Chemistry • Year 11 • Module 3 • Lesson 7
Metal Activity Series & Reactions of Metals
Build HSC Band 5–6 extended-response technique on the activity series, corrosion, and the evaluation of corrosion-protection strategies in Australian infrastructure contexts.
1. Data + scenario — corrosion protection of the Tomago aluminium smelter jetty (Band 5–6)
8 marks Band 5–6
Scenario. The Tomago Aluminium smelter in the Hunter Valley, NSW, operates a marine jetty used to unload raw bauxite shipments from vessels on the Hunter River. The jetty structure consists of mild steel piles (iron-based) submerged in tidal estuarine water, a moderately saline electrolyte. The original designers specified zinc sacrificial anodes attached to each steel pile at the waterline. After 12 years in service, an engineering review finds that a section of the jetty nearest the ship-unloading platform has experienced significantly greater anode consumption than the rest of the jetty, despite using identical zinc anodes. The review also considers whether aluminium-alloy anodes (which are lighter and cheaper per kilogram) could replace the zinc anodes going forward, given that aluminium is higher than zinc in the activity series. However, field engineers raise a concern about aluminium passivation.
Figure 1.1. Mean annual zinc anode mass loss per pile across four sections of the Tomago jetty. Hypothetical data, illustrative of tidal marine corrosion patterns.
Q1. Analyse and evaluate the data and scenario to assess the corrosion protection situation and recommend an approach for the jetty review. In your response you must:
- Use the data to identify a pattern in anode consumption and propose a chemical or physical reason for the higher consumption rate at Section 4.
- Explain, using the activity series and the concept of sacrificial protection, why zinc anodes protect the steel piles.
- Evaluate whether aluminium anodes would be a suitable replacement, explicitly addressing the passivation concern raised by the field engineers.
- Reach a justified engineering recommendation about which anode material to use going forward, weighing reactivity in the activity series against practical behaviour.
2. Source critique — Berzelius and the activity series (Band 5–6)
7 marks Band 5–6
“The activity series was originally developed in the early 19th century by Jöns Jacob Berzelius, who ranked metals by the vigour of their reaction with oxygen only. Because gold does not form an oxide and potassium burns explosively, Berzelius concluded that reactivity toward oxygen was the definitive measure of all metal reactivity. The same ranking was later confirmed to predict displacement reactions in solution, proving that reactivity with oxygen is the fundamental property from which all other metal reactivity is derived.”
Attributed: fictional secondary-school textbook, 2023.
Q2. Evaluate this claim. Identify any parts that are scientifically correct, identify any parts that are flawed or overstated, and explain the correct chemistry. In your response:
- Identify which part of the claim about Berzelius is broadly consistent with the historical development of the activity series.
- Identify and correct the specific scientific flaw in the claim that “reactivity with oxygen is the fundamental property from which all other metal reactivity is derived”.
- Explain what the activity series actually measures at the atomic/electronic level, and why the same series predicts reactivity with oxygen, water, and acids.
- Describe how displacement reaction experiments (not just oxygen reactions) are used to construct and verify the activity series.
Q1 — Sample Band 6 response (8 marks), with marking criteria
Data pattern and reason for Section 4: The graph shows a clear trend of increasing anode consumption from Section 1 (1.2 kg/pile/yr) to Section 4 (4.8 kg/pile/yr), with Section 4 consuming roughly four times as much zinc as Section 1. The most likely explanation is increased electrolyte conductivity or higher current demand at Section 4, driven by: (a) greater mechanical disturbance of the water (ship propellers, berthing activity) increasing dissolved oxygen and electrolyte mixing, (b) possible contamination of the estuarine water near the platform with salts from ballast water discharge, raising conductivity, or (c) stray electrical currents from the ship or docking equipment that accelerate electrochemical reactions. Any one well-reasoned explanation earns the mark. [2 marks: 1 for identifying the pattern, 1 for a reasonable physical/chemical reason]
Zinc sacrificial protection mechanism: Zinc (Zn) is higher than iron (Fe) in the NESA activity series, meaning zinc has a lower ionisation energy and greater tendency to be oxidised (lose electrons). When a zinc anode is electrically connected to a steel pile and both are immersed in an electrolyte (estuarine water), zinc acts as the anode (Zn → Zn2+ + 2e−) and preferentially loses electrons. Iron acts as the cathode and receives electrons — preventing iron from being oxidised to Fe2+ (rust). The zinc corrodes rather than the steel. [2 marks: 1 for connecting activity series ranking to electron-loss tendency; 1 for correctly describing Zn as anode and Fe as cathode]
Evaluation of aluminium anodes: Aluminium is above zinc in the NESA activity series (Al > Zn > Fe), which means Al has an even greater thermodynamic tendency to be oxidised than zinc — at first glance making Al an attractive sacrificial anode material that could protect both Zn and Fe. However, the field engineers' concern about passivation is critical: aluminium rapidly forms a dense, adherent Al2O3 layer in water that prevents further oxidation. This passivation means that despite Al's true reactivity, it may not function as an effective sacrificial anode in marine conditions because its surface becomes electrically inactive. Marine-grade aluminium anodes (aluminium-indium alloys) are engineered to disrupt passivation, but standard aluminium blocks would likely be ineffective. [2 marks: 1 for correctly noting Al is above Zn in the series so thermodynamically more active; 1 for correctly evaluating passivation as the practical barrier]
Engineering recommendation: Retain zinc anodes (or use marine-grade Al-In alloy anodes where cost savings are required). Standard aluminium should not replace zinc despite its higher position in the activity series because passivation makes it unreliable as a sacrificial anode in marine environments. For Section 4, the higher consumption rate suggests more frequent anode inspection and replacement intervals, or larger anode blocks — not a change of material. [2 marks: 1 for a justified recommendation consistent with the analysis; 1 for explicitly integrating both the activity series position and the passivation constraint]
Marking criteria summary:
- 2 marks — Identifies increasing consumption trend from data AND proposes a chemically/physically plausible reason for Section 4's elevated rate (e.g. higher salinity, increased current, stray electrical current).
- 2 marks — Correctly explains the sacrificial protection mechanism using activity series (Zn above Fe → Zn preferentially oxidised as anode → Fe protected as cathode).
- 2 marks — Evaluates aluminium anodes: correctly identifies Al is above Zn in the series AND correctly identifies that passivation is a practical barrier to aluminium functioning as an effective sacrificial anode.
- 2 marks — Reaches a justified, evidence-based recommendation that integrates activity series position and practical behaviour; does not simply say “use the most reactive metal”.
Q2 — Sample Band 6 response (7 marks)
What the claim gets right: It is historically true that early 19th-century chemists, including Berzelius, used reactivity with oxygen as one of the early criteria for ranking metal reactivity. The observation that potassium burns explosively while gold does not react with oxygen at all provided early evidence for a reactivity gradient. This part of the claim is broadly consistent with the historical development of the activity series. [1 mark]
Scientific flaw — oxygen as “fundamental”: The claim that “reactivity with oxygen is the fundamental property from which all other metal reactivity is derived” is a scientific overstatement and is misleading. The actual fundamental property measured by the activity series is the tendency of a metal atom to lose electrons to form cations — in other words, its electron-loss tendency (inversely related to ionisation energy and electronegativity). Oxygen is just one possible oxidising agent. The same electron-loss tendency that determines reactivity with O2 also determines reactivity with water (where H2O acts as the oxidising agent), with acids (where H+ acts as the oxidising agent), and with metal ions in solution (displacement reactions). The series does not “derive” from oxygen — it reflects a more fundamental atomic property. [2 marks: 1 for identifying the specific flaw; 1 for explaining the correct underlying atomic/electronic property]
What the activity series actually measures: The activity series ranks metals by their relative tendency to be oxidised (lose electrons). This is determined by atomic radius (larger radius → valence electron held more loosely), ionisation energy (lower IE → easier to remove the electron), and electronegativity (lower electronegativity → less tendency to hold onto electrons). Potassium ranks highest because it has the largest atomic radius among the listed metals and the lowest ionisation energy. Gold ranks lowest because it has strong nuclear hold on its valence electrons (small radius, high electronegativity), making it very difficult to oxidise. This one underlying property (electron-loss tendency) predicts behaviour with all oxidising reagents — oxygen, water, acid, or metal ions in solution. [2 marks: 1 for explaining the atomic/electronic basis of the series; 1 for explaining why it predicts multiple reaction types]
How displacement experiments verify the series: By systematically placing metal A in a solution of metal B's salt (e.g. copper wire in silver nitrate, iron nail in copper sulfate), chemists can observe whether a reaction occurs. If A displaces B, then A is more reactive than B. By performing a matrix of such experiments, the complete ranking can be confirmed and refined. Displacement experiments are important because they are conducted in aqueous solution at room temperature — conditions closer to real industrial applications (corrosion, electroplating) than high-temperature oxide formation, and they also verify the position of hydrogen as a reference point in the series. [2 marks: 1 for describing the displacement experiment method; 1 for identifying an advantage of displacement experiments over oxygen-only ranking]
Marking criteria summary:
- 1 mark — Correctly identifies the historically defensible element of the claim (early oxygen-reaction rankings were part of the series' development).
- 1 mark — Identifies the specific scientific flaw: the claim that oxygen reactivity is the “fundamental” source from which all other reactivity derives is incorrect.
- 1 mark — States the correct fundamental property: tendency to lose electrons / electron-loss tendency / relative ionisation energy / position relative to electronegativity.
- 2 marks — Explains why the same electron-loss tendency predicts behaviour toward oxygen, water, acid, and metal ions in solution (2 marks for clear explanation covering at least two reagent types; 1 mark for one reagent type).
- 2 marks — Describes how displacement reaction experiments are used to construct/verify the series, AND identifies an advantage over oxygen-only experiments (e.g. aqueous conditions, hydrogen reference, room temperature).