Year 11 Chemistry Module 3 ⏱ ~35 min Lesson 5 of 12

Acid-Base & Acid-Carbonate Reactions

Every time you take an antacid tablet for heartburn, you're triggering an acid-base reaction in your stomach — and the salt it produces determines whether the treatment works or makes things worse. The chemistry of neutralisation is in your medicine cabinet.

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Think First

You've just eaten a large meal and your stomach is burning with acid reflux. You reach for an antacid — the label says it contains calcium carbonate (CaCO₃). Within minutes the burning stops.

(1) What do you think happens chemically when calcium carbonate meets stomach acid (hydrochloric acid, HCl)? (2) You notice the tablet fizzes when it dissolves — what gas do you think is being produced, and where does it come from? Write your predictions now.

Type your initial response below — you will revisit this at the end of the lesson.

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📐

Key Patterns — This Lesson

$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O}$  (neutralisation)
$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$ — net ionic equation for any strong acid + strong base
$\text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{H}_2\text{O} + \text{CO}_2\uparrow$
$\text{CO}_2\uparrow$ gas always produced — visible fizzing; also: $\text{Acid} + \text{HCO}_3^- \rightarrow \text{Salt} + \text{H}_2\text{O} + \text{CO}_2\uparrow$
Salt identification: cation from base/carbonate  |  anion from acid  |  HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate
📖 Know

Key Facts

  • The products of acid-base neutralisation (salt + water)
  • The products of acid-carbonate reactions (salt + water + CO₂)
  • Which acid produces which anion in the salt
💡 Understand

Concepts

  • How to identify the salt from an acid-base reaction before balancing
  • Why carbonates produce CO₂ but hydroxides do not
  • How different antacids work chemically
✅ Can Do

Skills

  • Predict products and name the salt for acid-base reactions
  • Write balanced molecular equations for both reaction types
  • Explain antacid chemistry using reaction type and products

📚 Core Content

Key Terms — scan these before reading
acidA substance that donates protons (H⁺) or accepts electron pairs, according to context.
baseA substance that accepts protons (H⁺) or donates electron pairs, according to context.
soluble baseA substance that accepts protons (H⁺) or donates electron pairs, according to context.
alkaliAlkali vs base: NaOH and KOH are alkalis (soluble bases).
Common basesA substance that accepts protons (H⁺) or donates electron pairs, according to context.
Synthesis reactionA reaction where two or more reactants combine to form a single product.
⚗️

Acid-Base Neutralisation Reactions

Neutralisation: Acid + Base → Salt + Water. The H⁺ from the acid combines with the OH⁻ from the base to form water. The remaining ions form the salt.

Acid Base Salt Balanced Equation
HClNaOHNaCl HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
H₂SO₄Ca(OH)₂CaSO₄ H₂SO₄(aq) + Ca(OH)₂(aq) → CaSO₄(s) + 2H₂O(l)
HNO₃KOHKNO₃ HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)
HClMgOMgCl₂ 2HCl(aq) + MgO(s) → MgCl₂(aq) + H₂O(l)
Balancing tip: When the base has two OH⁻ groups (e.g. Ca(OH)₂, Mg(OH)₂), you need two moles of a monoprotic acid (HCl, HNO₃) to neutralise one mole of the base.
Common error: Students write H₂O₂ instead of H₂O as the product. Neutralisation always produces H₂O — one O from OH⁻ combined with one H⁺. H₂O₂ is hydrogen peroxide — a completely different compound.
Universal net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l) is the net ionic equation for every strong acid-strong base neutralisation, regardless of which specific acid and base are used. This is why all strong acid-strong base reactions release the same energy per mole of water formed.
NEUTRALISATION MECHANISM H⁺ (aq) from acid + OH⁻ (aq) from base bond forms H₂O (l) water + energy released 57 kJ/mol ΔH = −57 kJ mol⁻¹
02

Acid-Carbonate & Acid-Hydrogen Carbonate Reactions

When an acid reacts with a carbonate, three products are always formed: salt + water + CO₂ gas. The fizzing you observe is one of the most recognisable indicators of chemical change in chemistry.

SWC mnemonic: Salt, Water, Carbon dioxide. These three products are fixed for every acid-carbonate reaction. Forgetting one costs marks.

Examples:

2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Left: 2H, 2Cl, 1Ca, 1C, 3O. Right: 1Ca, 2Cl, 2H, 1O+2O=3O, 1C. ✓
HCl(aq) + NaHCO₃(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Left: 2H, 1Cl, 1Na, 1C, 3O. Right: 1Na, 1Cl, 2H, 1O+2O=3O, 1C. ✓
CO vs CO₂ confusion: Students sometimes write CO as the gas product of acid-carbonate reactions. This is wrong — CO is a product of incomplete combustion. The carbon in carbonate (CO₃²⁻) is already at oxidation state +4, so CO₂ is the only possible gaseous carbon product.
03

Antacids — Clinical Connection

Stomach acid is predominantly HCl at ~0.1 mol/L (pH ≈ 1–2). Excess acid causes heartburn. Antacids neutralise this excess HCl — but different formulations use different chemistry:

Quick-Eze / Tums

Active ingredient: CaCO₃
Reaction type: Acid-carbonate
CO₂ produced?: Yes — belching
Balanced equation with HCl: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

Mylanta

Active ingredient: Mg(OH)₂
Reaction type: Neutralisation
CO₂ produced?: No
Balanced equation with HCl: 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O

Baking soda

Active ingredient: NaHCO₃
Reaction type: Acid-H carbonate
CO₂ produced?: Yes — belching
Balanced equation with HCl: HCl + NaHCO₃ → NaCl + H₂O + CO₂
💊 Real-World Anchor — Antacids: This context appears in Short Answer Q3. Be ready to: write a balanced equation for a specific antacid with HCl, state the reaction type, name the salt produced, and explain whether CO₂ is generated and what this means for the patient.
Common error — Mg(OH)₂ classified as carbonate: Mylanta contains Mg(OH)₂, which is a hydroxide base — it undergoes neutralisation and produces NO CO₂. The OH⁻ groups, not a CO₃²⁻ ion, are what react with H⁺. Mg(OH)₂ is not a carbonate.
Choose acid + base (including antacid combinations) · drag to add base · watch pH curve and equivalence point Interactive

🧮 Worked Examples

Worked Example 1 — Predicting Products and Balancing Neutralisation

Stepwise
Sulfuric acid reacts with potassium hydroxide solution. (a) Identify the reaction type. (b) Predict the products and name the salt formed. (c) Write the balanced molecular equation with state symbols.
  1. 1
    (a) Identify reaction type
    Acid (H₂SO₄) + base (KOH) → neutralisation reaction.
  2. 2
    (b) Identify the salt
    Cation from base: K⁺ (from KOH). Anion from acid: SO₄²⁻ (from H₂SO₄). Salt = potassium sulfate, K₂SO₄. Other product: water.
  3. 3
    (c) Write unbalanced equation
    H₂SO₄(aq) + KOH(aq) → K₂SO₄(aq) + H₂O(l)
  4. 4
    Balance — need 2 KOH to supply 2 K⁺; then 2 H₂O
    H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)
    Check: Left — 4H, 2K, 1S, 6O. Right — 4H, 2K, 1S, 4+2=6O. ✓
✓ Answer Neutralisation. Salt: potassium sulfate. Equation: H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

Worked Example 2 — Acid-Carbonate Reaction

Stepwise
A student adds excess hydrochloric acid to sodium carbonate powder. (a) Write the balanced equation with state symbols. (b) Identify all three products. (c) Describe two observable indicators of chemical change.
  1. 1
    Identify salt: Na⁺ from Na₂CO₃ + Cl⁻ from HCl → NaCl
    Products: NaCl(aq), H₂O(l), CO₂(g) [SWC]
  2. 2
    Write and balance
    2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g)
    Check: Left — 2H, 2Cl, 2Na, 1C, 3O. Right — 2Na, 2Cl, 2H, 1O+2O=3O, 1C. ✓
  3. 3
    (c) Observable indicators
    Vigorous fizzing/bubbling as CO₂ gas is evolved; solid Na₂CO₃ disappears as it is consumed by the reaction.
✓ Answer 2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g). Products: NaCl(aq), H₂O(l), CO₂(g). Indicators: gas evolved, solid disappears.

📝 How are you completing this lesson?

Acid + Carbonate Reaction 2HCl(aq) + CaCO₃(s) acid + carbonate fizz! CaCl₂(aq) salt + H₂O(l) water + CO₂(g)↑ carbon dioxide

🧪 Activities

🔎 Activity 1 — Spot + Fix

Identifying Errors in Acid-Base Equations

Each equation below contains an error. Identify the error and write the correct equation.

  1. 1 Student writes: HNO₃(aq) + Ca(OH)₂(aq) → CaNO₃(aq) + H₂O(l)

    Error 1: Wrong salt formula — Ca²⁺ with NO₃⁻ gives Ca(NO₃)₂, not CaNO₃ (Ca is 2+ and NO₃⁻ is 1−, need two NO₃⁻ ions).
    Error 2: Equation is unbalanced — only 1 HNO₃ cannot provide both OH⁻ groups needed to neutralise Ca(OH)₂.
    Correct: 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
    Check: Left — 2H+2H=4H, 2N, 6O+2O=8O, 1Ca, 2O(Ca(OH)₂) but let's recount: Left: 2H(acid), 2N, 6O(from HNO₃) + 1Ca, 2O, 2H(from Ca(OH)₂) = 4H, 2N, 1Ca, 8O. Right: 1Ca, 2N, 6O + 4H, 2O = 4H, 2N, 1Ca, 8O. ✓

  2. 2 Student writes: HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO(g)

    Error 1: CO is written instead of CO₂. The carbon in CO₃²⁻ is at +4 oxidation state — it cannot be further oxidised and the product is CO₂, not CO.
    Error 2: Equation is unbalanced — need 2HCl to provide 2Cl⁻ for CaCl₂.
    Correct: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
    Check: Left — 2H, 2Cl, 1Ca, 1C, 3O. Right — 1Ca, 2Cl, 2H, 1O+2O=3O, 1C. ✓

  3. 3 Student writes: H₂SO₄(aq) + 2NaOH(aq) → 2NaOH(aq) + H₂SO₄(aq) + H₂O(l)

    Error: The student has written the reactants on both sides — no salt has been identified and the equation is just the reactants repeated. The reaction did not proceed to products.
    Salt identification: Na⁺ from NaOH + SO₄²⁻ from H₂SO₄ → Na₂SO₄
    Correct: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)
    Check: Left — 4H, 2Na, 1S, 4+2=6O. Right — 2Na, 1S, 4O + 4H, 2O = 4H, 2Na, 1S, 6O. ✓

Type your working below before revealing answers:

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💊 Activity 2 — Clinical Case: Antacid Selection

Choosing the Right Antacid

A pharmacist advises patients on antacid selection. Use your chemistry knowledge to complete the table and answer the questions.

Patient Antacid prescribed Active ingredient Reaction type with HCl Salt produced CO₂ produced?
Sophie (post-surgery, cannot burp) Mylanta Mg(OH)₂ Your answer Your answer Your answer
Tom (occasional heartburn, wants fast relief) Quick-Eze CaCO₃ Your answer Your answer Your answer
Priya (pregnant, uses baking soda) Baking soda solution NaHCO₃ Your answer Your answer Your answer
Question A: Write the balanced equation for Sophie's antacid (Mg(OH)₂) reacting with stomach acid (HCl). Include state symbols.
Question B: Explain in 2–3 sentences why Sophie is prescribed Mylanta (Mg(OH)₂) rather than Quick-Eze (CaCO₃), linking your answer to the specific chemistry of each reaction.

Complete the table and answer A and B below:

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✏️ Complete the table and answer in your workbook
Revisit Your Thinking

Earlier you were asked: What happens when CaCO₃ meets stomach acid (HCl)? And what gas causes the fizzing?

The key insight: CaCO₃ undergoes an acid-carbonate reaction — 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). The fizzing is CO₂ gas being released as the carbonate group (CO₃²⁻) picks up two H⁺ ions, forms H₂CO₃, which immediately decomposes to water and CO₂. The salt (CaCl₂) stays dissolved, and the excess acid is consumed. This is why antacids with carbonates cause belching — and why Mylanta (Mg(OH)₂) is preferred when CO₂ production is a problem.

Now revisit your initial response. What did you get right? What has changed in your thinking?

Look back at your initial response. Annotate it with what you now understand differently.

Annotate your initial response in your book
Saved
Revisit Your Initial Thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

Misconceptions to Fix

✗

Wrong: A base must contain OH⁻ to be a base.

✗

Right: The Arrhenius definition requires OH⁻, but the Brønsted-Lowry definition is broader: a base is any proton acceptor. NH₃, Na₂CO₃, and oxide ions are all bases despite containing no hydroxide. The Brønsted-Lowry definition is more widely applicable in HSC chemistry.

MC

Multiple Choice

5 random questions from a replayable lesson bank — feedback shown immediately

✍️ Short Answer

04

Extended Questions

UnderstandBand 3

8. Distinguish between acid-base neutralisation and acid-carbonate reactions. In your answer, state the products of each type and write one balanced equation (with state symbols) for each. 4 MARKS

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ApplyBand 4

9. When excess hydrochloric acid is added to potassium carbonate (K₂CO₃) solution: (a) Predict and name all products formed. (1 mark) (b) Write the balanced molecular equation with state symbols. (1 mark) (c) Write the net ionic equation. (1 mark) (d) Describe two observable indicators of chemical change. (1 mark) 4 MARKS

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EvaluateBand 5

10. A patient with severe heartburn is choosing between two antacids: Mylanta (contains Mg(OH)₂) and Quick-Eze (contains CaCO₃). (a) Write balanced equations for each antacid reacting with stomach acid (HCl). Include state symbols. (2 marks) (b) For each reaction, name the salt produced and identify whether CO₂ is generated. (2 marks) (c) Evaluate which antacid would be more appropriate for a patient who has recently had abdominal surgery and cannot belch comfortably. Justify your answer using the chemistry. (1 mark) 5 MARKS

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✅ Comprehensive Answers

▼

🔎 Activity 1 — Spot + Fix

1. Error: CaNO₃ (wrong formula — Ca²⁺ + NO₃⁻ requires 2 NO₃⁻ → Ca(NO₃)₂) and unbalanced. Correct: 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

2. Error: CO written instead of CO₂ (carbonate carbon is +4, produces CO₂); also unbalanced. Correct: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)

3. Error: Reactants written as products — no salt identified. Correct: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

💊 Activity 2 — Antacid Table

Row 1 (Mylanta/Mg(OH)₂): Neutralisation; Salt = MgCl₂; CO₂ = No

Row 2 (Quick-Eze/CaCO₃): Acid-carbonate; Salt = CaCl₂; CO₂ = Yes

Row 3 (Baking soda/NaHCO₃): Acid-hydrogen carbonate; Salt = NaCl; CO₂ = Yes

Question A: 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l)

Question B: Sophie is prescribed Mylanta because Mg(OH)₂ undergoes neutralisation (acid + base → salt + water), which produces no CO₂. Quick-Eze contains CaCO₃ which reacts via acid-carbonate reaction, producing CO₂ gas. Since Sophie cannot comfortably belch after surgery, the CO₂ produced by Quick-Eze would cause painful bloating. Mylanta's reaction type — neutralisation — avoids this problem entirely.

❓ Multiple Choice

1. B — Acid + base → neutralisation (salt + water, no CO₂). Salt: Ca²⁺ + NO₃⁻ → Ca(NO₃)₂; no carbonate → no CO₂.

2. C — CaCl₂ and H₂O are colourless — no blue colour is produced. Gas, disappearing solid, and warmth are all expected.

3. C — Mg(OH)₂ undergoes neutralisation (no CO₃²⁻ ion → no CO₂). All carbonate/hydrogen carbonate options produce CO₂.

4. D — H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Na₂SO₄ is correct (2 Na⁺ with SO₄²⁻). Option B is unbalanced (only 1 NaOH); Option C has wrong salt formula (NaSO₄ should be Na₂SO₄).

5. B — The net ionic equation applies universally because regardless of the specific acid and base, the actual chemical event is always H⁺ + OH⁻ → H₂O. The counterions (e.g. Na⁺, Cl⁻, K⁺, NO₃⁻) are spectators.

6. C (Band 5) — NaHCO₃ produces CO₂ (bloating), adds Na⁺ to the diet (problematic for hypertension patients), provides only short-term relief (HCO₃⁻ provides less buffering capacity than metal hydroxides), and long-term systemic absorption could affect blood pH homeostasis.

7. A (Band 6) — Al(OH)₃: reacts with HCl via neutralisation (no CO₂): Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l). Contains no sodium — safe for low-Na diets. Option C (NaOH) is too caustic and adds sodium. Options B and D produce CO₂ or sodium.

📝 Short Answer Model Answers

Q8 (4 marks): Neutralisation: acid + base → salt + water [1]. Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) [1]. Acid-carbonate reaction: acid + carbonate → salt + water + CO₂ gas [1]. Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1].

Q9 (4 marks): (a) Products: potassium chloride KCl(aq), water H₂O(l), carbon dioxide CO₂(g) [1]. (b) 2HCl(aq) + K₂CO₃(aq) → 2KCl(aq) + H₂O(l) + CO₂(g) [1 — balanced with state symbols]. (c) Net ionic: 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g) [1]. (d) Any two: gas evolved (vigorous fizzing), temperature change (slightly warm), [1].

Q10 (5 marks): (a) Mylanta: 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l) [1]. Quick-Eze: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1]. (b) Mylanta: salt = magnesium chloride MgCl₂, no CO₂ produced [1]. Quick-Eze: salt = calcium chloride CaCl₂, CO₂ produced [1]. (c) Mylanta is more appropriate because it undergoes neutralisation, producing no CO₂. Quick-Eze produces CO₂ gas that would need to be expelled by belching — which is painful and potentially harmful post-surgery. Mylanta eliminates this side effect entirely [1].

Interactive: Acid-Base Reaction Simulator
⚔️
Boss Battle

Acid-Base & Acid-Carbonate Reactions

Put your knowledge of Acid-Base & Acid-Carbonate Reactions to the test. Answer correctly to deal damage — get it wrong and the boss hits back. Pool: lessons 1–5.

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