Edexcel IGCSE Chemistry · Spec 2.18-2.20
Rusting & Redox
How rusting happens, how to prevent it, and oxidation and reduction (redox).
Chemistry revision video
Rusting & Redox
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Explained
Rusting, and the two definitions of redox
Rusting is the corrosion of iron. Iron reacts with oxygen and water to form hydrated iron(III) oxide, which is what rust is.
Both conditions are needed, and neither alone is enough. That is what the classic three test tube experiment shows: a nail in boiled water with oil on top has water but no oxygen, a nail in dry air with a drying agent has oxygen but no water, and only the nail with both present rusts.
Salt speeds rusting up without being one of the conditions, which is why cars rust faster where roads are salted and near the sea.
The word rust belongs to iron
Only iron and steel rust. Other metals corrode, and the reaction is chemically similar, but it is not rusting and the mark schemes hold to that.
Aluminium is the case worth knowing. It is more reactive than iron, so it ought to corrode faster, and it does not appear to. It reacts immediately with oxygen to form a thin layer of aluminium oxide that sticks tightly to the surface and stops anything reaching the metal underneath. Rust flakes off and exposes fresh iron; aluminium oxide stays put and protects.
Preventing it
Two approaches, and they work in different ways.
Barrier methods keep oxygen and water away from the iron: painting, greasing, coating in plastic, or plating with another metal. They are simple, and they fail as soon as the barrier is scratched, because the exposed iron then rusts normally.
Sacrificial protection uses a more reactive metal, usually zinc or magnesium, attached to or coating the iron. Being more reactive, it loses electrons in preference to the iron, so it corrodes and the iron does not. It goes on protecting even where the surface is broken, and it is used up over time and has to be replaced.
Galvanising is coating iron with zinc, and it is both at once. The zinc is a barrier, and if the barrier is scratched the zinc still protects sacrificially.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme covers this topic in a sequence of short questions, and its rulings line up neatly.
For the conditions, it credits oxygen and water, allowing air for oxygen and allowing moisture, water vapour or steam for water. For the product it wants hydrated iron(III) oxide, allowing ferric oxide or Fe2O3, and adds that if alternatives are offered all of them must be correct. Writing two answers and hoping one is right does not work; a wrong one alongside a right one loses the mark.
For barrier protection it credits that the paint acts as a barrier or protective layer, and that this prevents oxygen, air or water reaching the iron. Both halves again: what it is, and what it stops.
The sacrificial protection question is where the interesting refusal sits. It credits that zinc is more reactive than iron, allowing that zinc is higher in the reactivity series, and then credits that zinc oxidises, reacts, loses electrons in preference to iron, corrodes, forms zinc oxide, or acts as a reducing agent.
Six accepted ways of saying it, and one refused: reject zinc rusts.
Zinc does corrode, and describing it as rusting uses a word reserved for iron. The mark scheme has offered every reasonable alternative and still refuses this one, which tells you how firmly the distinction is held.
Oxidation and reduction
There are two definitions and both are examinable, so learn which applies where.
In terms of oxygen: oxidation is gaining oxygen and reduction is losing it. This is the version used in extraction questions, where iron(III) oxide loses oxygen and is reduced while carbon monoxide gains oxygen and is oxidised.
In terms of electrons: oxidation is loss of electrons and reduction is gain of electrons. OIL RIG is the usual reminder. This is the version used in displacement and electrolysis questions, where no oxygen is involved at all.
The two always happen together, which is why the reactions are called redox. If something is oxidised, something else in the same reaction is reduced.
The agents are named the other way round, which is the part that catches people. An oxidising agent is the substance that oxidises something else, so it is itself reduced. A reducing agent reduces something else, so it is itself oxidised. In the blast furnace, carbon monoxide is the reducing agent and it is oxidised to carbon dioxide.
Spec 2.18-2.20
What you need to know
- State the conditions needed for rusting
- Describe ways to prevent rusting
- Define oxidation and reduction
Active recall
Quick check
Answer each question before opening the answer.
What two things are needed for iron to rust?
Oxygen (from air) and water.
Define oxidation and reduction in terms of electrons.
Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).
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