Extraction of Metals and Redox

A few very unreactive metals, such as gold, are found in the Earth as the metal itself. These are called native metals: because gold reacts with almost nothing, it stays as pure element rather than combining with other substances.

Most metals are much more reactive than gold, so over millions of years they have reacted with other elements to form compounds. These metal compounds are dug up as ores — rocks containing enough of the compound to make extraction worthwhile. To get the pure metal out, you must reverse the reaction that formed the compound, using a chemical reaction to separate the metal from whatever it is joined to.

A metal's reactivity decides how it is found and how it is extracted. Unreactive metals are found native; reactive metals are found as compounds and need chemical extraction.

The position of a metal in the reactivity series therefore predicts the method needed to extract it. This links directly to the reactivity series you met in the previous lesson.

Many metals are found as oxides, or as ores that can be turned into oxides. A metal that is less reactive than carbon can be extracted from its oxide by heating it with carbon. The carbon is more reactive than the metal, so the carbon takes the oxygen away from the metal oxide. Removing oxygen is reduction, so the metal oxide is reduced to the metal.

The classic example is extracting iron from iron(III) oxide:

$2F{e}_2{O}_3+3C\to 4Fe+3C{O}_2$

Here the iron oxide loses its oxygen, so the iron is reduced. The carbon gains oxygen (it becomes carbon dioxide), so the carbon is oxidised.

Metal position vs carbon	Extraction method	Example
Below carbon	Reduction by heating with carbon	Iron, zinc, copper
Above carbon	Electrolysis	Aluminium, sodium

Carbon can only extract a metal that is less reactive than carbon. Metals above carbon (like aluminium) hold their oxygen too strongly for carbon to remove, so electrolysis is used instead.

You are only expected to know reduction of oxides with carbon; the fine detail of industrial furnaces is not required.

Because oxidation and reduction always happen together, a reaction where one substance loses oxygen must have another substance gaining it. A reaction involving both is called a redox reaction (reduction and oxidation).

Worked example. Copper oxide is heated with carbon:

$2CuO+C\to 2Cu+C{O}_2$

Work through it step by step:

1. The copper oxide loses oxygen to become copper, so copper is reduced.
2. The carbon gains oxygen to become carbon dioxide, so carbon is oxidised.
3. Both happen at once, so this is a redox reaction.

The general test: whichever substance ends up joined to oxygen has been oxidised; whichever loses its oxygen has been reduced. Using the oxygen definition, you can label any extraction equation correctly.

In every carbon-reduction extraction, the metal is reduced and the carbon is oxidised. Name both changes in a redox question, not just one.

(Higher Tier only) The electron definition of oxidation and reduction below is assessed at Higher Tier.

The deeper definition of oxidation and reduction is about electrons, not oxygen. Use the memory aid OIL RIG:

• Oxidation Is Loss of electrons
• Reduction Is Gain of electrons

When magnesium reacts with oxygen, the magnesium atoms lose electrons to become $M{g}^{2+}$ ions, so the magnesium is oxidised. The oxygen atoms gain those electrons to become $O^{2-}$ ions, so the oxygen is reduced. This matches the oxygen definition (magnesium gains oxygen and is oxidised) but explains what is really happening to the electrons.

$Mg\to M{g}^{2+}+2e^{-}\text{(oxidation: loses electrons)}$

$O_2+4e^{-}\to 2O^{2-}\text{(reduction: gains electrons)}$

(Higher Tier) The oxygen definition and the electron definition agree. Gaining oxygen and losing electrons both mean oxidation.

(Higher Tier only) Writing ionic equations for displacement reactions is assessed at Higher Tier.

A displacement reaction is a redox reaction: the more reactive metal loses electrons (is oxidised) and the ions of the less reactive metal gain electrons (are reduced). An ionic equation shows only the species that change, leaving out spectator ions.

Worked example. Magnesium is added to copper(II) sulfate solution. The full equation is:

$Mg+CuS{O}_4\to MgS{O}_4+Cu$

The sulfate ion ($S{O}_4^{2-}$) appears on both sides unchanged, so it is a spectator ion and is cancelled. The ionic equation is:

$Mg+C{u}^{2+}\to M{g}^{2+}+Cu$

Now identify the electron transfer using two half equations:

$Mg\to M{g}^{2+}+2e^{-}\text{(magnesium loses electrons: oxidised)}$

$C{u}^{2+}+2e^{-}\to Cu\text{(copper ions gain electrons: reduced)}$

The magnesium is oxidised and the copper ions are reduced. This is why displacement goes in one direction only: the more reactive metal gives up electrons more readily, so it becomes the ion while the less reactive metal is deposited as the element.

1. Saying carbon can extract any metal

Carbon can only reduce the oxide of a metal that is less reactive than carbon. Metals above carbon in the reactivity series, such as aluminium, are extracted by electrolysis, not by carbon.

2. Only naming one half of a redox change

A redox question usually wants both changes. In carbon reduction, state that the metal is reduced (loses oxygen) and that the carbon is oxidised (gains oxygen).

3. Confusing OIL RIG

Oxidation Is Loss of electrons; Reduction Is Gain of electrons. It is easy to swap them under pressure. Losing electrons means becoming more positive, which is oxidation.

4. Leaving spectator ions in an ionic equation (HT)

In an ionic equation, cancel any ion that is unchanged on both sides, such as the sulfate ion. Only the species that actually react should appear.

5. Mixing the two definitions incorrectly (HT)

Gain of oxygen and loss of electrons are both oxidation; loss of oxygen and gain of electrons are both reduction. Do not pair "gain of oxygen" with "gain of electrons".

6. Forgetting native metals exist

Not every metal must be chemically extracted. Very unreactive metals such as gold are found native, as the uncombined element, precisely because they are so unreactive.