HSC Exam Practice

Sample response. A synthesis reaction is one in which two or more reactants combine to form a single product — general pattern: A + B → AB. A decomposition reaction is one in which a single compound breaks down into two or more simpler substances — general pattern: AB → A + B.

Marking notes. 1 mark — defines synthesis with correct A + B → AB pattern; 1 mark — defines decomposition with correct AB → A + B pattern; 1 mark — correctly uses the words "single product" (synthesis) and "two or more" (decomposition) to distinguish them.

Sample response. Thermal decomposition is driven by heat; for example, CaCO₃(s) → CaO(s) + CO₂(g). Electrolytic decomposition is driven by an electric current; for example, 2H₂O(l) → 2H₂(g) + O₂(g) (Nicholson and Carlisle, 1800). The key distinction is the type of energy input: heat (thermal) versus electrical energy (electrolytic).

Marking notes. 1 mark — distinguishes on energy type (heat vs electricity); 1 mark — correct definition/description of thermal decomposition; 1 mark — correct balanced equation with state symbols for thermal example (accept CaCO₃ → CaO + CO₂, or CuCO₃ → CuO + CO₂, or 2KClO₃ → 2KCl + 3O₂); 1 mark — correct balanced equation with state symbols for electrolytic example (accept 2H₂O → 2H₂ + O₂).

Sample response. Changing a subscript inside a formula changes the identity of the substance — it creates a completely different (and often non-existent) compound rather than adjusting the quantity of the correct compound. For example, writing H₃O instead of 2H₂O would represent a different molecule (hydronium, which is not water), violating the requirement to use correct chemical formulas. Only coefficients — numbers placed in front of the formula — may be changed when balancing, because they adjust quantity while keeping the formula (and therefore the substance) the same.

Marking notes. 1 mark — states that changing a subscript changes the identity/nature of the substance; 1 mark — provides a specific example illustrating this (e.g. H₃O ≠ H₂O, or similar); 1 mark — explains that coefficients are the only acceptable method as they adjust quantity not formula.

Sample response (a). Synthesis — two reactants (Fe and Cl₂) combine to form one product (FeCl₃). Balanced: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s). Atom check: Left 2 Fe, 6 Cl. Right 2 Fe, 6 Cl ✓

Sample response (b). Decomposition — one reactant (KClO₃) breaks into two products (KCl and O₂). Balanced: 2KClO₃(s) → 2KCl(s) + 3O₂(g). Atom check: Left 2 K, 2 Cl, 6 O. Right 2 K, 2 Cl, 6 O ✓

Marking notes. 1 mark per correctly balanced equation with state symbols (×2 = 2 marks); 1 mark per correct reaction type identification with brief justification (×2 = 2 marks). Equations with correct coefficients but missing state symbols: deduct 0.5 per equation (accept half marks or follow school policy).

Sample response. As a fertiliser, NH₄NO₃ dissolves in soil water at ambient temperature to release ammonium (NH₄⁺) and nitrate (NO₃⁻) ions — no decomposition occurs because temperatures are far too low to initiate the reaction. As an explosive, NH₄NO₃ undergoes thermal decomposition when heated to high temperatures in confined conditions: 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g). The same compound, different conditions → very different outcomes.

Marking notes. 1 mark — describes fertiliser use correctly (dissolution, N-release, ambient temp, no decomposition); 1 mark — describes explosive behaviour correctly (high temperature, confinement, or uncontrolled decomposition, reference to large gas volume); 1 mark — explicitly links the difference to conditions (temperature and/or confinement) rather than to a difference in the compound itself.

Sample response. Step 1: Write the unbalanced equation with correct formulas and state symbols. Step 2: Count atoms of each element on both sides. Step 3: Add coefficients to balance, starting with the most complex molecule. Step 4: Balance hydrogen and oxygen last — they appear in the most compounds, so balancing them last avoids having to revise earlier work. Step 5: Verify by recounting all atoms on both sides.

Marking notes. 1 mark — identifies write (correct formulas) → count → coefficient adjustment as the three-stage process; 1 mark — states that H and O are balanced last; 1 mark — gives a reason for this (H and O appear in many compounds, so balancing them last avoids iterating earlier steps).

Part (a) — 3 marks. Both compounds show decreasing percentage remaining as energy input increases. CaCO₃ decreases more slowly at low energy inputs — at 550 °C approximately 88% remains, still near its starting value — then falls steeply between 700 °C and 900 °C, ending at about 8%. AgBr decreases more rapidly with increasing light: at the equivalent point (550 on the x-axis) only about 60% remains, and it approaches near-zero before the highest energy input. AgBr therefore has a lower energy threshold for decomposition than CaCO₃ requires for thermal decomposition. (1 mark — describes both trends as decreasing; 1 mark — compares rate/speed of decrease with data values; 1 mark — draws a comparative conclusion about energy threshold.)

Part (b) — 2 marks. CaCO₃(s) → CaO(s) + CO₂(g). Atom count: Left: 1 Ca, 1 C, 3 O. Right: 1 Ca (in CaO) + 1 O (in CaO) + 1 C + 2 O (in CO₂) = 1 Ca, 1 C, 3 O. ✓ Balanced. (1 mark — correct balanced equation with state symbols; 1 mark — correct atom count shown for both sides.)

Part (c) — 2 marks. Different types of energy trigger initiate different named categories of decomposition reaction — heat triggers thermal decomposition and light triggers photodecomposition. This demonstrates that the type of energy provided, not just the quantity, determines whether and how decomposition occurs. A compound that does not decompose under one energy type may readily decompose under another (e.g. AgBr is stable to moderate heating but decomposes quickly in light). (1 mark — states that different energy triggers lead to different named decomposition types; 1 mark — gives a substantive interpretation — e.g. trigger type is specific to reaction type; a compound responsive to one trigger may not respond to another.)

Part (a) — 2 marks. Left side: 2H₂O → 2 × 2 = 4 H atoms, 2 × 1 = 2 O atoms. Right side: 2H₂ → 4 H atoms; O₂ → 2 O atoms. Left: 4 H, 2 O. Right: 4 H, 2 O. ✓ Balanced. (1 mark — correct atom count for H on both sides; 1 mark — correct atom count for O on both sides, with explicit match confirmed.)

Part (b) — 2 marks. From the equation 2H₂O(l) → 2H₂(g) + O₂(g): the mole ratio of H₂ : O₂ = 2 : 1. This demonstrates the law of conservation of mass — atoms are neither created nor destroyed, so the number of H and O atoms on the product side must equal those on the reactant side, constraining the ratio of gaseous products. (1 mark — correctly states 2:1 mole ratio; 1 mark — correctly identifies and briefly explains the law of conservation of mass as the governing principle.)

Part (c) — 2 marks. The claim is incorrect. The electrolysis of water is a decomposition reaction (AB → A + B): one reactant (H₂O) breaks into two simpler products (H₂ and O₂). The fact that water can be broken down into hydrogen gas and oxygen gas shows that water is a compound — it is composed of hydrogen and oxygen chemically combined. An element cannot be decomposed into two different substances by any chemical reaction; the decomposition of water proves it contains at least two elements. (1 mark — correctly classifies electrolysis of water as decomposition with justification; 1 mark — explains that the products reveal water is a compound, not an element, since it can be broken into two different substances.)

Sample response. The claim that decomposition reactions are always dangerous and difficult to control is incorrect: the hazard and controllability of a decomposition reaction depend almost entirely on the conditions — particularly the energy trigger applied, the degree of confinement, and the nature of the products — not on the reaction type itself.

Decomposition reactions span a wide range of behaviours. Thermal decomposition of limestone — CaCO₃(s) → CaO(s) + CO₂(g) — is a routine, controlled industrial reaction. Heat is applied in a controlled manner; the rate of decomposition is managed by adjusting temperature, and the gaseous CO₂ disperses harmlessly into the atmosphere. Similarly, electrolytic decomposition of water — 2H₂O(l) → 2H₂(g) + O₂(g) — was first demonstrated safely by Nicholson and Carlisle in 1800. An electrical current is the energy trigger; by controlling the current precisely, the rate of decomposition is fully managed, and both gaseous products (hydrogen and oxygen) can be collected in an orderly manner.

Decomposition reactions become dangerous when conditions change. Ammonium nitrate (NH₄NO₃) is stable enough to store and apply as a fertiliser at ambient temperatures — no decomposition occurs because the thermal energy present is insufficient to initiate the reaction. However, if NH₄NO₃ is heated rapidly to high temperatures in a confined space, the decomposition switches to a more violent pathway: 2NH₄NO₃(s) → 2N₂(g) + O₂(g) + 4H₂O(g). This generates 3.5 moles of gas per mole of solid — a catastrophic pressure build-up if confined, as demonstrated by the 2020 Beirut explosion. The difference between the fertiliser bag and the disaster is not the compound but the conditions: temperature, confinement, and contamination.

In summary, decomposition reactions are not inherently dangerous or uncontrollable. They become so only when the energy trigger is applied too rapidly, in confinement, or when incompatible materials are present. When conditions are engineered correctly — controlled temperature, appropriate containment, correct energy type — decomposition reactions are among the most important and reliable chemical processes in modern industry. The claim is therefore rejected: it is conditions, not reaction type, that determine hazard.

Marking notes. 1 mark — states an explicit evaluative judgement rejecting the claim with a reason (conditions, not reaction type, determine hazard). 1 mark — names and describes at least one safe/controlled decomposition example from the lesson with a balanced equation and correct reaction type label (accept: CaCO₃ thermal decomposition, electrolysis of water, H₂O₂ catalytic decomposition, KClO₃ thermal decomposition, AgBr photodecomposition). 1 mark — names and describes a second, distinct decomposition example from the lesson using a different energy trigger type. 1 mark — identifies at least two named energy triggers (e.g. heat, electricity, light, catalyst) and links each to a named decomposition type and example from the lesson. 1 mark — explicitly assesses how conditions (temperature, confinement, rate) determine whether the same reaction (NH₄NO₃) is hazardous or useful, with reference to the two decomposition pathways taught in the lesson. 1 mark — reaches a justified overall conclusion using precise lesson vocabulary (thermal decomposition, electrolytic decomposition, catalytic decomposition, photodecomposition, balanced equation, energy trigger, products, law of conservation of mass). 1 mark — demonstrates Band 5–6 synthesis by integrating three or more concepts from the lesson (reaction type, energy trigger, conditions, products, law of conservation of mass, Beirut real-world context) into a coherent, evidence-based argument.