C13: Reactions of Acids
Acids, bases, neutralisation and making salts
📋 Key Definitions
Acid: A substance that produces H⁺ ions when dissolved in water. Acids have a pH below 7.
Base: A substance that neutralises an acid, producing a salt and water. Metal oxides, metal hydroxides and metal carbonates are bases.
Alkali: A base that dissolves in water, producing OH⁻ ions. Alkalis have a pH above 7.
Neutralisation: The reaction between an acid and a base to produce a salt and water only.
pH scale: A scale from 0 to 14 that measures how acidic or alkaline a solution is. pH 7 is neutral.
Neutralisation (ionic equation):
H⁺(aq) + OH⁻(aq) → H₂O(l)
Acid + base → salt + water
Acid + metal carbonate → salt + water + carbon dioxide
Acid + metal → salt + hydrogen
📝 Acids, Bases and Alkalis
| Property | Acids | Alkalis |
|---|---|---|
| pH range | Below 7 | Above 7 |
| Ions in water | Produce H⁺ ions | Produce OH⁻ ions |
| Universal indicator colour | Red / orange / yellow | Blue / purple |
| Effect on litmus | Turns blue litmus red | Turns red litmus blue |
| Examples | HCl, H₂SO₄, HNO₃ | NaOH, KOH, Ca(OH)₂ |
Strong acids completely ionise in water — all molecules split into H⁺ ions and the negative ion. Weak acids only partially ionise — only some molecules split into ions.
| Feature | Strong Acid | Weak Acid |
|---|---|---|
| Ionisation | Complete | Partial |
| pH for same concentration | Lower | Higher (closer to 7) |
| Reaction rate | Faster | Slower |
| Examples | HCl, H₂SO₄, HNO₃ | Ethanoic acid (CH₃COOH), citric acid, carbonic acid |
⚗️ Neutralisation Reactions
When an acid reacts with a base, the H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water. This is neutralisation.
Worked Example 1: Acid + alkali
Write the equation for the neutralisation of hydrochloric acid with sodium hydroxide.
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
Worked Example 2: Acid + metal oxide
Write the equation for the reaction of sulfuric acid with copper(II) oxide.
H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)
Worked Example 3: Acid + metal carbonate
Write the equation for the reaction of hydrochloric acid with calcium carbonate.
2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Observations: fizzing (CO₂ gas), solid dissolves, limewater turns milky if CO₂ is bubbled through it.
🧂 Making Soluble Salts
| Method | Reactants | Products | Notes |
|---|---|---|---|
| Acid + metal | Metal + acid | Salt + hydrogen | Only works with metals above hydrogen. Add metal until no more reacts (excess), then filter. |
| Acid + base (insoluble) | Metal oxide/hydroxide + acid | Salt + water | Add base until in excess (no more dissolves), then filter off excess solid. Evaporate to crystallise. |
| Acid + alkali (titration) | Soluble base + acid | Salt + water | Must use titration because both are in solution — need to measure exact volumes. Use indicator to find end point. |
Rules for naming salts: The first part comes from the metal (or ammonia). The second part comes from the acid: hydrochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate.
Worked Example 4: Making copper(II) sulfate
Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
1. Add copper(II) oxide to warm dilute sulfuric acid and stir.
2. Continue adding until no more reacts (base is in excess).
3. Filter to remove excess unreacted copper(II) oxide.
4. Heat the filtrate to evaporate some water and concentrate the solution.
5. Leave to cool so crystals form.
6. Filter off the crystals and pat dry between filter paper.
🧪 Making Insoluble Salts (Precipitation)
Some salts are insoluble in water. These can be made by mixing two soluble salts in a precipitation reaction.
Precipitation: When two soluble salts are mixed, the ions swap partners. If one of the new combinations is insoluble, it forms a solid precipitate.
Worked Example 5: Making lead(II) iodide
Describe how to make lead(II) iodide (an insoluble yellow salt).
Mix lead(II) nitrate solution with potassium iodide solution:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
The yellow precipitate of lead(II) iodide forms. Filter, wash with distilled water, and dry.
Ionic equation: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
🌈 The pH Scale
| pH | Type | Universal Indicator Colour | Example |
|---|---|---|---|
| 0–2 | Strongly acidic | Red | Stomach acid (pH 1) |
| 3–6 | Weakly acidic | Orange / yellow | Lemon juice (pH 3), vinegar (pH 4) |
| 7 | Neutral | Green | Pure water |
| 8–11 | Weakly alkaline | Blue | Baking soda (pH 9), soap (pH 10) |
| 12–14 | Strongly alkaline | Purple / dark blue | Bleach (pH 13), oven cleaner (pH 14) |
❓ Practice Questions
Q1: Foundation Write the word equation for the reaction between hydrochloric acid and sodium hydroxide.
Q2: Foundation What ion do all acids produce in water? What ion do all alkalis produce in water?
Q3: Higher Write the ionic equation for the neutralisation of nitric acid with potassium hydroxide.
Q4: Foundation Describe how to make pure, dry crystals of zinc sulfate from zinc oxide and dilute sulfuric acid.
Q5: Higher Lead(II) sulfate is insoluble. Describe how to make it using a precipitation reaction, including the ionic equation.
✅ Answers
- Hydrochloric acid + sodium hydroxide → sodium chloride + water
- Acids produce H⁺ ions. Alkalis produce OH⁻ ions.
- HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l). Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l) (the ionic equation is the same for all neutralisations between strong acids and alkalis)
- Add zinc oxide to warm dilute sulfuric acid and stir. Continue adding until no more dissolves (excess). Filter to remove unreacted zinc oxide. Heat the filtrate to evaporate some water. Leave to cool and crystallise. Filter off crystals and pat dry between filter paper.
- Mix lead(II) nitrate solution with sodium sulfate solution. Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq). The white precipitate of lead(II) sulfate forms. Filter, wash with distilled water, and dry. Ionic equation: Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s)
🎯 Exam Tips
- The ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) works for ALL neutralisations between strong acids and alkalis
- To name a salt: the metal comes first, then the acid name gives the second part (HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate)
- In making salts: use excess base method for insoluble bases (metal oxides), titration for soluble bases (alkalis)
- For precipitation questions, always give the ionic equation (only include the ions that form the precipitate)
- State symbols are important: (aq) for dissolved substances, (s) for precipitates and solid bases
- When describing salt-making methods, mention all steps including filtration, evaporation, crystallisation and drying
🔬 Required Practical
Required Practical Activity
Preparing a pure dry soluble salt: Add the insoluble base (e.g. copper(II) oxide) to warm dilute acid (e.g. sulfuric acid) and stir. Continue adding until no more reacts (base is in excess — some unreacted solid remains). Filter to remove the excess base. Heat the filtrate gently to evaporate some water and concentrate the solution. Leave to cool so crystals form. Filter off the crystals, wash with distilled water, and pat dry between filter paper.
Key points: Using an excess of the insoluble base ensures all the acid is neutralised. Filtration removes unreacted base. Crystallisation gives pure crystals.
🔢 Maths Skills
Mathematical Skills
Writing balanced neutralisation equations: Ensure the number of H⁺ ions from the acid matches the OH⁻ ions (or oxide ions) from the base.
Example: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O. Two HNO₃ are needed because Ca(OH)₂ provides two OH⁻ groups.
⚠️ Common Misconceptions
Watch Out!
Strong acid and concentrated acid mean the same thing. Wrong: strong acid = concentrated acid Correct: strength refers to how completely the acid ionises in water; concentration refers to how much acid is dissolved per unit volume — a dilute strong acid can have a lower pH than a concentrated weak acid
All acids are equally dangerous. Wrong: all acids are equally dangerous Correct: the danger depends on both the concentration and the strength — a concentrated strong acid is more hazardous than a dilute weak acid
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how to prepare a pure dry sample of a named soluble salt.
To prepare copper(II) sulfate crystals: add copper(II) oxide to warm dilute sulfuric acid and stir. The CuO reacts and dissolves. Continue adding CuO until no more dissolves — this means the acid is fully neutralised and the base is in excess. Filter the mixture to remove the unreacted solid CuO. Gently heat the filtrate to evaporate some water and concentrate the salt solution. Leave the solution to cool so that crystals form slowly (slow cooling gives larger, purer crystals). Filter off the crystals using a funnel and filter paper. Wash the crystals with a small amount of cold distilled water to remove impurities. Pat the crystals dry between sheets of filter paper.
Mark scheme: 1 mark for adding base to acid with stirring; 1 mark for using excess base; 1 mark for filtration to remove excess; 1 mark for evaporation and crystallisation; 1 mark for filtering/washing crystals; 1 mark for drying method.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student makes magnesium sulfate by adding magnesium ribbon to dilute sulfuric acid. After the reaction finishes, the student filters and crystallises the solution, but the crystals are contaminated with unreacted acid.
Question: Identify the error in the method. Explain how to correct it and why the corrected method gives a pure product.
Answer: The error is using magnesium ribbon (a soluble reactant), which makes it impossible to tell when the acid is fully neutralised — the magnesium could all dissolve before the acid is used up, leaving excess acid in the solution. The correct method is to use an insoluble base such as magnesium oxide. Add MgO to the acid until no more dissolves (excess base), then filter to remove the unreacted MgO. This ensures all the acid has been neutralised and the filtrate contains only magnesium sulfate and water, giving pure crystals after crystallisation.
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🎬 Video Resources
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