Where the Electrons Go: Oxidation and Reduction
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Oxidation is the loss of electrons and reduction is the gain; the two always happen together.
- Rusting needs both oxygen and water, and dissolved salt speeds it up.
- A battery separates the two halves of a redox reaction so the electrons must travel through a circuit.
- A more reactive metal can protect iron by being oxidised in its place.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Losing and gaining electrons
Rusting, burning and a battery running down look like three unrelated things. Chemically they are the same: in each, electrons move from one substance to another. The substance that loses electrons is oxidised, and the one that gains them is reduced. The word oxidation once meant joining with oxygen, but today it means losing electrons, even when no oxygen is involved. A common memory aid is OIL RIG: oxidation is loss, reduction is gain. The two always happen together, because an electron that leaves one atom has to arrive at another. Rust is the everyday example. Iron in damp air gives up electrons and becomes iron ions, while oxygen dissolved in the water takes those electrons. The iron ions end up as flaky orange-brown rust. Water matters because it lets ions move, and salt water matters even more, because dissolved salt carries charge well. That is why cars rust fastest where roads are salted. Remove either oxygen or water and the reaction stalls: a nail kept dry, or in boiled water sealed under oil, stays bright.
Making electrons do work
If electrons are going to move anyway, they can be made to do something on the way. Put a strip of zinc into copper sulfate solution and copper metal coats the zinc: zinc atoms hand electrons straight to copper ions. Now separate the two halves. Zinc sits in one beaker and copper in another, joined by a wire outside and a salt bridge between the liquids. The electrons can no longer jump straight across. The only way from zinc to copper is through the wire, and a bulb in that wire lights up. That is a battery: a redox reaction arranged so the electrons take the long way, through something useful. The salt bridge lets ions drift between the beakers so charge does not build up. Which way the electrons flow depends on which metal gives them up more readily — the same reactivity order as in metals and acids. The same ranking protects steel. A more reactive metal attached to iron is oxidised first, in the iron’s place. Ships and pipes carry blocks of zinc for this reason, and they are replaced as they wear away.
Curriculum links: NGSS HS-PS1-2 · NGSS HS-PS1-5