How a chemical cell makes electricity
A cell has two different metals (the electrodes) dipped in a liquid or paste that has ions (the electrolyte). A chemical reaction pushes electrons round a circuit.
- The more reactive metal loses electrons more easily. It becomes the negative terminal.
- Electrons flow through the wire to the less reactive metal (positive terminal).
- Chemical energy is changed into electrical energy.
What sets the voltage? The bigger the difference in reactivity between the two metals, the bigger the voltage. The type of electrolyte and its concentration also change the voltage. Two pieces of the same metal give 0 V.
| Metals | About |
|---|---|
| Zinc and copper | 1.1 V |
| Iron and copper | 0.8 V |
| Magnesium and copper | 2.7 V |
(Values for standard solutions; a simple lemon or salt-water cell gives less.)
Batteries, and rechargeable vs non-rechargeable cells
A battery is two or more cells joined in series (positive of one to negative of the next). The voltages add up, so three 1.5 V cells give 4.5 V.
- Non-rechargeable (like alkaline AA cells): the reaction goes one way only. When a reactant is used up, the cell is "flat" and must be recycled.
- Rechargeable (like lithium-ion and lead–acid): an outside electric current forces the reaction to go backwards, rebuilding the reactants. They can be used many times.
Hydrogen fuel cells
A fuel cell is supplied with a fuel and oxygen (or air) from outside. In a hydrogen fuel cell, hydrogen is oxidised to make water, and the energy comes out as electricity, not as a flame.
Overall: 2H₂ + O₂ → 2H₂O
Half equations (acid electrolyte):
- Negative electrode: 2H₂ → 4H⁺ + 4e⁻ (oxidation)
- Positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction)
The H⁺ ions travel through the electrolyte (a membrane) while the electrons travel through the outside circuit.
Fuel cells compared with rechargeable batteries
| Hydrogen fuel cell | Rechargeable battery | |
|---|---|---|
| Waste | Only water at the point of use | Toxic metals when thrown away |
| Running time | As long as fuel is supplied; refuel in minutes | Needs hours to recharge |
| Life | Lasts long | Wears out after many charges |
| Problems | Hydrogen is a flammable gas, hard to store; most hydrogen is still made from fossil fuels | Electricity to charge may come from fossil fuels |
So a fuel cell is only truly "clean" if the hydrogen is made using renewable electricity.
Try it: a lemon cell
Push a galvanised (zinc-coated) nail and a copper coin or copper wire into a lemon, about 2 cm apart. Touch the leads of a cheap multimeter to them: you should read about 0.9 V. Predict what happens with two nails of the same metal (about 0 V), or with three lemons joined in series (about three times more). Do not eat the lemon afterwards.
Key formulas and definitions
- Bigger difference in reactivity → bigger cell voltage
- Battery voltage (series) = sum of the cell voltages
- Same metal on both electrodes → 0 V
- Fuel cell overall: 2H₂ + O₂ → 2H₂O
- Negative electrode: 2H₂ → 4H⁺ + 4e⁻
- Positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
Worked examples
1. A cell is made with zinc and copper. Which metal is the negative terminal and which way do electrons flow?
Zinc is more reactive, so it loses electrons and is negative. Electrons flow through the wire from zinc to copper.
2. Order these pairs by voltage, smallest first: Fe–Cu, Mg–Cu, Zn–Cu.
Iron is closest to copper in reactivity, then zinc, then magnesium. So Fe–Cu < Zn–Cu < Mg–Cu.
3. A torch needs 4.5 V. How many 1.5 V cells are needed and how are they joined?
4.5 ÷ 1.5 = 3 cells, joined in series (+ to −) so that their voltages add.
4. Give two reasons a city might choose hydrogen fuel-cell buses, and one reason against.
For: only water comes out of the exhaust, and refuelling takes minutes. Against: hydrogen is hard to store safely and is often made from fossil fuels, so it is not always low-carbon.
Common mistakes
- Thinking a cell "stores electricity". It stores chemicals; the reaction makes the electric current.
- Saying any two pieces of metal make a cell. The metals must be different (and need an electrolyte).
- Saying fuel cells give no pollution at all. Making and storing hydrogen can produce pollution.
- Mixing up which metal is negative: the more reactive metal is the negative terminal.