Reacting zinc and copper(II) oxide

Copper(II) oxide and zinc metal react together in an exothermic reaction to produce zinc oxide and copper. By observing this reaction and its products, and noting the difference in reactivity between zinc and copper, students can familiarise themselves with the idea of competition reactions.

Equipment

Apparatus

• Eye protection
• Bunsen burner
• Tin lid sitting on a tripod (or a strip of ceramic paper)
• Beaker, 100 cm³
• Circuit tester (battery, bulb and leads) (optional)
• Safety screens (optional)
• Test tubes x2 (optional – see Procedure, step 7)
• Test tube rack
• Access to a balance weighing to the nearest 0.1 g

Chemicals

• Copper(II) oxide powder (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 2 g
• Zinc powder (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT), 1.6 g
• Dilute hydrochloric acid, approx 2 M (IRRITANT), 20 cm³
• Zinc oxide (DANGEROUS FOR THE ENVIRONMENT), a few grams
• Copper powder, a few grams
• Concentrated nitric acid (CORROSIVE, OXIDISING), 5 cm³ (optional – see Procedure, step 7)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout. Consider placing safety screens around the experiment (samples of zinc can vary considerably in reactivity, depending on particle size and the state of surface oxidation).
• Copper(II) oxide powder, CuO(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC026.
• Zinc powder, Zn(s), (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT)  – see CLEAPSS Hazcard HC107.
• Dilute hydrochloric acid, HCl(aq), (IRRITANT) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.
• Concentrated nitric acid, HNO₃(aq), (CORROSIVE, OXIDISING) – see CLEAPSS Hazcard HC067.
• Copper powder, Cu(s) – see CLEAPSS Hazcard HC026.
• Zinc oxide, ZnO(s), (DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC108b.

Procedure

1. Weigh out 2 g (0.025 mol) of copper(II) oxide and 1.6 g (0.025 mol) of zinc powder.
2. Mix thoroughly to give a uniformly grey powder.
3. Pour the mixture in the shape of a ‘sausage’ about 5 cm long onto a clean tin lid sitting on top of a tripod (or onto a strip of ceramic paper, folded lengthwise into a V-shape).
4. Heat one end of the ‘sausage’ from above with a roaring Bunsen flame until it begins to glow, then remove the flame. A glow will spread along the ‘sausage’ until it has all reacted. A white/grey mixture will remain.
5. Heat this to show that the white powder (zinc oxide) is yellow when hot and white when cool.
6. Pour the cool residue into a 100 cm³ beaker and add a little dilute hydrochloric acid to dissolve the zinc oxide (and also any unreacted zinc and copper oxide), warming if necessary. Red-brown copper will be left. This can be rinsed with water and passed around the class for observation. Show that the powder conducts electricity using a circuit tester.
7. If further confirmation of identity is required, treat a small amount of the red-brown powder with a few drops of concentrated nitric acid in a test tube in a fume cupboard. A brown gas, nitrogen dioxide, NO₂(g) (VERY TOXIC), is given off as the copper reacts and dissolves. After the reaction adding a little water makes the blue solution of copper(II) nitrate visible.

Teaching notes

The depth of discussion depends on the level of the students involved. Essentially it is a competition between metal(1) and metal(2) for oxygen in a reaction represented by:

Metal(1) + Metal(2) oxide → Metal(1) oxide + Metal(2)

The more reactive metal displaces the less reactive metal from its oxide, as in the case of zinc and copper(II) oxide, for example:

Zn(s) + CuO(s) → ZnO(s) + Cu(s)

Demonstrate that zinc oxide goes yellow when heated and returns to white when cool to help confirm the identity of this product. (This phenomenon is caused by a change in crystal structure – a genuine example of a physical change.)

Where appropriate, it could be pointed out that these reactions are redox reactions, with the more reactive metal behaving as a reducing agent, and the metal oxide acting as an oxidising agent. This could be extended to consider these redox reactions in terms of the loss and gain of electrons by the metals.

Other metals can be used, but take care to compare like with like. Coarse magnesium powder, for example, gives a less vigorous reaction than powdered zinc. Finely powdered magnesium gives a very vigorous reaction and should only be attempted with great care. The reaction between aluminium powder and copper oxide is almost explosive and must not be attempted.