Revision notes for AQA GCSE Chemistry Extraction of metals and reduction. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Extraction of metals and reduction
What you'll learn
- Why gold can be found as the metal itself, but most metals are found in compounds.
- How the reactivity series tells you whether carbon can extract a metal from its oxide.
- What reduction and oxidation mean in terms of oxygen.
- How to identify what has been reduced or oxidised in a metal extraction reaction.
Starting point: where metals are found
An element is a substance made of only one type of atom. A compound is a substance made when two or more different elements are chemically joined together.
Some very unreactive metals, such as gold, do not easily react with substances like oxygen or water. This means they can be found in the Earth as the metal itself.
Most metals are more reactive than gold. Over millions of years, they have reacted with other elements to form compounds, such as metal oxides.
A metal oxide is a compound containing a metal chemically joined to oxygen, for example copper(II) oxide, CuO(s), or iron(III) oxide, Fe₂O₃(s).
Definition
Native metal
A native metal is a metal found naturally as the uncombined element, rather than as part of a compound. Gold is the key GCSE example.
What “extraction” means
Extraction means obtaining a useful metal from the substances found in the Earth.
Many metals are found in rocks called ores. An ore contains enough of a metal, or a metal compound, for extraction to be worthwhile.
Definition
Ore
An ore is a rock that contains enough metal or metal compound to make it economic to extract the metal.
If the metal is already present as the element, like gold, extraction may mainly involve separating it from rock. But if the metal is locked inside a compound, a chemical reaction is needed to turn the compound into the metal.
The reactivity series decides the extraction method
The reactivity series lists elements in order of how readily they react. The most reactive are at the top.
Carbon and hydrogen are included as useful comparison points, even though they are not metals. For this topic, carbon is the important one.
Metals less reactive than carbon can be extracted from their oxides by heating with carbon. Metals more reactive than carbon cannot be extracted from their oxides using carbon; they need a more reactive method, such as electrolysis.
Key Idea
The carbon boundary
If a metal is below carbon in the reactivity series, carbon can remove oxygen from its oxide. If a metal is above carbon, carbon is not reactive enough to do this.
Example
Choosing whether carbon can extract a metal
A question says zinc oxide, aluminium oxide and copper(II) oxide are available. Which can be reduced by carbon?
- Compare each metal with carbon in the reactivity series: zinc and copper are below carbon, while aluminium is above carbon.
- Carbon is more reactive than zinc and copper, so it can take oxygen from zinc oxide and copper(II) oxide.
- Carbon is less reactive than aluminium, so it cannot extract aluminium from aluminium oxide.
- Therefore, zinc and copper can be extracted from their oxides using carbon, but aluminium cannot.
Tip
Carbon and hydrogen are reference points
Carbon and hydrogen appear in the reactivity series to help with comparisons. They are not metals.
Reduction: removing oxygen
In this topic, reduction means the loss of oxygen.
When carbon is used to extract a metal from a metal oxide, the metal oxide loses oxygen and becomes the metal. The carbon gains oxygen and usually forms carbon dioxide.
For example:
2CuO(s) + C(s) → 2Cu(s) + CO₂(g)
Copper(II) oxide has lost oxygen, so it has been reduced. Carbon has gained oxygen, so it has been oxidised.
Definition
Reduction, oxidation and redox
- Reduction is the loss of oxygen.
- Oxidation is the gain of oxygen.
- A redox reaction is a reaction where reduction and oxidation happen at the same time.
A reducing agent is a substance that causes another substance to be reduced. In these reactions, carbon is the reducing agent because it removes oxygen from the metal oxide.
Key Idea
Carbon’s job
Carbon reduces the metal oxide by removing oxygen from it. Carbon itself is oxidised because it gains oxygen.
Example
Identifying oxygen transfer
Use the equation: 2PbO(s) + C(s) → 2Pb(s) + CO₂(g). Which substance is reduced and which substance is oxidised?
- Compare the lead substance at the start and end: lead starts in lead(II) oxide, PbO(s), and ends as lead, Pb(s), so the lead compound has lost oxygen.
- A loss of oxygen is reduction, so PbO(s) is reduced to Pb(s).
- Compare carbon at the start and end: C(s) becomes CO₂(g), so carbon has gained oxygen.
- A gain of oxygen is oxidation, so C(s) is oxidised. Carbon is also the reducing agent because it causes the metal oxide to lose oxygen.
Common Mistake
Saying carbon is reduced
In carbon extraction reactions, carbon is usually oxidised, not reduced. Carbon gains oxygen to form carbon dioxide, CO₂(g).
Writing equations for reduction with carbon
A common general pattern is:
metal oxide + carbon → metal + carbon dioxide
The exact balanced equation depends on the formula of the metal oxide. You may be asked to balance an equation, so remember: balance atoms by changing the large numbers in front of formulas, not the small numbers inside formulas.
Example
Balancing a carbon reduction equation
Balance this skeleton equation: Fe₂O₃(s) + C(s) → Fe(s) + CO₂(g).
- Balance oxygen first by making both sides contain 6 oxygen atoms: put 2 in front of Fe₂O₃(s) and 3 in front of CO₂(g).
- Now balance iron: 2Fe₂O₃(s) contains 4 iron atoms, so put 4 in front of Fe(s).
- Now balance carbon: 3CO₂(g) contains 3 carbon atoms, so put 3 in front of C(s).
- The balanced equation is: 2Fe₂O₃(s) + 3C(s) → 4Fe(s) + 3CO₂(g).
Common Mistake
Changing the small numbers
Never change a formula such as Fe₂O₃(s) to FeO(s) just to make balancing easier. That would change the substance. Only change the big numbers in front.
Evaluating extraction methods
You do not need to memorise the detailed industrial processes for extracting metals in this section. However, you may be given information about a process and asked to interpret or evaluate it.
When evaluating, think about:
- whether the metal is above or below carbon in the reactivity series
- how much energy the process needs
- cost of raw materials and equipment
- environmental impact, such as carbon dioxide emissions
- purity of the metal produced
- safety and waste products
Example
Evaluating two extraction methods
A question says metal X is below carbon. Method A heats X oxide with carbon and releases carbon dioxide. Method B uses electrolysis and needs much more electricity. Which method is likely better for large-scale cheap production?
- Use the reactivity information first: because metal X is below carbon, reduction with carbon is chemically possible.
- Compare energy and cost: Method B needs much more electricity, so it is likely to be more expensive unless electricity is very cheap.
- Compare environmental effects: Method A releases carbon dioxide, while Method B may still cause emissions if the electricity is generated from fossil fuels.
- A sensible evaluation would choose Method A for lower cost and energy use, but mention the carbon dioxide drawback.
Tip
Evaluate means weigh up
If a question says evaluate, do not just describe the method. Compare advantages and disadvantages, then make a justified judgement using the information given.
The whole story in one chain
For metals below carbon, the extraction idea is:
- The metal is found in a compound, often a metal oxide.
- Carbon is more reactive than the metal.
- Carbon removes oxygen from the metal oxide.
- The metal oxide is reduced to the metal.
- Carbon is oxidised because it gains oxygen.
So for copper(II) oxide:
2CuO(s) + C(s) → 2Cu(s) + CO₂(g)
The useful product is copper, Cu(s).
Exam technique
In the exam
- Compare the metal with carbon in the reactivity series before choosing an extraction method.
- For redox questions in this topic, track oxygen: loss of oxygen means reduction, gain of oxygen means oxidation.
- If asked to evaluate a process, use the information given and include both a benefit and a drawback before your judgement.
Self review
Check yourself
- Why is gold often found as the metal itself, rather than as a compound?
- Why can carbon extract iron from iron oxide, but not aluminium from aluminium oxide?
- In 2ZnO(s) + C(s) → 2Zn(s) + CO₂(g), which substance is reduced and which is oxidised?
Reactivity of metals
Guide 3 of 4
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?