Chemistry • Year 11 • Module 3 • Lesson 2
Synthesis & Decomposition
Build HSC Band 5–6 extended-response technique — synthesise data, a real-world scenario, and lesson concepts into an evaluated, evidence-based response.
1. Evaluate a chemical incident — two decomposition pathways (Band 5–6)
8 marks Band 5–6
Scenario. On 4 August 2020, an explosion at the port of Beirut, Lebanon, killed over 200 people and injured more than 6,000. Investigators found that a fire in an adjacent warehouse had raised temperatures in a sealed warehouse containing 2,750 tonnes of improperly stored ammonium nitrate (NH₄NO₃). The ammonium nitrate underwent uncontrolled thermal decomposition. Depending on temperature and confinement, NH₄NO₃ can decompose via two pathways:
Pathway 2 (high temperature, confined): 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g)
Ammonium nitrate is also used as a common agricultural fertiliser in Australia because it releases nitrogen ions (NH₄⁺ and NO₃⁻) in soil water when it dissolves.
Data table — moles of gas produced per mole of NH₄NO₃ decomposed
| Pathway | Moles of NH₄NO₃ (input) | Total moles of gas produced | Gas moles per mole NH₄NO₃ |
|---|---|---|---|
| 1 (controlled) | 1 | 1 (N₂O) + 2 (H₂O) = 3 | 3.0 |
| 2 (explosive) | 2 | 2 (N₂) + 1 (O₂) + 4 (H₂O) = 7 | 3.5 |
Q1. Using the scenario, the data table, and your knowledge from this lesson, evaluate why the Beirut warehouse explosion was so catastrophic and assess what storage conditions would have reduced the risk of an explosive decomposition event. In your response you must:
- Verify that Pathway 2 is balanced by performing an atom count on both sides.
- Use the data table to explain why Pathway 2 produces a more destructive outcome than Pathway 1, in terms of moles of gas per mole of solid consumed.
- Explain why the combination of high temperature and confinement — not either factor alone — was critical to the Beirut disaster, using the concept of decomposition rate and gas expansion.
- Assess two specific storage conditions that would have reduced risk, with reference to the chemistry of decomposition triggers covered in the lesson.
- Clarify why ammonium nitrate is used safely as a fertiliser, even though it is a reactive compound — linking to lesson content about conditions required for decomposition.
2. Evaluate a set of classroom reactions — applying synthesis and decomposition criteria (Band 5–6)
7 marks Band 5–6
Scenario. A Year 11 class performed four reactions from this lesson and recorded the following summary data:
Reaction 2: 2KClO₃(s) → 2KCl(s) + 3O₂(g) [requires sustained heating]
Reaction 3: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s) [iron glows red in chlorine gas]
Reaction 4: 2H₂O₂(aq) → 2H₂O(l) + O₂(g) [rate greatly increased by MnO₂]
Data table — reaction classification summary
| Reaction | Number of reactants | Number of products | Energy trigger |
|---|---|---|---|
| 1 | 2 | 1 | Heat (ignition) |
| 2 | 1 | 2 | Heat (sustained) |
| 3 | 2 | 1 | Heat (ignition) |
| 4 | 1 | 2 | Catalyst (MnO₂) |
Q2. Using the scenario, the data table, and your knowledge from this lesson, analyse how synthesis and decomposition reactions can be distinguished and evaluated using the data above. In your response you must:
- Define synthesis and decomposition reactions using the A + B → AB / AB → A + B framework, and classify each of the four reactions with justification.
- Verify that Reaction 2 is balanced by performing an atom count on both sides.
- Explain how the data table (number of reactants/products) supports the classification of each reaction, and identify which data column is the most reliable criterion and why.
- Compare Reactions 2 and 4: both are decomposition reactions, but they use different energy triggers. Explain what a catalyst does and why MnO₂ does not appear as a reactant in Reaction 4's balanced equation, while heat does not appear at all in any balanced equation.
- Use the law of conservation of mass to explain why the total atom count must be identical on both sides of any balanced equation, using one of the four reactions as a specific example.
Q1 — Sample Band 6 response (8 marks), annotated
Atom count — Pathway 2: Left: 2 × NH₄NO₃ → 2N + 4H + 1N + 3O per molecule × 2 molecules = 4N, 8H, 6O. Right: 2N₂ (4N) + O₂ (2O) + 4H₂O (8H, 4O) = 4N, 8H, 6O. Balanced. [1 — atom count both sides shown]
The data table shows Pathway 2 produces 3.5 moles of gas per mole of solid NH₄NO₃, compared to 3.0 for Pathway 1 — a 17% greater gas yield per mole consumed. At Beirut scale (2,750 tonnes ≈ 34,375 moles of NH₄NO₃ per tonne × 2,750 = massive scale), this translates to tens of millions of moles of rapidly produced gas. [1 — moles of gas comparison quantitatively linked to destructiveness]
High temperature alone increases decomposition rate but if the container is open, gases can expand freely and the pressure does not build catastrophically. Confinement alone at low temperature produces only modest gas volumes. Together, high temperature drives extremely rapid decomposition (via the Pathway 2 mechanism) while the sealed warehouse traps the hot product gases, creating a positive feedback: trapped gas heats the remaining solid further, accelerating decomposition further. This runaway process rapidly produces an enormous volume of high-pressure gas that bursts the container as an explosive shock wave. [1 — synergistic effect of temp + confinement explained mechanistically]
Two storage conditions that would have reduced risk: (1) Store at low ambient temperature, well away from heat sources and other flammable materials — the lesson shows decomposition requires sufficient thermal energy to reach the activation energy; low temperatures prevent this. (2) Store in well-ventilated, open-sided structures — without confinement, any gases produced disperse harmlessly and pressure cannot build. [1 per condition = 2 marks, each linked to a specific decomposition trigger or confinement concept]
As a fertiliser, NH₄NO₃ is dissolved in soil water at ambient temperature (roughly 10–30 °C) — far below the 200 °C threshold for even the mild Pathway 1 decomposition. Under these conditions, it simply dissolves to release NH₄⁺ and NO₃⁻ ions that plants absorb as nitrogen nutrients; it does not decompose at all. Decomposition requires a specific energy trigger (sufficient heat, confinement, or contamination) that is simply not present in normal agricultural or storage use. [1 — fertiliser safety explanation linked to absence of decomposition trigger]
Marking criteria.
- 1 mark — Atom count performed correctly for Pathway 2 (both sides shown, totals matched).
- 1 mark — Uses data table to quantitatively compare gas yield (Pathway 2 = 3.5 mol/mol vs Pathway 1 = 3.0 mol/mol) and links higher yield to greater destructive potential.
- 1 mark — Explains the synergistic effect of temperature AND confinement: rapid gas production + inability to expand = catastrophic pressure buildup and explosion.
- 1 mark — Identifies and explains storage condition 1 (cool temperatures / away from heat sources) with explicit link to decomposition trigger concept.
- 1 mark — Identifies and explains storage condition 2 (ventilated / open structure) with explicit link to confinement as a necessary factor for explosion.
- 1 mark — Assesses a third or higher criterion demonstrating synthesis-level thinking (e.g. contamination with fuels lowers activation energy; isolation from ignition sources; risk assessment using the lesson's trigger categories).
- 1 mark — Explains why NH₄NO₃ is safe as a fertiliser — ambient temperatures are far below decomposition threshold; it dissolves rather than decomposes in soil water; specific energy trigger is absent in normal use.
- 1 mark — Overall evaluative judgement: reaches an evidence-based conclusion about the relative contribution of each factor (temp, confinement, scale) to the disaster, using lesson vocabulary precisely.
Q2 — Sample Band 6 response (7 marks), annotated
A synthesis reaction combines two or more reactants into a single product (A + B → AB); a decomposition reaction breaks a single compound into two or more products (AB → A + B). Reactions 1 and 3 are synthesis (two reactants → one product each); Reactions 2 and 4 are decomposition (one reactant → two products each). [1 — definition of both types using A/B notation + correct classification of all four reactions]
Reaction 2 atom count: Left: 2KClO₃ → 2K, 2Cl, 6O. Right: 2KCl (2K, 2Cl) + 3O₂ (6O) = 2K, 2Cl, 6O. Balanced. [1 — correct atom count shown on both sides, totals matched]
The most reliable criterion in the data table is the number of reactants and products: synthesis = many reactants → one product; decomposition = one reactant → many products. The energy trigger column alone is insufficient, because multiple different triggers (heat, catalyst) can drive decomposition — the trigger type distinguishes the sub-type of decomposition, not the main classification. [1 — identifies number of reactants/products as most reliable criterion with reasoning]
Comparing Reactions 2 and 4: both are decomposition (one reactant splits into two products) but they use different energy triggers. Reaction 2 uses sustained heat (thermal decomposition); Reaction 4 uses a catalyst (MnO₂). A catalyst speeds up the reaction by providing an alternative pathway with lower activation energy, but is not consumed — it is regenerated at the end. This is why MnO₂ does not appear as a reactant or product in Reaction 4's balanced equation (2H₂O₂ → 2H₂O + O₂). Heat is an energy input that also does not appear in any balanced equation because balanced equations show only the chemical substances (reactants and products), not the conditions under which the reaction occurs. [1 — explains catalyst correctly + why MnO₂ absent from equation; 1 — explains why neither heat nor catalyst appears in a balanced equation]
The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction — the total number of each type of atom must be equal on both sides of the balanced equation. Using Reaction 2 (2KClO₃ → 2KCl + 3O₂): Left = 2K, 2Cl, 6O; Right = 2K, 2Cl, 6O — the atom counts match exactly, confirming conservation of mass. This is why we balance by adjusting coefficients, never subscripts. [1 — applies conservation of mass with explicit atom count to a named example from the lesson]
Marking criteria.
- 1 mark — Defines both synthesis and decomposition using the A + B → AB / AB → A + B framework and correctly classifies all four reactions.
- 1 mark — Performs a correct atom count for Reaction 2 (both sides shown, totals matched).
- 1 mark — Identifies the number of reactants/products as the most reliable classification criterion and explains why the energy trigger column alone is not sufficient.
- 1 mark — Explains what a catalyst does and why MnO₂ does not appear in Reaction 4's balanced equation.
- 1 mark — Explains why neither heat nor a catalyst appears in a balanced equation (balanced equations record substances, not conditions).
- 1 mark — Applies the law of conservation of mass using a specific lesson example with atom counts shown.
- 1 mark — Uses precise lesson vocabulary throughout and reaches an integrated, evidence-based conclusion distinguishing the two decomposition reactions on at least two criteria.