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This lesson covers neutralisation reactions for Edexcel GCSE Chemistry (1CH0). You need to understand the general equations for neutralisation, write word and balanced symbol equations, and know the ionic equation for neutralisation. This topic is fundamental and links to making salts, which is covered in the next lesson.
Neutralisation is a chemical reaction between an acid and a base to produce a salt and water. The acid and base cancel each other out — the resulting solution is closer to pH 7 (neutral).
The general equation for neutralisation is:
acid + base → salt + water
Neutralisation is an exothermic reaction — it releases heat energy to the surroundings. You can detect this by measuring the temperature rise of the solution during the reaction.
There are several types of neutralisation reaction, depending on the type of base used.
A metal oxide is a base. When it reacts with an acid, the products are a salt and water.
Word equation: acid + metal oxide → salt + water
Examples:
| Acid | Metal Oxide | Salt | Balanced Symbol Equation |
|---|---|---|---|
| Hydrochloric acid | Copper oxide | Copper chloride | 2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l) |
| Sulfuric acid | Zinc oxide | Zinc sulfate | H₂SO₄(aq) + ZnO(s) → ZnSO₄(aq) + H₂O(l) |
| Nitric acid | Magnesium oxide | Magnesium nitrate | 2HNO₃(aq) + MgO(s) → Mg(NO₃)₂(aq) + H₂O(l) |
A metal hydroxide is also a base. Soluble metal hydroxides are alkalis.
Word equation: acid + metal hydroxide → salt + water
Examples:
| Acid | Metal Hydroxide | Salt | Balanced Symbol Equation |
|---|---|---|---|
| Hydrochloric acid | Sodium hydroxide | Sodium chloride | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) |
| Sulfuric acid | Potassium hydroxide | Potassium sulfate | H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l) |
| Nitric acid | Calcium hydroxide | Calcium nitrate | 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l) |
A metal carbonate also acts as a base. This reaction produces three products: a salt, water, and carbon dioxide gas.
Word equation: acid + metal carbonate → salt + water + carbon dioxide
Examples:
| Acid | Metal Carbonate | Salt | Balanced Symbol Equation |
|---|---|---|---|
| Hydrochloric acid | Calcium carbonate | Calcium chloride | 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) |
| Sulfuric acid | Sodium carbonate | Sodium sulfate | H₂SO₄(aq) + Na₂CO₃(s) → Na₂SO₄(aq) + H₂O(l) + CO₂(g) |
| Nitric acid | Copper carbonate | Copper nitrate | 2HNO₃(aq) + CuCO₃(s) → Cu(NO₃)₂(aq) + H₂O(l) + CO₂(g) |
Observations: Fizzing/effervescence (bubbles of CO₂ gas produced), solid carbonate dissolves, solution may change colour (e.g. green copper carbonate gives a blue copper salt solution).
Test for carbon dioxide: Bubble the gas through limewater (calcium hydroxide solution). If CO₂ is present, the limewater turns milky/cloudy (white precipitate of calcium carbonate forms).
Exam Tip: Learn all three types of neutralisation reaction. A common 3-mark question asks you to give the general word equation, then write a specific balanced symbol equation. Don't forget state symbols — they are often worth a mark.
In any neutralisation reaction between an acid and an alkali, the essential reaction is between hydrogen ions (from the acid) and hydroxide ions (from the alkali):
H⁺(aq) + OH⁻(aq) → H₂O(l)
This is called the ionic equation for neutralisation. It shows that neutralisation is fundamentally the reaction of H⁺ ions with OH⁻ ions to form water.
All the other ions (e.g. Na⁺, Cl⁻, K⁺, SO₄²⁻) are spectator ions — they are present in solution but do not take part in the reaction.
The ionic equation tells us that the heat energy released during neutralisation of any strong acid with any strong alkali is approximately the same per mole of water formed, because the same reaction (H⁺ + OH⁻ → H₂O) is occurring each time.
Exam Tip: The ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) appears frequently in Edexcel exams. You must know it, include the state symbols, and be able to explain that it represents the essential reaction in neutralisation.
Edexcel requires you to understand how to measure the temperature change during a neutralisation reaction. This links to the topic of energy changes.
Exam Tip: Questions about this practical often ask you to explain why the temperature increases (because neutralisation is exothermic — energy is transferred from the reaction to the surroundings), identify sources of error, and suggest improvements.
Neutralisation has many practical applications:
| Application | Acid | Base/Alkali | Purpose |
|---|---|---|---|
| Treating indigestion | Stomach acid (HCl) | Antacid (e.g. Mg(OH)₂, CaCO₃, NaHCO₃) | Neutralises excess acid to relieve pain |
| Treating acidic soil | Acidic soil (H⁺ ions) | Lime (CaO or Ca(OH)₂) | Raises soil pH for crop growth |
| Treating acid rain in lakes | Acidic lake water | Powdered limestone (CaCO₃) | Raises lake pH to protect wildlife |
| Wastewater treatment | Acidic industrial waste | Sodium hydroxide (NaOH) or lime | Neutralises before safe discharge |
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