Reactivity of Metals » Extracting Metals by Reduction

Where metals are found

Metals come from the Earth's crust, but most of them are not lying around as shiny metal. How a metal is found depends on how reactive it is.

Unreactive metals such as gold are found in the Earth as the metal itself. Gold does not react with oxygen or water, so over millions of years it stays as gold. Miners can find it as flakes, grains or nuggets. A metal found like this is called a native metal. Silver and platinum can also be found native.

Most metals are more reactive than this. They have reacted with other elements, such as oxygen or sulfur, and are found as compounds in rocks. To get the metal out, a chemical reaction is needed. Getting a metal out of its compound is called extraction.

A rock that contains enough of a metal compound to make extracting the metal worthwhile is called an ore. Many ores contain metal oxides, which is why this lesson focuses on oxides.

Key terms:

• Native metal: a metal found in the Earth as the uncombined metal itself, not as a compound.
• Extraction: getting a metal out of its compound using a chemical reaction.
• Ore: a rock containing enough of a metal or metal compound to make extracting the metal worthwhile.

Using carbon to extract metals

Remember, reduction is the loss of oxygen. To get a metal from its oxide, we need to take the oxygen away, so the metal oxide must be reduced.

Carbon can do this job. In the reactivity series, carbon sits between magnesium and zinc. The key rule is:

When a metal oxide is heated with carbon, the carbon takes the oxygen away from the metal. This only works if carbon is more reactive than the metal, because the more reactive element ends up with the oxygen.

Carbon is a good choice for the metals it can extract because it is cheap and easy to get, for example as coke made from coal.

Spotting oxidation and reduction

In every reduction with carbon, two things happen at once. The metal oxide loses oxygen, so it is reduced. The carbon gains oxygen, so it is oxidised. The carbon usually becomes carbon dioxide or carbon monoxide.

Here is copper oxide heated with carbon powder:

copper oxide + carbon → copper + carbon dioxide
2CuO + C → 2Cu + CO₂

The black copper oxide powder turns into pink-brown copper.

• Copper oxide (CuO) loses oxygen and becomes copper (Cu). Copper oxide is reduced.
• Carbon (C) gains oxygen and becomes carbon dioxide (CO₂). Carbon is oxidised.

A quick way to do this: look at each substance on the left, then find it on the right. If it has lost its oxygen it was reduced. If it has picked up oxygen it was oxidised.

Using information about extraction

You do not need to learn the details of any industrial extraction process. But you may be given information about one and asked to explain or judge it. Use the reactivity series and the idea of oxidation and reduction.

When you evaluate a method, think about points like these:

• Will it work? Carbon only reduces oxides of metals below it in the reactivity series.
• Cost: carbon is cheap, so reduction with carbon is usually a cheaper way to extract a metal than other methods.
• Energy: the oxide and carbon must be heated to a high temperature, which uses fuel.
• Waste gases: the carbon becomes carbon dioxide or carbon monoxide. Carbon monoxide is toxic, and carbon dioxide is a greenhouse gas.

Only use the facts you are given plus what you know. Do not guess at details of a process that the question does not tell you.

Exam-style question

Nickel can be extracted by heating nickel oxide with carbon.

NiO + C → Ni + CO

(a) Name the substance that is reduced in this reaction. Explain your answer. [2 marks]

(b) What does this reaction tell you about the reactivity of nickel compared with carbon? [1 mark]

(c) Gold is found in the Earth as the metal itself. Explain why gold does not need to be extracted by reduction. [2 marks]