How do metals, acids and bases react, and how can we use electrolysis to extract elements?
The reactivity series and metal extraction, the reactions of acids with metals, bases and carbonates, neutralisation and the pH scale, making soluble salts, and electrolysis of molten compounds and aqueous solutions.
A focused answer to the AQA GCSE Combined Science: Trilogy Chemical changes topic, covering the reactivity series and metal extraction, reactions of acids, neutralisation and the pH scale, making salts, and electrolysis of molten and aqueous compounds.
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What this topic is asking
AQA wants you to use the reactivity series, describe how metals are extracted, describe the reactions of acids with metals, bases and carbonates, explain neutralisation and the pH scale, describe how soluble salts are made, and explain electrolysis of molten and aqueous compounds.
Reactivity and metal extraction
Metals less reactive than carbon (such as iron, zinc and copper) are extracted by reduction with carbon: the carbon is more reactive and takes the oxygen away from the metal oxide. Metals more reactive than carbon (such as aluminium) cannot be displaced by carbon and so are extracted by electrolysis of the molten ore, which uses a great deal of electricity and is therefore more expensive. Oxidation is the loss of electrons and reduction is the gain of electrons (the rule OILRIG: oxidation is loss, reduction is gain); a displacement reaction is a redox reaction because the more reactive metal is oxidised (loses electrons) and the less reactive metal ion is reduced (gains electrons).
Acids, bases and neutralisation
The three general reactions of acids that AQA expects you to know:
- Acid + metal salt + hydrogen (the hydrogen can be tested with a lit splint).
- Acid + base (metal oxide or metal hydroxide) salt + water (this is neutralisation).
- Acid + metal carbonate salt + water + carbon dioxide (the carbon dioxide turns limewater milky).
The salt made depends on the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates.
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is, using universal indicator or a pH probe. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is alkaline. Each whole step on the pH scale is a tenfold change in hydrogen ion concentration, so pH 2 has ten times the hydrogen ion concentration of pH 3. A strong acid (such as hydrochloric, sulfuric or nitric) is fully ionised in water, whereas a weak acid (such as ethanoic or citric) is only partly ionised, which is a separate idea from how concentrated the acid is.
Making salts and electrolysis
Soluble salts are made by reacting a suitable acid with an excess of an insoluble base or metal carbonate so that all the acid is used up, then filtering off the unreacted excess, and finally crystallising the solution by evaporating some water and cooling.
For molten ionic compounds the metal forms at the cathode and the non-metal at the anode. For aqueous solutions it is more complex because water also provides and ions: at the cathode, hydrogen gas is produced unless the metal is less reactive than hydrogen (in which case the metal is deposited); at the anode, oxygen is produced unless a halide ion (chloride, bromide, iodide) is present, in which case the halogen is produced.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain why aluminium is extracted from its ore by electrolysis, whereas iron is extracted by heating its ore with carbon.Show worked answer →
A Chemistry Paper 1 explanation testing the reactivity series. Reward: a metal can be displaced from its compound only by a more reactive element. Iron is less reactive than carbon, so carbon can reduce iron oxide to iron (the carbon takes the oxygen), which is cheaper. Aluminium is more reactive than carbon, so carbon cannot displace it from its oxide; instead the aluminium oxide must be melted and electrolysed, where electricity supplies the energy to reduce the aluminium ions at the cathode. Markers credit the comparison with carbon's position in the reactivity series and the link to which method is possible, and may reward that electrolysis is more expensive because of the energy needed.
AQA 20214 marksA student adds excess copper oxide (a base) to warm dilute sulfuric acid to make copper sulfate crystals. Describe the steps the student should take after the reaction to obtain pure, dry crystals of copper sulfate.Show worked answer →
A Chemistry Paper 1 required-practical question on making a soluble salt. Reward the sequence: filter the mixture to remove the excess (unreacted) copper oxide, leaving a copper sulfate solution; heat the solution gently to evaporate some of the water and concentrate it (until the point of crystallisation); leave the solution to cool so that crystals form; then dry the crystals, for example by patting between filter paper or leaving in a warm place. Markers credit filtering off the excess base, evaporating to concentrate, crystallising on cooling, and drying. Using excess base ensures all the acid is used up so the salt is not contaminated with acid.
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