 5 below)
• 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 6 below)

Chemicals

• Copper(II) sulfate(VI) solution, 1.0 M (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 100 cm³
• Powdered or finely-divided metals, using two spatula measures of each:
  • Iron (HIGHLY FLAMMABLE)
  • Magnesium (HIGHLY FLAMMABLE)
  • Zinc (HIGHLY FLAMMABLE)
  • Tin

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Copper(II) sulfate(VI) solution, CuSO₄(aq), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.
• Powdered or finely divided metals: iron, Fe(s), magnesium, Mg(s), zinc, Zn(s), (all HIGHLY FLAMMABLE) and tin, Sn(s) – see CLEAPSS Hazcards HC055A, HC059A, HC107 and HC102A. Iron filings tend to be greasy and should be degreased using propanone (HIGHLY FLAMMABLE, IRRITANT, see CLEAPSS Hazcard HC085A) and dried before being used. Carry out degreasing in a fume cupboard.
• 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.
• 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.

Procedure

1. Measure 20 cm³ of the copper(II) sulfate(VI) solution into a polystyrene cup.
2. Put the cup into a beaker so that it does not fall over.
3. Measure and record the temperature of the solution.
4. Add the first of the powdered metals and stir the mixture with the thermometer. 
5. Observe the temperature over the next few minutes until a maximum temperature is reached.
6. Record the temperature rise.
7. 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.