Questions and Answers
What forms a coating on the metal surface during corrosion?
Metal Oxide or other salt of the metal
Which element acts as the anode in the rusting of iron?
Iron (Fe)
What is the cathodic reaction during the rusting of iron?
O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
Write the overall reaction for the rusting of iron.
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What is the role of a sacrificial electrode in preventing corrosion?
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How many moles of mercury will be produced by electrolyzing 10M Hg(NO₃)₂ solution with a current of 2000 A for three hours?
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Study Notes
Corrosion
- Corrosion occurs when a metal oxide or salt forms a coating on the metal surface.
- Examples of corrosion include tarnishing of silver and rusting of iron.
Rusting of Iron
- Rusting involves the oxidation of Fe (acting as an anode) and the reduction of O₂ (acting as a cathode).
- Oxidation at anode: 2Fe(s) → 2Fe²⁺(aq) + 4e⁻.
- Reduction at cathode: O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l).
- The overall reaction: 2Fe(s) + O₂ + 4H⁺(aq) → 2Fe²⁺(aq) + 2H₂O(l).
Atmospheric Oxidation
- Atmospheric oxidation occurs when Fe²⁺ ions are further oxidized by atmospheric oxygen.
- Reaction: 2Fe²⁺(aq) + 2H₂O(l) + ¹/₂O₂(g) → Fe₂O₃(s) + 4H⁺(aq).
Methods for Prevention of Corrosion
- Electrochemical method: uses a sacrificial electrode of another metal (like Mg, Zn) that corrodes itself to save the object.
- Sacrificial metals should be more electro positive than iron.
- Other methods: covering the surface with paint or using a chemical (e.g., bisphenol).
Example Problem
- Electrolysing 10M Hg(NO₃)₂ solution with a current of 2000 A for three hours produces mercury.
- Mol of Hg produced depends on the reaction and current used.
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