The same chemical logic that detonated 2,750 tonnes of ammonium nitrate in Beirut in 2020 is used every day in mining — decomposition reactions release enormous energy when bonds break apart. Understanding these two fundamental reaction types gives you the power to predict what gets built and what breaks down.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
In 2020, a warehouse in Beirut exploded with the force of a small nuclear weapon. The cause was 2,750 tonnes of ammonium nitrate — a white crystalline solid used as fertiliser. It had been stored for years without incident, then suddenly decomposed catastrophically.
Here's the puzzle: ammonium nitrate is stable enough to pour onto fields, yet unstable enough to level a city. What do you think triggered the decomposition? And what do you predict the products were? Write your ideas before reading on.
Type your initial response below — you will revisit this at the end of the lesson.
Write your initial response in your book. You will revisit it at the end of the lesson.
📚 Core Content
Balancing an equation applies the Law of Conservation of Mass systematically, one element at a time. The method by inspection works for both reaction types.
Example — electrolysis of water:
The same decomposition reaction that makes ammonium nitrate useful as a fertiliser makes it catastrophically dangerous when it decomposes uncontrollably.
Ammonium nitrate (NH₄NO₃) can decompose via two pathways depending on conditions:
🧮 Worked Examples
🧪 Activities
1 Iron(III) chloride forms when iron reacts with chlorine gas: Fe(s) + Cl₂(g) → FeCl₃(s) [unbalanced]
2 Potassium chlorate decomposes on heating: KClO₃(s) → KCl(s) + O₂(g) [unbalanced]
3 Sulfur dioxide reacts with oxygen to form sulfur trioxide: SO₂(g) + O₂(g) → SO₃(g) [unbalanced]
Type your working below before revealing answers:
Complete in your workbook.
| Pathway | Conditions | Equation | Gas volumes produced |
|---|---|---|---|
| Controlled | Gentle heating (~200°C) | NH₄NO₃(s) → N₂O(g) + 2H₂O(g) | 2 moles gas per mole NH₄NO₃ |
| Explosive | High temp / confined | 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g) | 3.5 moles gas per mole NH₄NO₃ |
Type your responses below:
Answer A, B, and C in your workbook.
Earlier you were asked: What triggered the Beirut decomposition, and what were the products?
The key insight: the trigger was a fire that initiated rapid thermal decomposition in a confined space. The products were N₂(g), O₂(g), and H₂O(g) — 3.5 moles of hot gas per mole of ammonium nitrate. This enormous rapid gas expansion, confined in a warehouse, produced a pressure wave equivalent to a small nuclear weapon. The chemistry is the same decomposition reaction you balanced today: 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g).
Now revisit your initial response. What did you get right? What has changed in your thinking?
Look back at your initial response in your book. Annotate it with what you now understand differently.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Wrong: Synthesis reactions always produce a single product from two elements.
Right: Synthesis reactions combine two or more reactants into a single product, but the reactants need not be elements. Compounds can also combine in synthesis reactions (e.g., SO₃ + H₂O → H₂SO₄). The defining feature is one product forming from multiple reactants.
5 random questions from a replayable lesson bank — feedback shown immediately
✍️ Short Answer
8. Distinguish between synthesis and decomposition reactions. For each type, provide one example equation (balanced, with state symbols) and state one type of energy that can drive the reaction. 4 MARKS
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Answer in your workbook.
9. Hydrogen peroxide (H₂O₂) decomposes to form water and oxygen gas. (a) Write the balanced equation for this reaction with state symbols. (1 mark) (b) Classify the reaction type and justify. (1 mark) (c) This reaction is catalysed by MnO₂. Explain what a catalyst does and why MnO₂ is not written as a reactant in the equation. (2 marks) 4 MARKS
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Answer in your workbook.
10. In 2020, the Beirut explosion was caused by the uncontrolled decomposition of 2,750 tonnes of ammonium nitrate (NH₄NO₃). The explosive decomposition equation is: 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g). (a) Verify this equation is balanced by showing atom counts on both sides. (2 marks) (b) Calculate the number of moles of gas produced per mole of ammonium nitrate that decomposes. Explain why a large volume of gas produced rapidly contributes to an explosion. (2 marks) (c) Under normal conditions, ammonium nitrate is used as a fertiliser. Identify which element in NH₄NO₃ makes it useful for plant growth. (1 mark) 5 MARKS
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Answer in your workbook.
1. Fe + Cl₂ → FeCl₃: Synthesis (A + B → AB). Balanced: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s). Check: 2 Fe, 6 Cl each side ✓
2. KClO₃ → KCl + O₂: Decomposition (AB → A + B). Balanced: 2KClO₃(s) → 2KCl(s) + 3O₂(g). Check: 2 K, 2 Cl, 6 O each side ✓
3. SO₂ + O₂ → SO₃: Synthesis. Balanced: 2SO₂(g) + O₂(g) → 2SO₃(g). Check: 2 S, 6 O each side ✓
Question A (atom counts):
Controlled: NH₄NO₃ → N₂O + 2H₂O. Left: 2N, 4H, 3O. Right: 2N (in N₂O) + 0, 4H, 1O (in N₂O) + 2O (in 2H₂O) = 2N, 4H, 3O. ✓
Explosive: 2NH₄NO₃ → 2N₂ + O₂ + 4H₂O. Left: 4N, 8H, 6O. Right: 4N + 2O + 8H + 4O = 4N, 8H, 6O. ✓
Question B: The explosive pathway produces 7 moles of gas (2N₂ + O₂ + 4H₂O) from 2 moles of solid, or 3.5 moles of gas per mole of NH₄NO₃. Gases occupy much greater volume than solids — rapid expansion of hot gas in a confined space generates enormous pressure, producing an explosive shock wave. The Beirut warehouse confinement prevented the gas from dispersing, maximising this pressure build-up.
Question C: Both pathways are decomposition reactions — in each case, one reactant (NH₄NO₃) breaks down into two or more products. They are distinct reactions (different products, different conditions) but belong to the same reaction type.
1. C — Two reactants (Fe, O₂) combine to form one product (Fe₂O₃): synthesis.
2. B — Coefficient 4 in front of Ag: 4 × 1 = 4 silver atoms.
3. B — H₂O₂ is hydrogen peroxide, a different compound. The subscript was changed, not the coefficient.
4. D — CaCO₃(s) → CaO(s) + CO₂(g): 1 Ca, 1 C, 3 O each side. ✓
5. A — SO₃ + H₂O → H₂SO₄: two reactants form one product = synthesis.
6. C (Band 5) — Same starting material does not mean same reaction. Different temperature/confinement conditions cause different bonds to break, producing different products. Both are decomposition reactions but are chemically distinct.
7. B (Band 6) — Fe(s) + S(s) → FeS(s) is correctly balanced (1 Fe, 1 S each side). Sulfur in this solid-state reaction is treated as S (monoatomic for simplicity). Option A uses S₂ which is a gas-phase species; option D uses S₈ which is the standard allotrope but makes the equation unnecessarily complex for this context.
Q8 (4 marks): Synthesis: two or more reactants combine to form one product (A + B → AB) [1]. Example: 2Mg(s) + O₂(g) → 2MgO(s). Energy: heat (combustion) [1]. Decomposition: one reactant breaks into two or more products (AB → A + B) [1]. Example: CaCO₃(s) → CaO(s) + CO₂(g). Energy: heat (thermal decomposition) [1]. Accept any correct balanced equations with state symbols and valid energy types.
Q9 (4 marks): (a) 2H₂O₂(aq) → 2H₂O(l) + O₂(g) [1]. (b) Decomposition — one reactant breaks into two products [1]. (c) A catalyst speeds up the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy [1]. MnO₂ is not consumed in the reaction — it is regenerated at the end, so it does not appear as a reactant or product in the balanced equation [1].
Q10 (5 marks): (a) Left: 2N + 2×4H + 2×3O = 4N, 8H, 6O. Right: 2N₂ (4N) + O₂ (2O) + 4H₂O (8H, 4O) = 4N, 8H, 6O [1 for left, 1 for right showing match ✓]. (b) 2 moles of NH₄NO₃ produce 7 moles of gas → 3.5 mol gas per mol NH₄NO₃ [1]. Rapid production of hot gas in a confined space creates enormous pressure far greater than atmospheric; the sudden, uncontrolled pressure release is the explosion [1]. (c) Nitrogen (N) — essential plant nutrient for protein and chlorophyll synthesis [1].
Answer questions on Synthesis & Decomposition before your opponents cross the line. Fast answers = faster car. Pool: lessons 1–2.
Tick when you've finished all activities and checked your answers.