Exothermic metal–acid reactions

In this set of experiments, students add powdered or finely-divided metals – including magnesium, zinc, iron and copper – to hydrochloric acid and measure the temperature changes.

Try them with groups of students, or set them up as teacher demonstrations, to reinforce key ideas about energy changes during reactions, the reactivity series of the metals and the chemical behaviour of acids.

The hydrochloric acid used in this experiment is relatively concentrated, and the temperature rises can be quite significant. This experiment is, therefore, best suited to students who can be trusted to behave sensibly. Appropriate eye protection must be worn throughout the experiment and during any clearing up session.

Alternatively, the experiments could be done as demonstrations – perhaps making use of a flexicamera, digital temperature probe and projector, so that all the students can observe the reactions as well as view the temperature changes. An additional teacher-only demonstration of the reaction of aluminium powder with the acid could be included.

Hydrogen gas (EXTREMELY FLAMMABLE) is released in each reaction. Care should be taken to ensure that there are no naked flames or other sources of ignition in the laboratory.

Equipment

Apparatus

• Eye protection
• Polystyrene cup or cups (see note 6 below)
• Beaker, 250 cm³, to stand the polystyrene cup in for support
• Measuring cylinder, 25 cm³, x4
• Thermometer, -10 ˚C–110˚C, or similar
• Spatula
• Absorbent paper

Chemicals

• Hydrochloric acid, approximately 2 M (IRRITANT) (see note 3 below)
• Powdered or finely-divided metals: magnesium, zinc, iron, copper (ALL HIGHLY FLAMMABLE) (see notes 4 and 7)
• Large bowl for collecting the waste residues from the experiments (see note 8)

For the additional teacher-only demonstration

• Set of apparatus and chemicals as above
• Aluminium powder (HIGHLY FLAMMABLE) (see note 4)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Dilute hydrochloric acid, HCl(aq), (IRRITANT) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043. Each group of students can be provided with a 250 cm³ beaker containing about 100 cm³ of 2 M hydrochloric acid.
• Powdered metals: aluminium, Al(s), copper, Cu(s), iron, Fe(s), magnesium, Mg(s) and zinc, Zn(s), (all HIGHLY FLAMMABLE) – see CLEAPSS Hazcards HC001A, HC026, HC055A, HC059A and HC107. The metals should be provided as powders or fine filings, or fine turnings, rather than fine powders or ‘dust’ (which are likely to be significantly oxidised). ‘Powder’ refers to metal which is obviously ‘gritty’. Iron filings tend to be greasy and may need to be degreased with propanone (HIGHLY FLAMMABLE, IRRITANT – refer to CLEAPSS Hazcard HC085A) and dried before being provided to students. Carry out degreasing in a fume cupboard.
• Hydrogen gas, H₂(g), (EXTREMELY FLAMMABLE) – see CLEAPSS Hazcard HC048. 
• Typical expanded polystyrene cups fit snugly into 250 cm³ squat form beakers. This provides a more stable reaction vessel. It also prevents spillage if the student accidentally pokes the thermometer through the bottom of the polystyrene cup. A desirable, but not essential, addition is the provision of 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.
• The metal samples can be provided in labelled plastic weighing boats, small beakers or watch glasses to prevent cross-contamination or wastage. Two spatula measures of each metal should be more than sufficient for each group.
• For disposal, provide a number of bowls (eg washing-up bowls) for the metal residues. Do not let students discard residues into sinks. Any unreacted metal can be dissolved in dilute acid (in a fume cupboard). After neutralisation, the residue can then be poured down the foul water drain with a bucket of water.

Procedure

1. Put the cup into the empty 250 cm³ beaker so that the cup is more stable.
2. Measure 20 cm³ of hydrochloric acid into the polystyrene cup.
3. Measure and record the starting temperature of the acid.
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. Record the rise in temperature.
6. Discard the residues into the plastic bowl provided. Do not pour the metal and acid mixture into the sink.
7. Rinse and dry the polystyrene cup. Then repeat the experiment using each of the other metals in turn. Use a fresh cup if necessary.

Teaching notes

Sample results 

The rise in temperature and vigour of the reactions will obviously depend mainly on the surface area of the metals and the actual concentration of the acid. Here are likely results:

• Magnesium, 60 ˚C (vigorous reaction)
• Zinc, 9 ˚C (moderately brisk reaction)
• Iron, 5 ˚C (slow to start, then fairly brisk reaction)
• Copper, 0–8 ˚C (but the copper will almost certainly appear cleaner as any oxide film reacts with the acid)

The reaction with aluminium (optional teacher-only demonstration) is quite spectacular! The temperature rises only by around 5 ˚C over about 15 minutes and then, after about 25–35 minutes, accelerates to a vigorous reaction with a temperature rise approaching 80 ˚C.

Aluminium has a protective oxide film on its surface. When this has reacted, the aluminium shows something of its true reactivity. It may make sense to demonstrate this reaction at the beginning of the activity (before the class start their own experiments) and then report the progress from time to time.