Oxidation and reduction
There are two ways to define oxidation and reduction. The older definition is in terms of oxygen: oxidation is gaining oxygen, reduction is losing oxygen. So when magnesium burns, Mg gains oxygen and is oxidised: 2Mg + O2 → 2MgO.
The more powerful definition is in terms of electrons, and it is summed up by OIL RIG:
- OIL RIG
- Oxidation Is Loss of electrons; Reduction Is Gain of electrons.
The two always happen together, one species loses the electrons that the other gains, which is why we call the whole thing a redox reaction. For example, when magnesium reacts: Mg → Mg2+ + 2e- (loses electrons, oxidised) while each oxygen atom gains those electrons (reduced).
Track the electrons
If a particle's charge becomes more positive, it has lost electrons, so it was oxidised. If its charge becomes more negative (or less positive), it gained electrons and was reduced.
Reactivity & displacement
The reactivity series lists metals in order of how readily they lose electrons to form positive ions. A rough order, most reactive first: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold.
A more reactive metal will displace a less reactive one from its compound, because it holds onto its electrons less tightly and pushes the other metal out.
Iron is placed in copper(II) sulfate solution. Write what happens and identify what is oxidised.
Extracting iron
How a metal is extracted depends on its reactivity. Metals below carbon (such as iron, zinc, copper) can be extracted by reduction with carbon, which is cheap. Metals above carbon (such as aluminium) are too reactive for this and must be extracted by electrolysis.
Iron is extracted in the blast furnace. The raw materials are iron ore (haematite, mostly Fe2O3), coke (carbon) and limestone. The key steps:
- Coke burns in hot air to make carbon dioxide: C + O2 → CO2.
- More coke reduces this to carbon monoxide: CO2 + C → 2CO.
- Carbon monoxide is the main reducing agent. It removes oxygen from the ore: Fe2O3 + 3CO → 2Fe + 3CO2.
The iron ore is reduced (loses oxygen) while the carbon monoxide is oxidised (gains oxygen). Molten iron runs to the bottom and is tapped off. Limestone removes sandy impurities as molten slag.
Common slip
In the furnace it is mainly carbon monoxide, not solid carbon, that reduces the ore. Learn the equation Fe2O3 + 3CO → 2Fe + 3CO2, and remember all arrows point one way, never write a dash for the arrow.
Electrolysis of a molten salt
Electrolysis uses electricity to break down an ionic compound into its elements. The compound (the electrolyte) must be molten or dissolved so its ions are free to move. Two electrodes dip into it:
- the cathode is the negative electrode, and
- the anode is the positive electrode.
Opposite charges attract, so positive metal ions (cations) move to the negative cathode, and negative non-metal ions (anions) move to the positive anode.
Take molten lead(II) bromide, PbBr2, which contains Pb2+ and Br- ions:
| Electrode | Ion attracted | What happens | Half idea |
|---|---|---|---|
| Cathode (−) | Pb2+ | Gains electrons, reduced to lead metal | Pb2+ + 2e- → Pb |
| Anode (+) | Br- | Loses electrons, oxidised to bromine gas | 2Br- → Br2 + 2e- |
So at the cathode you get a metal (reduction, gain of electrons) and at the anode a non-metal (oxidation, loss of electrons). Electrolysis is redox forced to happen by an electric current. A memory hook: PANIC, Positive is Anode, Negative Is Cathode.
Where this is assessed
Identifying oxidation and reduction and writing electrode processes is Criterion A. Explaining electrolysis or displacement results from experimental observations draws on Criterion C (Processing and evaluating).
Check yourself
1. In the reaction Zn → Zn2+ + 2e-, is zinc oxidised or reduced? +
Zinc loses two electrons. By OIL RIG, oxidation is loss, so zinc is oxidised.
2. Why can copper be extracted using carbon but aluminium cannot? +
Copper is below carbon in the reactivity series, so carbon can reduce copper oxide. Aluminium is above carbon and holds its oxygen too strongly for carbon to remove, so it must be extracted by electrolysis.
3. During electrolysis of molten PbBr2, what forms at the negative electrode and why? +
The positive Pb2+ ions are attracted to the negative cathode, where they gain electrons and are reduced: Pb2+ + 2e- → Pb. So lead metal forms at the cathode.
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