GCSE · Chemistry · Edexcel · Spec 1CH0
Corrosion of metals as oxidation
An iron nail rusts. Lithium very quickly goes dull. Aluminium shrugs it off. Three metals meet oxygen and get three different endings. Why?
Chemistry · Corrosion
Same idea, different endings
Pick a situation, then pick the category it belongs in. You'll see the reason straight after.
Still to sort
Rusting (0)
Iron or steel, with oxygen and water together.
Where the line is: Only iron and steel rust. And they need oxygen (air) and water at the same time.
Tarnishing (0)
A Group 1 metal oxidises very quickly and gets a thin, dull oxide coat.
Where the line is: Tarnish is a thin discoloured layer on the surface. It causes no structural damage, so it is not rusting.
Protective oxide layer (0)
A thin, stable oxide that sticks strongly and stops further oxidation.
Where the line is: Rust is porous and flakes off, so corrosion carries on. A protective layer stays put and stops it.
No rusting (0)
Iron or steel, but a needed reactant is missing.
Where the line is: Rusting needs iron, oxygen and water together. Take one away and it doesn't rust.
Every metal here can meet oxygen. Decide what actually happens to each one, then read why.
Rusting, half 1: what iron does
Follow the electrons. Tap each part to see what it tells you.
Tap a molecule to see what it does in the reaction.
Rusting, half 2: what oxygen does
Now the other side of the swap. Those 4e⁻ have to come from somewhere.
Tap a molecule to see what it does in the reaction.
Predict, then check
Two iron nails, both with plenty of air. Only the water is different.
One nail is wet with pure water. The other is wet with salty water. Which rusts faster?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
iron + oxygen + water → hydrated iron(III) oxide
That's rusting. Iron gains oxygen, so iron is oxidised — and not every metal's story ends this way.
What you need to know
- Metals react with oxygen to form metal oxides, and that reaction is called oxidation.
- Corrosion is the gradual deterioration of a substance as it reacts with its environment, like a metal oxidising in air.
- Corrosive chemicals are different: they damage living tissue and materials immediately, on contact.
Have a goSam says: "Corrosion and corrosive chemicals are the same thing — both are slow." Is Sam right?
No. Corrosive chemicals damage on contact, straight away.
Sam has merged two different things: corrosion is gradual, whereas corrosive chemicals act immediately.
- Most metals oxidise slowly, but Group 1 metals such as lithium oxidise very quickly, so their surface tarnishes.
- Tarnishing is a thin, dull coat of metal oxide, and it causes no structural damage.
Have a goLithium and a typical metal are left in air. Which one would you expect to go dull faster?
Lithium.
Group 1 metals oxidise very quickly, whereas most metals oxidise slowly — so "all metals tarnish at the same speed" is wrong.
- Rusting is corrosion of iron and steel only, and it needs iron, oxygen (air) and water together.
Have a goMaya announces that her aluminium bike frame is rusting. Has she picked the right word?
No. Only iron and steel rust.
Other metals can still oxidise, but "rusting" is the corrosion of iron and steel only.
- In rusting, iron gains oxygen to form iron oxide, so iron is oxidised: iron + oxygen + water → hydrated iron(III) oxide.
- Electron view: iron loses electrons (oxidation), oxygen gains them (reduction), so electrons move from iron to oxygen.
- Rust is porous and flakes off, so more iron corrodes; aluminium's thin, stable oxide layer sticks strongly and stops further oxidation.
- Salty water speeds up rusting, because as an electrolyte its ions help the electrochemical reactions along.
The big picture
Corrosion is the gradual oxidation of a metal. Iron and steel rust when oxygen and water are both present, Group 1 metals tarnish, and aluminium is protected by its own oxide layer. Rusting is oxidation both as iron gaining oxygen and as iron losing electrons to oxygen.
Key points
Worked example
Problem
A classmate writes: "In rusting, oxygen is oxidised because it gains electrons." Use the half equations Fe → Fe²⁺ + 2e⁻ and O₂ + 2H₂O + 4e⁻ → 4OH⁻ to decide whether that's right.
⚠ Watch out
Calling any dull or flaky metal surface "rust". Rust is only for iron and steel. A Group 1 metal going dull is tarnishing, and a corrosive chemical is a different thing again.
Memory hook
Iron hands its electrons over, oxygen catches them. The giver is oxidised; the catcher is reduced.
Check yourself
Cover the page. Rusting is oxidation in two senses. What does iron gain, what does it lose, and which way do the electrons go?
Flashcards
(14)What is oxidation, when a metal reacts with oxygen?
What is corrosion?
What can corrosion do to a structure?
How is a corrosive chemical different from corrosion?
What is tarnishing?
Does tarnishing cause structural damage?
Which metals can rust?
What three things does rusting need together?
Word equation for rusting?
Iron half equation, and what is it called?
Oxygen half equation, and what is it called?
Why does iron keep corroding once it has rusted?
Why is aluminium protected?
What does salty water do to rusting, and why?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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