Chemical cells and fuel cells

Chemistry · Energy changes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A simple cell is made from two different metals dipped in an electrolyte, a solution that contains ions. The more reactive metal gives up electrons more easily, so a voltage is produced between the two metals. The bigger the difference in reactivity, the bigger the voltage. Two electrodes of the same metal give no voltage. A battery is two or more cells joined in series.

In a non-rechargeable cell the reaction stops when one reactant is used up. In a rechargeable cell an external current reverses the reactions.

A hydrogen fuel cell is supplied with hydrogen and oxygen. The hydrogen is oxidised and the only product is water.

Rules to remember

Cell = two different metals + an electrolyte
Voltage depends on the two metals and on the electrolyte; a bigger gap in reactivity gives a bigger voltage
Non-rechargeable cells stop when a reactant is used up; rechargeable cells are recharged by an external current
Fuel cell overall: 2H₂ + O₂ → 2H₂O
Negative electrode: H₂ → 2H⁺ + 2e⁻
Positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
Fuel cells: only water is made and no recharging is needed, but hydrogen is flammable and hard to store

Common mistake

Students say a cell stops working because the electrons run out. Electrons are not used up. The cell stops because one of the reacting chemicals has been used up.

Worked example

The order of reactivity is magnesium > zinc > iron > copper. A cell made from zinc and copper gives a certain voltage. The copper is replaced by iron. What happens to the voltage?

The voltage depends on the difference in reactivity between the two metals.
Zinc and copper are far apart in the order, so their difference is large.
Zinc and iron are next to each other in the order, so their difference is small.
A smaller difference in reactivity gives a smaller voltage.

Answer: The voltage becomes smaller.

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