3B_Rusting as a Redox Reaction

7/31/2019 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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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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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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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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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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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.

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3B_Rusting as a Redox Reaction