What it shows
A chemical cell turns the energy of a redox reaction into electricity. Zinc and copper in their salt solutions, joined by a salt bridge, give a voltage of about 1 volt that falls as the reaction proceeds.
The science
Zinc is more reactive than copper. At the zinc strip, Zn gives Zn2+ + 2 e-, which pass through the wire. At the copper strip, Cu2+ + 2 e- gives Cu. The salt bridge lets ions move between the glasses so charge does not build up. Under standard conditions the cell voltage is 0.76 + 0.34 = 1.10 volts. Real voltage is a little lower and falls when the solutions change in concentration, as the Nernst equation predicts.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A zinc strip (from a dead dry cell case or a galvanised nail ground clean), 5 cm
- A copper strip or thick copper wire, 5 cm
- Copper sulphate, 5 g, in 100 ml of water
- Epsom salt (magnesium sulphate) or zinc sulphate, 10 g, in 100 ml of water
- Two glasses
- A strip of filter paper or tissue soaked in table salt solution (the salt bridge)
- A digital multimeter and two crocodile clips
- A red LED for a series test
Safety
For this project: An adult must supervise. Copper sulphate is harmful if swallowed and irritates the skin and eyes; wear gloves and goggles. Do not put cells in the mouth. Dispose of the solution as the teacher directs.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sand both metal strips until bright, then rinse and dry them.
Put the copper sulphate solution in one glass and the zinc side solution in the other.
Stand the zinc strip in its solution and the copper strip in its solution.
Fold the wet salt-water paper strip into a bridge and place one end in each glass so it dips into both liquids.
Connect a multimeter on the voltage range, with the red lead on copper and the black on zinc. Record the open-circuit voltage.
Remove the bridge and see that the reading drops to zero. Put it back and see it return, to show that the bridge completes the circuit.
Dilute the copper sulphate by half and read the voltage again to see how the concentration changes the reading.
Make two cells and connect them in series. Try to light a red LED, which needs about 1.8 to 2 volts to glow, and record what happens.
What you should see
A voltage close to 1 volt between zinc and copper, falling to zero without the bridge. Two cells in series reach about 2 volts and make the LED glow faintly.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which metal is the negative electrode?
Zinc, because it loses electrons.
2What is the job of the salt bridge?
It lets ions flow to keep both solutions electrically neutral and completes the circuit.
3Why does the cell voltage fall with use?
The concentrations of reactants change and the reaction moves towards equilibrium.
4What is the standard cell potential of the Daniell cell?
About 1.10 volts.
Take it further
Replace zinc with an iron nail or an aluminium strip and compare the voltages to build a small electrochemical series.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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