Reactivity, Acids and Salts
The reactivity series orders metals by how readily they form positive ions, determining which metals react with water and acid, which metal displaces which from a solution of its salt, and how each metal is extracted: metals below carbon in the series are extracted by reduction with carbon, while those above it require electrolysis. Acids react with metals, metal oxides, metal hydroxides and metal carbonates to form salts, with neutralisation being the reaction of hydrogen ions from the acid with hydroxide ions from the alkali to form water.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the reactivity series in order (potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen, copper) and use it to predict every reaction in this topic rather than memorising each one separately.
- Predict displacement by position: a more reactive metal displaces a less reactive one from its compound, so zinc displaces copper from copper sulfate solution and the blue solution fades while a pink-brown solid appears.
- Choose the extraction method from the position of carbon: metals below carbon (zinc, iron, copper) are extracted by reduction with carbon, and metals above carbon (aluminium and above) must be extracted by electrolysis, which is more expensive because of the energy needed to melt the ore and to drive the process.
- Identify oxidation and reduction by electrons: oxidation is loss of electrons, reduction is gain. In a displacement reaction the metal atom loses electrons and is oxidised, while the metal ion in solution gains them and is reduced.
- Learn the four salt-making reactions with their products: acid + metal gives salt + hydrogen; acid + metal oxide gives salt + water; acid + metal hydroxide gives salt + water; acid + metal carbonate gives salt + water + carbon dioxide. The salt's name comes from the acid: hydrochloric gives chlorides, sulfuric gives sulfates, nitric gives nitrates.
- Distinguish strong from concentrated, which is examined nearly every year: a strong acid is fully ionised in aqueous solution, a weak acid only partly ionised; concentration is how many moles of acid are dissolved in a given volume. A dilute strong acid and a concentrated weak acid can have similar pH values. As hydrogen ion concentration increases by a factor of 10, the pH falls by 1.
Worked example
Describe how to make pure, dry crystals of copper sulfate from copper oxide and dilute sulfuric acid, and name the type of reaction.
- Warm the dilute sulfuric acid gently, then add copper oxide a little at a time and stir, until some solid remains unreacted, which shows all the acid has been used up.
- Filter the mixture to remove the excess copper oxide, leaving a blue copper sulfate solution in the filtrate.
- Pour the solution into an evaporating basin and heat gently to evaporate some of the water, until the point of crystallisation, when crystals begin to appear at the edge.
- Leave the solution to cool slowly so that crystals form, then remove them and pat them dry between filter papers rather than heating strongly, which would drive off the water of crystallisation.
- Name the reaction: neutralisation, an acid reacting with a metal oxide (a base) to give a salt and water.
- Write the equation: CuO + H2SO4 gives CuSO4 + H2O.
Practice questions
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Q1Complete: acid + metal carbonate gives ...Show answer
Answer: Salt + water + carbon dioxide.
Q2Name the salt made from zinc oxide and nitric acid.Show answer
Answer: Zinc nitrate.
Q3Define oxidation in terms of electrons.Show answer
Answer: The loss of electrons.
Q4Why is aluminium extracted by electrolysis rather than by reduction with carbon?Show answer
Answer: Aluminium is more reactive than carbon, so carbon cannot displace it from its oxide. Electrolysis is used instead, which requires a large amount of energy and is therefore expensive.
Q5State what is observed when magnesium ribbon is added to copper sulfate solution.Show answer
Answer: The blue colour of the solution fades, a pink-brown solid (copper) is deposited, and the mixture warms up.
Q6Give the ionic equation for neutralisation.Show answer
Answer: H+ + OH- gives H2O.
Q7Explain the difference between a strong acid and a concentrated acid.Show answer
Answer: A strong acid is fully ionised in solution, releasing all of its hydrogen ions; a concentrated acid simply has a large amount of acid dissolved per cubic decimetre. A weak acid can be concentrated, and a strong acid can be dilute.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Iron can be extracted from iron oxide by heating with carbon. Explain why this method works for iron but not for aluminium, and state which substance is oxidised and which is reduced in the extraction of iron.
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Two solutions have the same concentration: one is hydrochloric acid and one is ethanoic acid. The hydrochloric acid has pH 1 and the ethanoic acid pH 3. Explain this difference.
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Free printable worksheet
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This topic is chapter 14 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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