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O-Level Chemistry · Secondary 3 and 4

Acids and Bases

Acids and bases appear everywhere in chemistry, from the hydrochloric acid in your stomach to the lime farmers spread on acidic soil. In O-Level Chemistry you learn what makes a substance acidic or alkaline, how acids react with metals, bases and carbonates, how to read the pH scale and how to classify oxides. This topic leads straight into making salts, qualitative analysis and titration calculations, so the equations here are worth knowing well.

What the syllabus expects

  • Describe acids as substances that produce hydrogen ions, H⁺, in water and alkalis as substances that produce hydroxide ions, OH⁻, in water
  • Describe the reactions of dilute acids with metals, bases and carbonates, and write equations, including ionic equations
  • Use the pH scale and universal indicator to describe how acidic or alkaline a solution is
  • Distinguish between strong and weak acids and alkalis in terms of the degree of ionisation, and between strong and concentrated
  • Classify oxides as acidic, basic, amphoteric or neutral
  • Describe the reaction of ammonium salts with alkalis to release ammonia

What acids, bases and alkalis are

An acid is a substance that produces hydrogen ions, H⁺, when dissolved in water. Hydrogen chloride gas ionises in water as HCl gives H⁺ + Cl⁻, and it is the H⁺ ions that give acids their properties. Without water there are no free H⁺ ions, so dry hydrogen chloride does not turn dry blue litmus red. A base is a substance that reacts with an acid to form a salt and water only, usually a metal oxide or hydroxide such as copper(II) oxide. An alkali is a base that dissolves in water to produce hydroxide ions, OH⁻, such as sodium hydroxide or aqueous ammonia. So all alkalis are bases, but not all bases are alkalis. The pH scale runs from about 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is alkaline. The lower the pH, the higher the concentration of H⁺ ions. Universal indicator gives a colour for each pH, while litmus only shows acid (red) or alkali (blue).

Reactions of dilute acids

Dilute acids have three standard reactions. With a reactive metal they give a salt and hydrogen: Mg + 2HCl gives MgCl₂ + H₂, and the gas gives a 'pop' with a lighted splint. Copper and other metals below hydrogen in the reactivity series do not react with dilute acids. With a base they give a salt and water: CuO + H₂SO₄ gives CuSO₄ + H₂O. This is neutralisation, and the ionic equation for an acid reacting with an alkali is H⁺ + OH⁻ gives H₂O. With a carbonate they give a salt, water and carbon dioxide: CaCO₃ + 2HCl gives CaCl₂ + H₂O + CO₂, and the gas forms a white precipitate with limewater. When writing ionic equations, cancel the spectator ions that appear unchanged on both sides, such as Cl⁻ in the reaction of magnesium with hydrochloric acid, which leaves Mg + 2H⁺ gives Mg²⁺ + H₂.

Strong, weak, concentrated and dilute

Strong and weak describe how far an acid ionises in water. A strong acid such as hydrochloric, sulfuric or nitric acid ionises completely. A weak acid such as ethanoic acid only partly ionises, so most of its molecules stay un-ionised. Strong alkalis such as sodium hydroxide ionise completely, while aqueous ammonia is a weak alkali. Concentrated and dilute describe something different: how many moles of acid are dissolved in each dm³. You can have a dilute solution of a strong acid or a concentrated solution of a weak acid. When two acids have the same concentration, the weak acid has a higher pH and reacts more slowly with magnesium or a carbonate, because it has fewer H⁺ ions in solution at any moment. With excess magnesium, the same volume of each acid still gives the same total volume of hydrogen, since the same number of moles of acid is present.

Types of oxides

Oxides are classified by how they react with acids and bases. Basic oxides are metal oxides, such as magnesium oxide, calcium oxide and copper(II) oxide, which react with acids to form a salt and water. Acidic oxides are usually non-metal oxides, such as carbon dioxide, sulfur dioxide and phosphorus(V) oxide, which react with alkalis to form a salt and water, and many dissolve in water to give acidic solutions. Amphoteric oxides, such as aluminium oxide and zinc oxide, react with both acids and alkalis. Neutral oxides, such as carbon monoxide, nitrogen monoxide and water, react with neither. A common slip is to call aluminium oxide basic simply because aluminium is a metal.

Alkalis and ammonium salts

When an ammonium salt is warmed with an alkali, ammonia gas is given off: NH₄Cl + NaOH gives NaCl + NH₃ + H₂O. Ammonia is the only common alkaline gas, and it turns damp red litmus paper blue. This reaction is the basis of the test for ammonium ions in qualitative analysis. It also explains why farmers should not add lime (calcium hydroxide) to soil at the same time as an ammonium fertiliser: the base reacts with the ammonium salt, and nitrogen is lost from the soil as ammonia gas.

Worked example

0.24 g of magnesium ribbon is added to excess dilute hydrochloric acid. Write the balanced equation and the ionic equation, and calculate the volume of hydrogen produced at room temperature and pressure. (Ar: Mg = 24, molar gas volume = 24 dm³ at r.t.p.)

  1. Balanced equation: Mg + 2HCl gives MgCl₂ + H₂.
  2. Cl⁻ ions appear unchanged on both sides, so remove them: Mg + 2H⁺ gives Mg²⁺ + H₂.
  3. Moles of Mg = 0.24 ÷ 24 = 0.010 mol.
  4. The ratio of Mg to H₂ is 1 : 1, so 0.010 mol of H₂ forms, because the acid is in excess and magnesium is limiting.
  5. Volume of H₂ = 0.010 × 24 = 0.24 dm³ = 240 cm³.

Answer: Mg + 2HCl gives MgCl₂ + H₂, ionic equation Mg + 2H⁺ gives Mg²⁺ + H₂, and 240 cm³ (0.24 dm³) of hydrogen forms at r.t.p.

Common mistakes

  • Treating strong as meaning concentrated. Strong means fully ionised, while concentrated means a lot of acid per dm³.
  • Saying that all bases are alkalis. Copper(II) oxide is a base but is insoluble in water, so it is not an alkali.
  • Thinking that a lower pH means fewer hydrogen ions. A lower pH means a higher concentration of H⁺ ions.
  • Calling aluminium oxide or zinc oxide a basic oxide. They are amphoteric because they react with both acids and alkalis.
  • Leaving spectator ions such as Na⁺ and Cl⁻ in an ionic equation, or writing H instead of H₂ for hydrogen gas.
  • Expecting copper to fizz in dilute acid. Copper is below hydrogen in the reactivity series and does not react with dilute acids.

Exam tips

  • Learn the three acid reactions as word patterns (metal gives salt and hydrogen, base gives salt and water, carbonate gives salt, water and carbon dioxide). Then any new equation is a matter of filling in names and balancing.
  • When asked to compare a strong and a weak acid, mention the degree of ionisation and the concentration of H⁺ ions in the same answer.
  • For oxide classification questions, test the oxide against both an acid and an alkali in your answer rather than relying on whether the element is a metal.

Practice questions

1. Easy: Name the products formed when zinc carbonate reacts with dilute sulfuric acid, and write the balanced equation.

Show answer

Zinc sulfate, water and carbon dioxide. ZnCO₃ + H₂SO₄ gives ZnSO₄ + H₂O + CO₂. The acid reacts with a carbonate, so it gives a salt, water and carbon dioxide.

2. Medium: Solution A is 0.10 mol/dm³ hydrochloric acid and solution B is 0.10 mol/dm³ ethanoic acid. Which has the lower pH, and which reacts faster with magnesium? Explain.

Show answer

Solution A has the lower pH and reacts faster. Hydrochloric acid is strong and ionises completely, while ethanoic acid is weak and only partly ionises. At the same concentration, A has a higher concentration of H⁺ ions, so its pH is lower and particles of magnesium collide with H⁺ ions more often.

3. Harder: 1.32 g of ammonium sulfate is warmed with excess sodium hydroxide solution. (NH₄)₂SO₄ + 2NaOH gives Na₂SO₄ + 2NH₃ + 2H₂O. Calculate the volume of ammonia given off at r.t.p. and state a test for the gas. (Ar: N = 14, H = 1, S = 32, O = 16)

Show answer

0.480 dm³ (480 cm³). Mr of (NH₄)₂SO₄ = 2 × 18 + 32 + 4 × 16 = 132, so moles = 1.32 ÷ 132 = 0.0100 mol. The ratio is 1 : 2, so 0.0200 mol of NH₃ forms, and 0.0200 × 24 = 0.480 dm³. Ammonia turns damp red litmus paper blue.

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