SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 8)

by BerryBerryTeacher

in Berry Reference (Notes)

Berry Readers will definitely be familiar with the concept of rusting. But do you know that rusting is also a redox reaction? Presented in this post is Part 8 of SPM Chemistry Form 5 notes on “Oxidation and Reduction” from Berry Berry Easy. In this post, you will get to read about the simplified 4-step rust process. Also important are the ways to prevent rust. Ways to prevent rust is typically found in exams on multiple choice questions (especially those that group answers together) and also in the structured paper. It’s popularity in exams is cyclical, so do read up on them.

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction (Part 8 )

Rusting of Iron Nails

Rusting of Iron Nails

Rusting as a Redox Reaction

  • Rust / hydrated iron(III) oxide, Fe2O3•xH2O – formed slowly at the surface of iron when it exposed to the damp air.
  • Rusting – a redox reaction that take places between iron and oxygen to form hydrated iron(III) oxide and this is a slow reaction.
    4Fe(s) + 3O2(g) + 2xH2O(l) –> Fe2O3•xH2O(s)

Corrosion – a redox reaction that take places between a metal and the gases in air. Metal is oxidised to form an oxide layer on the surface. Metal atoms lose electrons to form positive ions.

  1. Group 1 metals are very reactive.
  2. Metals are exposed to air will corrode rapidly and become tarnished.
  3. Aluminium, lead and zinc corrode rapidly in the air and forms an oxide layer. The oxide layer is hard, non-porous, impermeable and difficult to crack. This protects the aluminium, lead and zinc below it from further corrosion.

Example: Corrosion of metal.
Zn(s) –> Zn2+(aq) + 2e
Cu(s) –> Cu2+(aq) + 2e

K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Hg, Ag, Au
<————Tendency of metal to corrode increases.

Process of Rusting of Iron

  1. Anode (negative terminal) – concentration of oxygen is lower and iron rust (oxidation process) to form iron(II) ions:
    Fe(s) –> Fe2+(aq) + 2e
  2. Cathode (positive terminal) – concentration of oxygen is higher and oxygen gains electrons that reduced to hydroxide ions:
    O2(g) + 2H2O(l) + 4e –> 4OH-(aq)
  3. Fe2+ ions and OH- ions combine to form iron(II) hydroxide, Fe(OH)2
  4. Oxygen further oxidises iron(II) hydroxide, Fe(OH)2 to hydrated iron(III) oxide, Fe2O3•xH2O.
    2Fe(OH)2(s) –> Fe2O3•xH2O(s)

The Effect of Other Metals on the Rusting of Iron

Potassium hexacyanoferrate(III), K3Fe(NO)6 is used to detect Fe2+ ions
(produces dark blue colour in the presence of Fe2+).

Phenolphthalein is used to detect OH- ions
(produces pink colour in the presence of OH-).

Test tube Observation Reaction
Fe only Intensity of blue colour is low. Oxidation:Fe(s) –> Fe2+(aq) + 2e
Control Pink colour is not present. Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
Inference Fe nail rusts a little.
Test tube Observation Reaction
Fe & Mg Blue colour is not present. Oxidation:Mg(s) –> Mg2+(aq) + 2e
Intensity of pink colour is very high. Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)
Inference Mg is corroded and Fe nail does not rust.

i) Fe act as the (+) terminal (cathode)
ii) Mg act as the (-) terminal (anode)

Test tube Observation Reaction
Fe & Zn Blue colour is not present. Oxidation:Zn(s) –> Zn2+(aq) + 2e
Intensity of pink colour is high. Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)
Inference Zn is corroded and Fe nail does not rust.

i) Fe act as the (+) terminal (cathode)
ii) Zn act as the (-) terminal (anode)

Test tube Observation Reaction
Fe & Sn Intensity of blue colour is high. Oxidation:Fe (s) –> Fe2+(aq) + 2e
Pink colour is not present. Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
Inference Fe nail rusts quickly (high rate).

i) Sn act as the (+) terminal (cathode)
ii) Fe act as the (-) terminal (anode)

Test tube Observation Reaction
Fe & Cu Intensity of blue colour is very high. Oxidation:Fe (s) –> Fe2+(aq) + 2e
Pink colour is not present. Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
Inference Fe nail rusts very quickly (the highest rate).

i) Cu act as the (+) terminal (cathode)
ii) Fe act as the (-) terminal (anode)

  • Iron nail does not rust if it has contacted with more electropositive metals (Mg and Zn).
  • Iron nail rusts quickly if it has contacted with less electropositive metals (Sn and Cu).

Prevention of Rusting of Iron

The rate of rusting of iron decreases if the iron (Fe) in contact with any of these metals: K, Na, Ca, Mg, Al and Zn.
The rate of rusting of iron increases if the iron (Fe) in contact

  • with any of these metals: Sn, Pb, Cu, Hg, Ag and Au.
  • a strong electrolyte (salt and acid) is present.

Ways Used for Prevention of Rusting

  1. Painting – Protect iron surface (prevent from contacting with air and water)
  2. Coat with plastic – Used in metal netting
  3. Apply oil and grease – Protective coating for machine part
  4. Alloying the iron – Alloying the iron with 18% chromium and 8% nickel that provide a protective oxide coating.
  5. a) Tin plating (less electropositive metal) – Cans of food (iron) is covered with a thin layer of tin to provide a protective oxide coating to the cans.
    b) Chrome plating
  6. Cathodic protection / Electrical protection (more electropositive metal)
    a) Galvanising (coat with zinc metal) – Zinc layer provides a protective oxide coating and zinc is oxidized instead of iron. Iron cannot form ions, so it will not rust.
    b) Sacrificial protection – Blocks of magnesium are attached at the intervals of the water piping system & zinc bars are attached to the part of the ship submerged in sea water.

Stay tune and log in back again for Part 9 of this series on reactivity series of metal and their applications only at BerryBerryEasy

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