Exothermic metal-acid reactions

Class practical and Demonstration

Students add powdered or finely-divided metals to hydrochloric acid and measure the temperature changes. The experiment reinforces ideas about energy changes during reactions, the reactivity series of the metals, and the chemical behaviour of acids.
 

Lesson organisation

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.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection throughout. 

Dilute hydrochloric acid, HCl(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Powdered metals: aluminium, Al(s), copper, Cu(s), iron, Fe(s), magnesium, Mg(s) and zinc, Zn(s), (all HIGHLY FLAMMABLE) - see CLEAPSS Hazcards.

Hydrogen gas, H₂(g), (EXTREMELY FLAMMABLE) - see CLEAPSS Hazcard. 

1 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.

2 Each group of students can be provided with a 250 cm³ beaker containing about 100 cm³ of 2 M hydrochloric acid.

3 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) and dried before being provided to students. Carry out degreasing in a fume cupboard.

4 The metal samples can be provided in labelled plastic weighing boats, small beakers or watch glasses to prevent cross-contamination or wastage. 2 spatula measures of each metal should be more than sufficient for each group.

5 Disposal: provide a number of bowls (e.g. 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

a Put the cup into the empty 250 cm³ beaker so that the cup is more stable.

b Measure 20 cm³ of hydrochloric acid into the polystyrene cup.

c Measure and record the starting temperature of the acid.

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. Record the rise in temperature.

f Discard the residues into the plastic bowl provided. Do not pour the metal and acid mixture into the sink.

g 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.

Health & Safety checked February 2008