Download - 3B_Rusting as a Redox Reaction
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(B) Rusting as a Redox Reaction
OBJECTIVESAfter this lesson, you should be able to:
State the conditions for the rusting of iron
State what corrosion of metal is
Describe the process of rusting in terms of oxidation andreduction
Generate ideas on the use of other metals to control rusting
Explain with examples on the use of a more electropositivemetal to control metal corrosion
Explain with examples on the use of a less electropositivemetal to control metal corrosion
Describe efforts to prevent corrosion of metals
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What is Rusting?
Rusting is the corrosion of iron.
When iron corrodes, it forms a flaky or
powdery brown coating called rust.
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Condition for Rusting of Iron
Two conditions are required at the same
time for rusting to occur :(a) the presence of air(oxygen)
(b) the presence of water
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Examples of corrosion of metal
Iron rusting
Silver tarnish (dull)
Green substance formed on brass or bronze
Gold and platinum are called noble metals
because they are very unreactive metal and
never corrode.
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When corrosion occurred, the metal
surface loses its luster ( shine ) and become
tarnished (dull).
If corrosion continues , the metal will be
eaten up by rust and break
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Corrosion of Metal
The more electropositive a metal is, the
easier for it to corrode.
This is because a more electropositive
metal can easily loses its electrons to form
positive ions.
M(s) Mn+(aq) + ne-
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Sodium and potassium are very reactiveand must be kept in paraffin oil to protect
them from oxidized by air and water.
Aluminium corrodes quickly in the air toform a coating of a tightly packed andnon-porous aluminium oxide, Al2O3
This aluminium oxide prevents the
aluminium underneath from furthercorrosion.
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Lead, zinc, chromium and nickel also
form hard metal oxides that are
impermeable to water and air.
These protective oxide coating prevent
the metals from further corrosion.
Magnesium and iron when corroded, form
oxide layers that are porous and nottightly packed, weak and easily crack.
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Rusting as a Redox Reaction
Oxidizing Agent : oxygen
Reducing Agent : Iron
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Mechanism of Rusting
The surface of iron in the middle of thewater droplet act as anode.
The iron is oxidized to iron(II) ion, Fe2+ bythe loss of e-.
Fe(s) Fe2+ (aq) + 2e-
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Mechanism of Rusting
The e- released by iron flow to the edge ofthe water droplet where there is plenty ofdissolved oxygen.
The iron surface there acts as cathode.
The e- are gained by oxygen and is reducedto hydroxide ion, OH
O2 (g) + 2H2O(l) + 4e- 4OH(aq)
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Mechanism of Rusting
The iron(II) ions, Fe2+ combined with thehydroxide ions, OH to form iron(II)
hydroxide, Fe(OH)2.
2Fe(s) + O2 (g) + 2H2O(l) 2Fe(OH)2 (s)
iron(II) hydroxide
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Mechanism of Rusting
The iron(II) hydroxide, Fe(OH)2 is notstable.
It is then rapidly oxidized by oxygen to formbrown hydrated iron(III) oxide, Fe2O3.xH2O.
This hydrated iron(III) oxide is called rust.
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Factors affect the rate of Rusting
The presence of acids
The presence of electrolytes
(salt solution)
The presence of contact metal
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Factors affect the rate of Rusting
Rusting occurs faster in the presence of acids andsalts solution.
These substance increase the electrical
conductivity of water, making it a betterelectrolyte.
Thus, rusting of iron occurs more rapidly in areas
near the sea (contains salts vapours)
in industrial areas (polluted by acidic gases such as NO2and SO2 )
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Factors affect the rate of Rusting
When iron is in contact with a moreelectropositive metal, such as Mg, rusting isprevented.
This is because Mg can lose its e- more readily
than iron to form Mg2+.
The e- release by Mg flow to the iron. The oxygengains the electrons and is reduced to hydroxide
ions.
Mg corrodes instead of the iron.
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Prevention of Rusting Apply paint, oil, grease or put on plastic coating
to protects the iron surface from in contact with airand water.
Plating iron with tin or chromium
Galvanizing iron with zinc
Using the sacrificial metal (more electropositive
metal)
Alloying iron with carbon, chromium or nickel.
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Practice B1
1. State the condition required for metal to rust?
2. (a) Write the chemical equation for the rustingof iron. Assume the formula of rust asFe2O3.H2O
(b) explain why a layer of grease applied on to an
iron object will prevent iron from rusting.
3(a) What is meant by Galvanized iron?
(b) Explain why galvanized iron does not rust whenits surface is scratched.