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 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two zinc strips, 5 cm each (from a hardware shop, or from the case of a dead zinc-carbon cell opened by an adult, or a galvanised nail ground clean)
- Two copper strips or pieces of thick copper wire, 5 cm each
- Copper sulphate, 10 g, in 200 ml of water
- Zinc sulphate (agri zinc micronutrient), 20 g, in 200 ml of water (Epsom salt also works as a substitute, but the voltage may differ a little)
- Five small glasses or cups
- Two strips of filter paper or tissue soaked in table salt solution (the salt bridges)
- A digital multimeter and four crocodile clips
- A red LED and a 100 ohm resistor for a series test
Safety
For this project: An adult must supervise. Copper sulphate and zinc sulphate are harmful if swallowed and irritate the skin and eyes; wear gloves and goggles. Do not put cells in the mouth. Dispose of the solutions 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.
Step by step
9 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.
For the first cell, put 100 ml of the copper sulphate solution in one glass and 100 ml of the zinc sulphate solution in another. Keep the rest for the second cell.
Stand one zinc strip in the zinc solution and one copper strip in the copper 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 or becomes unsteady. Put it back and see it return, to show that the bridge completes the circuit.
In the fifth glass make a ten times weaker copper sulphate solution (take 10 ml from the first copper glass and top it up to 100 ml with water). Move the copper strip and one end of the bridge into it and read the voltage again. It should fall by only about 0.03 volt, which shows that concentration has a small effect.
Build the second cell in two more glasses with its own salt bridge and connect the two cells in series, copper of one to zinc of the other. Read the total voltage across the two free ends, about 2 volts.
Join a red LED and the 100 ohm resistor across the two free ends, with the LED’s longer leg towards the copper end. Record what happens: the cells have high internal resistance, so the LED may glow only faintly or not at all.
What you should see
A voltage close to 1 volt between zinc and copper, falling to zero or flickering without the bridge. Two cells in series read about 2 volts when nothing is connected, but the voltage drops when the LED is joined, and the LED glows faintly or not at all.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Which metal is the negative electrode?
Zinc, because it loses electrons.
02What is the job of the salt bridge?
It lets ions flow to keep both solutions electrically neutral and completes the circuit.
03Why does the cell voltage fall with use?
The concentrations of reactants change and the reaction moves towards equilibrium.
04What 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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