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Difficulty: MediumIron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

Underground steel pipelines are often connected to sacrificial blocks of magnesium to prevent rusting. Which statement best explains the electrochemical principle behind this method of corrosion protection?

  1. A
    Carbon(II) oxide in the surrounding soil reduces any oxidized iron ions back to metallic iron, preventing corrosion.
  2. Magnesium is more electropositive than iron, so it loses electrons preferentially and serves as a sacrificial anode.Answer
  3. C
    Magnesium acts as the cathode where oxidation takes place, while the iron pipe functions as the protective anode.
  4. D
    The rate of iron oxidation is halted because magnesium acts as the limiting reactant based purely on its initial mass.

Answer

Magnesium is more electropositive than iron, so it loses electrons preferentially and serves as a sacrificial anode.
Magnesium is higher than iron in the electrochemical series. When connected electrically in a moist environment, magnesium oxidizes preferentially by donating electrons to the iron structure, causing magnesium to dissolve as sacrificial anode while keeping iron uncorroded as the cathode.

Step-by-Step Solution

1
Compare the positions of magnesium and iron in the electrochemical series.
Magnesium (MgMg) has a higher standard oxidation potential than iron (FeFe).
Metals positioned higher in the activity series lose valence electrons more readily.
2
Determine which metal undergoes oxidation in the galvanic cell formed by moisture.
Magnesium undergoes oxidation at the sacrificial anode (Mg→Mg2++2e−Mg \rightarrow Mg^{2+} + 2e^-).
The more electropositive metal preferentially oxidizes, suppressing the oxidation of iron (Fe→Fe2++2e−Fe \rightarrow Fe^{2+} + 2e^-).
3
Identify the overall protection mechanism.
Cathodic protection (sacrificial protection) keeps the iron pipeline safe as long as magnesium is present to corrode sacrificial anodes.
Iron is forced to remain the cathode, maintaining its elemental state.

Key Concept

Cathodic (Sacrificial) Protection of Iron
Estimated Time:1m 0s
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Topics

Iron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

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