The reactivity series and metal extraction

This page covers the final Form 5 content standard in Redox Equilibrium: the reactivity series and metal extraction. It gathers up everything else in the chapter, oxidation as electron loss, displacement, cells and electrolysis, and applies it to a practical question: how do we get a pure metal out of the rock it is found in? The answer always comes back to the metal’s place in the reactivity series.

The reactivity series

The reactivity series arranges metals in order of how readily they lose electrons to form positive ions, that is, how easily they are oxidised. A common order, most reactive first, is:

potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, tin, lead, (hydrogen), copper, silver, gold.

Carbon and hydrogen are non-metals, but they are placed in the series as useful reference points. A metal higher in the series is more reactive, loses electrons more readily, forms a more stable compound, and is therefore harder to extract. A metal lower in the series (such as gold) is so unreactive it can be found native, as the uncombined element.

You can establish the order experimentally using the redox reactions from earlier in the chapter: displacement (a metal displaces any metal below it from solution), and reactions with oxygen, water and dilute acid (the more reactive the metal, the more vigorous the reaction).

How position decides the extraction method

The more reactive a metal, the more strongly it holds its compound together, so the more energy is needed to free it. This gives three broad routes:

• Very reactive metals, above carbon (potassium, sodium, calcium, magnesium, aluminium). Carbon is not reactive enough to remove their oxygen, so they are extracted by electrolysis of the molten compound. For example, aluminium is obtained by the electrolysis of molten aluminium oxide.
• Moderately reactive metals, below carbon (zinc, iron, tin, lead, copper). Carbon is reactive enough to take the oxygen, so they are extracted by reduction of their oxides with carbon (or carbon monoxide), which is cheaper. This is why iron is smelted with coke in a blast furnace.
• Unreactive metals (silver, gold). They occur native and need little or no chemical extraction.

Extraction by carbon is a redox reaction

Reducing a metal oxide with carbon is textbook redox. The metal oxide loses oxygen (it is reduced) and carbon gains oxygen (it is oxidised), so carbon is the reducing agent. For tin(II) oxide, for example:

SnO₂ + 2C → Sn + 2CO

Tin’s oxidation number falls from +4 to 0 (reduced); carbon’s rises from 0 to +2 (oxidised). The same logic runs the thermite reaction, where aluminium, being above iron, reduces iron(III) oxide to molten iron: 2Al + Fe₂O₃ → 2Fe + Al₂O₃, with aluminium as the reducing agent.

Worked example

Question. Iron is extracted by reducing iron(III) oxide with carbon in a blast furnace: 2Fe₂O₃ + 3C → 4Fe + 3CO₂. State which substance is oxidised and which is reduced, name the reducing agent, and explain in terms of the reactivity series why carbon can be used to extract iron but not aluminium.

Step 1, track oxygen. Iron(III) oxide loses oxygen to become iron, so iron(III) oxide is reduced. Carbon gains oxygen to become carbon dioxide, so carbon is oxidised.

Step 2, check with oxidation numbers. Iron falls from +3 (in Fe₂O₃) to 0 (in Fe): a decrease, so reduction. Carbon rises from 0 to +4 (in CO₂): an increase, so oxidation.

Step 3, name the reducing agent. The substance oxidised is the reducing agent, so carbon is the reducing agent (it reduces the iron(III) oxide).

Step 4, explain the reactivity-series point. Iron is below carbon in the reactivity series, so carbon is more reactive than iron and can pull the oxygen away from iron(III) oxide. Aluminium is above carbon, so carbon is not reactive enough to remove aluminium’s oxygen; aluminium must be extracted by electrolysis of its molten oxide instead.

Answer. Iron(III) oxide is reduced (Fe: +3 to 0); carbon is oxidised (C: 0 to +4) and is the reducing agent. Carbon works for iron because iron is below carbon in the series, but not for aluminium, which is above carbon and needs electrolysis.

Practice question

Copper can be extracted by heating copper(II) oxide with carbon: 2CuO + C → 2Cu + CO₂. Identify the oxidising agent and explain your choice using oxidation numbers.

Answer. Copper falls from +2 (in CuO) to 0 (in Cu), a decrease, so copper(II) oxide is reduced. Carbon rises from 0 to +4, an increase, so carbon is oxidised. The oxidising agent is the species that is reduced, so copper(II) oxide is the oxidising agent (and carbon is the reducing agent).

Exam tip

Learn the reactivity series in order and remember where carbon sits, that single fact answers most extraction questions. State the reason as a comparison: “iron is below carbon, so carbon can reduce its oxide; aluminium is above carbon, so electrolysis is used”. In any carbon-reduction equation, prove the redox with oxygen transfer and oxidation numbers, and name the reducing agent (carbon) explicitly. These are dependable SPM Chemistry marks, and the reasoning links straight back to displacement and cells from earlier standards.

Where this fits in the chapter

This standard closes the loop: the same order of metals you met in displacement, rusting and voltaic cells now explains industrial extraction. If you can rank metals by reactivity, you can predict displacement, cell polarity, corrosion protection and extraction method, the whole chapter in one idea.

• Up to the chapter hub: Redox Equilibrium.
• Sideways: the chapter’s revision notes and the Paper 2 essay guide for extraction essays.

Our online 1-to-1 SPM Chemistry teachers, English medium, from RM50 per hour, drill the “above or below carbon” reasoning so your extraction answers are always justified by the reactivity series, which is exactly what the marking scheme looks for.