Chemistry • Year 12 • Module 6 • Lesson 4
Neutralisation in Everyday Life & Industry
Lock in the core vocabulary, the three antacid equations, the two fertiliser equations, the FGD equation, and the agricultural liming reactions.
1. Label the neutralisation applications diagram
The diagram below shows five everyday and industrial applications of neutralisation. Write the missing labels into boxes A–H. Labels are drawn from the lesson Key Terms and Cards 1–5. 8 marks
- A — Type of chemical that antacids are (acid / base / salt?) ___________________
- B — Gas produced when CaCO₃ reacts with HCl ___________________
- C — Chemical used in agricultural liming to raise acidic soil pH ___________________
- D — Diprotic acid used to produce ammonium sulfate fertiliser ___________________
- E — Acidic gas neutralised in flue gas desulfurisation ___________________
- F — Base used in a wet FGD scrubber ___________________
- G — Safe pH range for industrial effluent discharge to waterways ___________________
- H — Example of a heavy metal ion that precipitates as pH rises in wastewater treatment ___________________
2. Term–definition match
The definitions below are shuffled. Write the matching term in the right-hand column. Terms: antacid, agricultural liming, diprotic acid, monoprotic acid, salt, wastewater treatment, flue gas desulfurisation, ammonium sulfate, acid rain, acidic oxide. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | An acid with two ionisable hydrogen atoms per molecule that can donate two protons; e.g. H₂SO₄. | |
| 2.2 | A base (such as Mg(OH)₂ or CaCO₃) that neutralises excess stomach acid (HCl); used medicinally. | |
| 2.3 | The ionic compound formed from the cation of a base and the anion of an acid during a neutralisation reaction. | |
| 2.4 | The process of neutralising industrial or mining effluent before it is discharged into waterways; often uses Ca(OH)₂. | |
| 2.5 | The application of CaCO₃ or Ca(OH)₂ to acidic soil to raise pH toward the optimal range of 6.0–7.5 for crop growth. | |
| 2.6 | An acid with only one ionisable hydrogen atom per molecule; e.g. HCl, HNO₃. | |
| 2.7 | The industrial process that removes SO₂ from power station exhaust gases by reacting it with Ca(OH)₂; prevents acid rain. | |
| 2.8 | A precipitation formed when SO₂ and SO₃ dissolve in atmospheric water droplets to produce H₂SO₃ and H₂SO₄; pH typically 4–5. | |
| 2.9 | A solid fertiliser salt produced by the reaction 2NH₃ + H₂SO₄ → (NH₄)₂SO₄; contains 21% nitrogen by mass. | |
| 2.10 | A non-metal oxide (e.g. SO₂, CO₂) that reacts with water to produce an acid, or directly with a base to form a salt and water. |
3. True or false — with correction
Circle T or F. If false, write the corrected statement. 8 marks (1 T/F + 1 correction where false)
3.1 Mg(OH)₂ and Al(OH)₃ both produce CO₂ gas when they react with hydrochloric acid. T / F
3.2 The reaction 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ requires 2 mol of NH₃ because H₂SO₄ is diprotic and donates two protons. T / F
3.3 The gas neutralised in flue gas desulfurisation is SO₃, not SO₂. T / F
3.4 Agricultural liming raises acidic soil pH because CaCO₃ releases OH⁻ ions when dissolved. T / F
4. Cloze — fill the blanks
Fill each blank using a word or formula from the word bank. Each word is used once. 10 marks
Word bank: CaSO₃ | diprotic | CaCO₃ | SO₂ | 6.5–8.5 | carbonate | base | NH₃ | 90–99 | Al(OH)₃
When excess stomach acid (HCl) needs to be neutralised, a patient may take an antacid. Antacids are (1) ________ compounds that react with HCl. Three common active ingredients are CaCO₃, Mg(OH)₂, and (2) ________. Only the (3) ________ -based antacid (CaCO₃) produces CO₂ gas as a product, because only carbonate ions react with H⁺ to form H₂CO₃, which decomposes.
In industry, ammonia reacts with H₂SO₄ to produce ammonium sulfate fertiliser. H₂SO₄ is a (4) ________ acid, meaning two moles of (5) ________ are needed per mole of acid. The product (NH₄)₂SO₄ contains 21% nitrogen by mass.
Coal-fired power stations produce (6) ________ gas, which dissolves in water to form acid rain. Flue gas desulfurisation (FGD) uses Ca(OH)₂ to neutralise this gas, producing (7) ________ as a solid byproduct. Modern FGD systems remove (8) ________% of the acidic gas before release.
Soil pH correction uses (9) ________ or Ca(OH)₂ to neutralise acidic H⁺ ions in soil. Industrial wastewater must be neutralised before discharge to bring pH within the safe range of (10) ________ for aquatic life.
5. Function recall
Answer each in 1–2 sentences using precise lesson terms. 10 marks, 2 each
5.1 What is the role of Ca(OH)₂ in flue gas desulfurisation?
5.2 Why does CaCO₃ raise acidic soil pH when applied?
5.3 Why does the production of ammonium sulfate require 2 mol of NH₃ per mole of H₂SO₄?
5.4 What additional environmental benefit (beyond neutralisation) occurs when Ca(OH)₂ is added to acidic wastewater containing Fe³⁺ ions?
5.5 What is the role of fluoride ions in toothpaste that goes beyond simple acid neutralisation?
Q1 — Labelled diagram
A: base (antacids are bases that neutralise HCl). B: CO₂ (from CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂). C: CaCO₃ or Ca(OH)₂. D: H₂SO₄ (sulfuric acid). E: SO₂ (sulfur dioxide, primary combustion product of sulfur in coal). F: Ca(OH)₂ (calcium hydroxide / slaked lime). G: 6.5–8.5. H: Fe³⁺ (or Cu²⁺, Pb²⁺, Zn²⁺).
Q2 — Term–definition matches
2.1 diprotic acid • 2.2 antacid • 2.3 salt • 2.4 wastewater treatment • 2.5 agricultural liming • 2.6 monoprotic acid • 2.7 flue gas desulfurisation • 2.8 acid rain • 2.9 ammonium sulfate • 2.10 acidic oxide.
Q3 — True / false with correction
3.1 False. Correction: Only CaCO₃ (a carbonate antacid) produces CO₂ when it reacts with HCl; Mg(OH)₂ and Al(OH)₃ are hydroxide bases that react by acid + base → salt + water, with no CO₂ produced because there is no carbonate ion present.
3.2 True. H₂SO₄ is diprotic — it can donate two protons, each accepted by one NH₃ molecule; hence 2 mol NH₃ per mole of H₂SO₄.
3.3 False. Correction: The gas primarily neutralised in FGD is SO₂ (sulfur dioxide), the main combustion product of sulfur in coal. The required equation is Ca(OH)₂ + SO₂ → CaSO₃ + H₂O. SO₃ is a secondary oxidation product and reacts by a different equation.
3.4 False. Correction: CaCO₃ does not release OH⁻ ions. It reacts directly with H⁺ ions in acidic soil: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. This consumes H⁺ ions (reducing [H⁺]) and raises pH without producing any OH⁻.
Q4 — Cloze answers
(1) base (2) Al(OH)₃ (3) carbonate (4) diprotic (5) NH₃ (6) SO₂ (7) CaSO₃ (8) 90–99 (9) CaCO₃ (10) 6.5–8.5
Q5.1 — Role of Ca(OH)₂ in FGD
Ca(OH)₂ acts as a base in the FGD scrubber. It reacts with SO₂ (an acidic oxide) by the equation Ca(OH)₂ + SO₂ → CaSO₃ + H₂O, converting the gaseous pollutant into solid calcium sulfite, which can be collected. This prevents SO₂ from reaching the atmosphere and forming acid rain.
Q5.2 — Why CaCO₃ raises soil pH
CaCO₃ reacts with the excess H⁺ ions in acidic soil water: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. This consumes H⁺ ions, reducing their concentration [H⁺] in the soil solution, which raises pH. The COtwo gas escapes, and the Ca²⁺ ions remain in solution as a harmless nutrient.
Q5.3 — Why 2 mol NH₃ per mole H₂SO₄
H₂SO₄ is diprotic — each molecule can donate two protons (H⁺). NH₃ acts as a Brønsted–Lowry base: each NH₃ molecule accepts exactly one proton to form NH₄⁺. Therefore, two NH₃ molecules are required to accept both protons from one H₂SO₄ molecule, producing (NH₄)₂SO₄.
Q5.4 — Additional benefit of Ca(OH)₂ in wastewater
As Ca(OH)₂ raises the pH of the wastewater, the increasing [OH⁻] causes dissolved heavy metal ions such as Fe³⁺ to precipitate as insoluble metal hydroxides: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s). These precipitates can then be filtered out, removing toxic heavy metals from the effluent that would otherwise harm aquatic organisms.
Q5.5 — Role of fluoride in toothpaste
Fluoride ions (F⁻) from NaF in toothpaste replace OH⁻ ions in hydroxyapatite (the mineral forming tooth enamel), converting it to the harder, more acid-resistant fluorapatite. This structural modification makes enamel more resistant to dissolution by bacterial acids even when mouth pH drops, providing protection beyond simple neutralisation of lactic acid.