Chemistry • Year 11 • Module 3 • Lesson 2
Synthesis & Decomposition
Apply synthesis and decomposition concepts to real-world data, cause-and-effect reasoning, and case-study scenarios at Band 4–5.
1. Interpret data — ammonium nitrate decomposition temperature trials
A safety research laboratory measured the proportion of ammonium nitrate (NH₄NO₃) that fully decomposed within 10 minutes at five different temperatures. Two experimental conditions were trialled: Condition 1 (open container, unconfined) and Condition 2 (sealed container, confined). The results are shown in the graph below. 8 marks
Figure 1.1. Hypothetical laboratory data illustrating the effect of temperature and confinement on NH₄NO₃ decomposition rate.
1.1 Describe the trend in each condition as temperature increases from 150 °C to 350 °C. 2 marks
1.2 At 250 °C, estimate the difference in percentage decomposed between the two conditions. What does this difference suggest about the effect of confinement on decomposition rate? 2 marks
1.3 The lesson states that ammonium nitrate in Beirut decomposed explosively because of confinement combined with rising temperature. Use the data to explain why these two factors together — not either one alone — are dangerous. 2 marks
1.4 An engineer proposes storing ammonium nitrate in sealed containers at 200 °C as a safety measure. Evaluate this claim using the data. 2 marks
2. Cause-and-effect chain — electrolysis of water by Nicholson and Carlisle (1800)
In 1800, William Nicholson and Anthony Carlisle passed an electric current through water and demonstrated that it decomposed into two gases. Complete the cause-and-effect chain below by filling in the empty effect boxes. 5 marks
Cause 1
An electric current (direct current) is passed through liquid water.
Effect 1 (fill in)
Effect 2 (fill in)
Cause 3 (link)
The balanced equation for this reaction is 2H₂O(l) → 2H₂(g) + O₂(g).
Effect 3 (fill in)
Effect 4 (fill in)
Overall outcome (so…):
Nicholson and Carlisle's experiment confirmed that water was a compound, not an element, because…
3. Interpret a data table — comparing synthesis reactions
A Year 11 class carried out three synthesis reactions and recorded data in the table below. Use the data to answer the questions that follow. 7 marks
| Reaction | Reactants | Product | Mass of reactants (g) | Mass of product (g) | Observation |
|---|---|---|---|---|---|
| 1 | Mg(s) + O₂(g) | MgO(s) | 24.3 g Mg + 16.0 g O₂ = 40.3 g | 40.3 g | Bright white flame; dazzling white solid forms |
| 2 | Fe(s) + S(s) | FeS(s) | 55.8 g Fe + 32.1 g S = 87.9 g | 87.9 g | Glowing exothermic reaction; black solid forms |
| 3 | SO₂(g) + O₂(g) | SO₃(g) | 64.1 g SO₂ + 16.0 g O₂ = 80.1 g | 80.1 g | Requires V₂O₅ catalyst; sharp smell |
3.1 Identify which principle is illustrated by the data in columns 4 and 5 across all three reactions, and explain its significance. 2 marks
3.2 Reaction 3 uses a V₂O₅ catalyst. Explain what a catalyst does and why it does not appear as a reactant in the balanced equation. 2 marks
3.3 The products of each synthesis reaction have different properties from the reactants. Using Reaction 1 as an example, identify one property of MgO that differs from the properties of either Mg or O₂. 1 mark
3.4 Reaction 3 is the key step in the Contact Process for manufacturing sulfuric acid (H₂SO₄) — an industry of major economic importance in Australia. Write a word equation for the next step, in which SO₃ reacts with water to form sulfuric acid. Classify this next step as synthesis or decomposition and justify. 2 marks
4. Predict and justify — two pathways for one compound
Read the scenario, then answer the question below. 4 marks
Scenario. In Australian mining, ammonium nitrate fuel oil (ANFO) is used as a controlled explosive to break rock. Miners mix ammonium nitrate (NH₄NO₃) with diesel fuel oil and detonate it in a borehole. Under these conditions, the decomposition follows the explosive pathway: 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g). However, if NH₄NO₃ is gently heated in an industrial process to about 200 °C, it follows a different pathway: NH₄NO₃(s) → N₂O(g) + 2H₂O(g).
4.1 Predict what would happen to the moles of gas produced if a miner accidentally ignited a large stockpile of NH₄NO₃ in a confined underground tunnel instead of a borehole. Use the explosive decomposition equation to justify your prediction quantitatively, and explain why confinement makes the outcome worse. 4 marks
Q1.1 — Trend description (2 marks)
In both conditions, the percentage of NH₄NO₃ decomposed increases as temperature increases [1]. Condition 2 (sealed) shows a steeper increase at all temperatures — at 350 °C it reaches 100% decomposition, while Condition 1 (open) reaches approximately 82% [1]. Accept ±5% for estimated values.
Q1.2 — Difference at 250 °C (2 marks)
At 250 °C: Condition 1 ≈ 42%, Condition 2 ≈ 65% — a difference of approximately 23 percentage points [1]. The greater decomposition in the sealed container suggests that confinement accelerates the decomposition rate, likely because accumulated product gases increase pressure and heat, accelerating further decomposition [1]. Accept ±5% for estimates.
Q1.3 — Why confinement + high temperature together are dangerous (2 marks)
From the data, high temperature alone (open container) gives rapid but not complete decomposition [1]. Confinement alone at low temperature (Condition 2 at 150 °C) gives only ~8% decomposition. Together, high temperature increases decomposition rate while confinement traps the hot product gases, which build pressure and raise the temperature further — a positive feedback loop — producing far more complete and rapid decomposition than either factor alone [1].
Q1.4 — Evaluate the engineer's claim (2 marks)
The claim is flawed [1]. The data show that at 200 °C in a sealed container approximately 28% of the NH₄NO₃ has already decomposed in just 10 minutes. Storing it hot in a sealed container actually accelerates decomposition compared to an open container at the same temperature — this is the opposite of safe. The safer strategy is to store NH₄NO₃ cool and in well-ventilated conditions [1].
Q2 — Cause-and-effect chain: electrolysis of water (5 marks)
Effect 1: The electrical energy breaks the bonds in water molecules, splitting them into hydrogen and oxygen atoms [1].
Effect 2: Hydrogen gas (H₂) forms at one electrode and oxygen gas (O₂) forms at the other electrode [1].
Effect 3: The ratio of hydrogen to oxygen gas produced is 2:1 by volume, consistent with the equation 2H₂O → 2H₂ + O₂ [1].
Effect 4: The combined mass of H₂ and O₂ produced equals the mass of water consumed, confirming conservation of mass [1].
Overall outcome: …it could be split by electricity into two simpler substances (hydrogen and oxygen), proving it is a compound composed of those elements rather than a pure element [1].
Q3 — Data table interpretation (7 marks)
3.1 The law of conservation of mass — the total mass of reactants equals the total mass of products in all three reactions [1]. This is significant because it confirms that no atoms are created or destroyed in a chemical reaction, only rearranged [1].
3.2 A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy [1]. V₂O₅ is regenerated at the end of the reaction, so it is not consumed — it does not appear as a reactant or product in the balanced equation because its quantity is unchanged [1].
3.3 Accept any one valid property, e.g. MgO is a white ionic solid that does not burn, whereas Mg is a shiny metallic solid that burns brightly and O₂ is a colourless gas. The new compound has properties distinct from either element [1].
3.4 Word equation: sulfur trioxide + water → sulfuric acid. Synthesis — two reactants (SO₃ and H₂O) combine to form one product (H₂SO₄), matching the A + B → AB pattern [1 + 1].
Q4 — Predict and justify (4 marks)
From the equation 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g): 2 moles of solid produce 7 moles of gas — 3.5 moles of gas per mole of NH₄NO₃ [1]. In a confined tunnel, this enormous volume of rapidly produced hot gas cannot expand freely [1]. The trapped gas creates a rapid, catastrophic pressure increase — an explosion — with a destructive shock wave [1]. Confinement prevents gas dispersion, so all the pressure builds in one place, making the outcome far more destructive than if the same reaction occurred in the open air [1]. Accept equivalent reasoning referencing lesson Beirut content or the lesson's explosive vs controlled pathway comparison.