HSC Exam Practice

Sample response. The activity series is a ranking of metals from most to least reactive, based on their relative tendency to lose electrons (be oxidised) under comparable conditions.

Marking notes. 1 mark for identifying it as a ranking by tendency to lose electrons / relative reactivity / tendency to be oxidised. “Ranking by reactivity” alone scores 0 without a reference to electron loss or oxidation tendency.

Sample response. Aluminium forms a thin, dense, impermeable layer of aluminium oxide (Al₂O₃) on its surface almost instantly on contact with oxygen. This passivation layer physically prevents the acid from reaching the underlying aluminium metal. Therefore aluminium does not appear to react readily with dilute HCl despite being genuinely reactive (above zinc in the NESA series).

Marking notes. 1 mark for identifying the Al₂O₃ layer (passivation). 1 mark for explaining that this layer physically prevents acid contact — not that Al is intrinsically unreactive.

Sample response. (a) Reaction. Fe is above Ag in the NESA activity series, so Fe can displace Ag⁺ ions from solution. (b) No reaction. Cu is below Zn in the NESA activity series, so Cu cannot displace Zn²⁺ ions from solution.

Marking notes. 1 mark per part: correct decision + correct activity series reasoning. A correct decision without reasoning scores 0.

Sample response. Potassium has a larger atomic radius than magnesium because K is in Period 4 and has more electron shells, while Mg is in Period 3. The larger radius means K's outermost electron is further from the nucleus and more shielded, requiring less energy to remove — K has a lower first ionisation energy (419 kJ mol⁻¹) than Mg (738 kJ mol⁻¹). K also has lower electronegativity than Mg, so it has less tendency to hold its valence electron. Together these properties mean K loses its valence electron far more readily under the same conditions, driving a faster, more vigorous reaction with water.

Marking notes. 1 mark for atomic radius comparison and structural reason (more electron shells → outermost e⁻ further from nucleus). 1 mark for ionisation energy comparison and link to electron loss. 1 mark for electronegativity and overall conclusion linking all three properties to greater reactivity of K.

Sample response. (a) Reaction occurs: Cu is above Ag in the NESA activity series, so Cu will displace Ag⁺ from AgNO₃ solution. (b) Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s). Check: 1Cu, 2Ag, 2N, 6O each side. (c) Solution gradually turns blue as Cu²⁺ ions form; silver crystals (grey metallic solid) deposit on the copper wire surface.

Marking notes. 1 mark for correct prediction with activity series reference. 1 mark for balanced equation with correct state symbols (accept ionic equation Cu + 2Ag⁺ → Cu²⁺ + 2Ag). 1 mark for two correct observable changes (must name both colour change and solid deposit).

Sample response. Similarity: Both methods use zinc as a more reactive metal (higher in the activity series than iron) that is preferentially oxidised, protecting the underlying iron/steel. Difference: Galvanising applies a zinc coating that acts as both a physical barrier AND a sacrificial layer; sacrificial protection (external zinc anodes) has no physical barrier — the anode is separate from the structure and works purely electrochemically, even at a distance.

Marking notes. 1 mark for a correct similarity (activity series — both use Zn as more reactive metal / sacrificial oxidation). 1 mark for a correct difference (physical barrier in galvanising vs purely electrochemical protection in sacrificial anodes; or galvanising coats the surface vs anodes are attached separately).

(a) Trend in initial rate. Metal A has the steepest initial slope (highest rate of H₂ production in the first 2 minutes); Metal B has a moderately steep slope; Metal C has the least steep slope — it produces H₂ at the slowest initial rate. The trend confirms A > B > C in reactivity (electron-loss tendency) with HCl. [2 marks: 1 for correctly ordering the initial rates; 1 for linking to relative reactivity.]

(b) Plateau vs no plateau. Metal A's graph levels off because all of the 0.10 g sample has been consumed (completely dissolved) — no more metal remains to react with the excess HCl, so no more H₂ is produced. Metal C is far less reactive; its reaction with HCl is slow, and 0.10 g of Metal C has not been fully consumed within 10 minutes, so the volume of H₂ is still rising at the end of the experiment. [2 marks: 1 for explaining A reaches plateau because metal is consumed; 1 for explaining C does not plateau because reaction is too slow to consume the metal in 10 min.]

(c) Identify the metals. Metal A = Mg: highest initial rate; graph plateau at ~92 mL within 3 minutes — magnesium is the most reactive of the three in the NESA series (Mg > Zn > Fe). Metal B = Zn: intermediate rate; Zn is above Fe in the series but below Mg. Metal C = Fe: lowest rate; Fe is the least reactive of the three and reacts slowly with dilute HCl. [3 marks: 1 per correct identification with clear link to activity series position and graph evidence; partial credit for 2 correct.]

(a) Percentage reduction. Reduction = 0.45 − 0.02 = 0.43 mm yr⁻¹. Percentage reduction = (0.43 / 0.45) × 100 = 95.6%. Zinc anodes reduce the corrosion rate of steel by approximately 96%. [2 marks: 1 for correct subtraction and setup; 1 for correct percentage answer (accept 95%–96%).]

(b) Copper cladding accelerates corrosion. Copper is below iron in the NESA activity series, meaning Cu has a lower tendency to be oxidised than Fe. When copper cladding is in direct metallic contact with the steel pile and both are immersed in seawater (a conducting electrolyte), a galvanic cell forms [1]. In this cell, iron acts as the anode (more reactive / higher in the series) and copper acts as the cathode [1]. Electrons flow from Fe to Cu: Fe → Fe²⁺ + 2e⁻. The large area of copper (cathode) relative to the steel contact zone accelerates the anodic dissolution of iron, explaining why the steel corrodes faster (0.68 mm yr⁻¹) than unprotected steel (0.45 mm yr⁻¹) [1]. This is the reverse of the intended effect — copper cladding is counterproductive for iron protection in marine environments.

Sample Band 6 response. The claim is fundamentally flawed because it conflates high reactivity with high usefulness as a construction material — a misapplication of the activity series that ignores both practical chemistry and Australian infrastructure evidence.

At the atomic level, reactivity is determined by a metal's tendency to lose electrons, which is controlled by atomic radius, ionisation energy, and electronegativity. Potassium (K), at the top of the NESA activity series, has the largest atomic radius in the listed metals, the lowest ionisation energy, and the lowest electronegativity — it loses its single valence electron almost instantaneously on contact with water or air. This extreme reactivity makes potassium completely unusable as a construction or infrastructure material: it explodes on contact with water and must be stored under oil. The claim that “more reactive equals more useful” collapses immediately at this example.

By contrast, iron (Fe) is moderately placed in the activity series — below Mg, Al, and Zn but above Cu, Ag, and Au. Its moderate reactivity means iron (as steel) is strong, machinable, weldable, and commercially accessible, while being reactive enough to act as the cathode in a galvanic pair when more reactive metals are attached. This is precisely why BHP-grade structural steel is the backbone of Australian construction from the Sydney Harbour Bridge to the Snowy Mountains hydroelectric scheme — moderate reactivity combined with engineerable corrosion control is more useful than extreme reactivity or extreme inertness.

Gold (Au), near the bottom of the activity series, has almost no tendency to lose electrons — it does not corrode in air, water, or dilute acids. This is useful for electrical contacts and fine jewellery but useless for structural engineering due to cost and low tensile strength. High inertness is not the same as “more useful” either.

In the context of corrosion protection, the activity series is used to engineer protection systems rather than simply to select the most reactive or least reactive metal. Zinc (above Fe) is used as a sacrificial anode because it preferentially corrodes, protecting steel. If the claim were correct, we would use potassium or sodium as the sacrificial metal — which would be explosively dangerous and impractical. The optimal material for a given function depends on matching a metal's position in the activity series to the task: high position for sacrificial roles (Zn anodes), moderate position for structural use (Fe/steel), and low position for long-term inertness in corrosive environments (Cu pipes, Au contacts).

In summary, the claim is rejected. Usefulness as a construction material depends on the entire profile of a metal — mechanical properties, cost, availability, and position in the activity series relative to its environment — not simply on being “most reactive.” The activity series is a predictive tool for designing material systems, not a usefulness ranking.

Marking criteria.

• 1 mark — Provides an explicit evaluative judgement (accepts or rejects the claim with a clear stance).
• 1 mark — Names and correctly places at least one metal high in the activity series (K, Na, or Ca) and explains why high reactivity makes it unsuitable for construction (explosive, unstable).
• 1 mark — Names and correctly places at least one metal lower in the series (Cu, Fe, or Au) and gives a specific use case where moderate or low reactivity is advantageous.
• 1 mark — Correctly explains the atomic basis of reactivity (at least two of: atomic radius, ionisation energy, electronegativity) for one named metal.
• 1 mark — Uses the activity series in the context of corrosion or protection (e.g. sacrificial anodes use a more reactive metal above the protected metal in the series).
• 1 mark — Identifies a counter-example to the claim: a highly reactive metal (e.g. K) is dangerous or impractical as a construction material; or a metal with lower activity series position is more useful (e.g. steel vs Na).
• 1 mark — Identifies that usefulness depends on matching position in the series to function, not on maximising reactivity.
• 1 mark — Reaches an evidence-based, nuanced conclusion using precise lesson terminology (activity series, sacrificial protection, electron-loss tendency, oxidation, and at least one Australian infrastructure reference or named example).