4CH1

The Reactivity Series

Inorganic Chemistry · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable
Low
Stable6%

Displacement reactions and determining order of reactivity from experimental data.

Conditions for rusting

  • Rust is the orange-brown flaky oxide that forms on iron and steel
  • Both air (oxygen) and water must be present for iron to rust — neither on its own is enough
  • The chemical name for rust is hydrated iron(III) oxide, Fe2O3·xH2O

Investigating the conditions

  • Set up three test tubes, each containing a clean iron nail:
    • Tube A — open to the air, partly filled with tap water (contact with both air and water)
    • Tube B — partly filled with water that has been boiled to drive off dissolved oxygen, then sealed with a layer of oil on top (contact with water but not air)
    • Tube C — contains a drying agent such as anhydrous calcium chloride, sealed with a stopper (contact with air but not water)
  • After several days:
    • The nail in tube A is rusty
    • The nails in tubes B and C are unchanged
  • Conclusion: both oxygen and water are required; removing either prevents rusting
Three test tubes investigating rusting conditions: iron nail in air and water (rusts), in boiled water sealed with oil (water, no air), and in air over calcium chloride (air, no water)
Source: Rusting of iron by Save My Exams

Why rusting is a problem

  • Rust is soft and flakes off the metal, exposing a fresh iron surface underneath
  • Once started, rusting therefore continues right through the bulk of the iron, weakening structures over time
  • This is different from the protective oxide that forms on aluminium, which seals the surface and stops further reaction

Prevention by barrier methods

  • Coating iron physically isolates the metal from oxygen and water
  • Common barrier coatings: paint, oil, grease, plastic, and an electroplated layer of an unreactive metal (such as tin on tin cans)
  • Drawback: if the coating is scratched, the bare iron is exposed and rust begins, often spreading under the coating

Prevention by sacrificial protection

  • A more reactive metal is bolted to the iron object
  • The more reactive metal corrodes preferentially because it loses electrons more readily, so the iron stays intact
  • Zinc blocks attached to the steel hulls of ships are a textbook example: the zinc corrodes away first and is replaced periodically
Sacrificial protection of a ship's steel hull by zinc blocks bolted below the waterline, which corrode in preference to the iron
Source: Rusting of iron by Save My Exams

Galvanising

  • coats the iron with a layer of zinc, applied by dipping the iron into molten zinc or by electroplating
  • The zinc layer protects in two ways at once:
    • As a barrier, it keeps oxygen and water away from the iron underneath
    • As , if the coating is scratched the zinc still corrodes preferentially, so the exposed iron is not attacked
Exam tip

Explaining why galvanising prevents rusting (two marks)

What comes up: the exam asks you to explain why galvanising works as a method of rust prevention, often in the context of a scratched zinc coating still protecting the iron beneath.

Write (two marks): (1) zinc is more reactive than iron (or: zinc is higher in the reactivity series than iron). (2) zinc therefore oxidises, corrodes, or reacts with oxygen in preference to iron — so the iron is protected even when the coating is damaged.

Watch out: the mark scheme rejects "zinc rusts" as a description of what the zinc does — rust refers specifically to iron. Say zinc oxidises or corrodes instead.

A note on terminology

  • Corrosion is the general term for the slow chemical attack of a metal surface by its environment
  • Rusting is specifically the corrosion of iron — every rusting reaction is a corrosion reaction, but not every corrosion reaction is rusting