CORROSION OF IRON
INTRODUCTION:
When the surface of iron is wet, regions of differing electrical potential may act as electrodes, resulting in
oxidation of the Fe to give ferrous, Fe2+, ion. These electrodes or anodic (oxidation) points are usually
regions where the crystal lattice is stressed and distorted, so that atoms can be easily removed.
Anode (oxidation)
Write the equation for the half-reaction of iron to ferrous ion, including the cell potential.
1.
The ferrous ions, Fe2+, which form may be further oxidized by oxygen to form ferric, Fe3+, ions.
Write an equation for the half-reaction in which oxidation of ferrous ion to ferric ion occurs,
including the cell potential.
2.
Add the equations 1 and 2 to create the overall reaction of iron to the ferric ion performed in two
steps. Include the cell potential (reference sheet).
3.
Cathode (reduction)
Cathodic (reduction) points occur where the iron is in contact with a metal which has a higher reduction
potential, or where the water has a high concentration of some oxidizing agent.
Write the half-reaction of neutral water, acting as an oxidizing agent. Include the cell potential.
4.
In acidic solutions, hydrogen ions may act as the oxidizing agent. Write this half-reaction including the
cell potential.
5.
If oxygen is present in the water, it may be the oxidizing agent. Write this half-reaction including the
cell potential.
6.
Complete Redox reaction
In water, OH- ions produced in the cathode (reduction) reaction, will combine with the iron ions from the
anode (oxidation) reaction and precipitate from the solution. The ferrous hydroxide is not seen, but the
ferric hydroxide precipitates. Write an equation for the precipitation of ferric hydroxide from a
solution of ferric ion.
7.
If these hydroxide compounds are dehydrated (lose water), they form ferrous oxide (black) and ferric
oxide (red-brown). Rust is the solid corrosion product consisting of iron oxides.
Write an equation for the dehydration of ferric hydroxide.
8.
Indicators
Potassium ferricyanide, K3Fe(CN)6, can be used to detect the presence of ferrous ion, Fe2+. (You will
have to research the answer to this one). Give both the chemical equation and the 2 colours in
solution, not solid.
9.
Phenolphthalein, a common indicator test (often used in titrations) that could be used to identify the
products of the cathodic half-reaction. Give both the equation and the 2 colours.
10.
Overall
If the solution is acidic, the OH- ions will be neutralized, consider acid/base reactions. If the solution is
basic, the production of the OH- is suppressed, consider common ion and equilibrium situations. But if
the solution is neutral, the OH- ions produced in this half-reaction migrate toward the anodic points. Fe2+
and Fe2+ ions migrate from the anode to the cathode. Somewhere in-between the iron ions meet
hydroxide ions and form the hydroxides, dehydrate and form rust. The water on the surface of the iron
acts as an electrolyte, transporting ions between the anodic and cathodic points. Any dissolved salts
present in the water will aid in this charge transfer and so accelerate the corrosion.
Prediction: Use the half reactions from the previous page. Anode: 1 and 3 Cathode: 4, 5 and 6
1. Write all the possible reactions for an Fe nail in water in an open test tube. Calculate the overall cell
potentials, determine spontaneity and describe the expected reaction observations. Verify your answers
by using the reduction table. (Note, if reaction 1 can’t occur, then 3 can’t happen either)
2. Write all the possible reactions for the spontaneous reactions for an Fe nail in acidic solution in an
open test tube. Calculate the overall cell potentials, determine spontaneity and describe the expected
reaction observations. Verify your answers by using the reduction table.
3. Write all the possible half reactions for the spontaneous reactions for an Fe nail partially covered with
Cu metal. Calculate the overall cell potentials and determine the spontaneity of the reactions.
4. Write all the possible half reactions for the spontaneous reactions for an Fe nail partially covered with
Zn metal. Calculate the overall cell potentials and determine the spontaneity of the reactions.
PROCEDURE:
1.
Heat 100 mL of distilled water to the boil in a 250 mL beaker. Remove from the heat
and stir in 1 tsp of agar. Continue heating gently and stirring until the agar is
dispersed. Share between two groups.
2.
Add 5 drops of 0.1 mol/L potassium ferricyanide solution and 30 drops of 0.1%
phenolphthalein solution. Stir thoroughly. Allow to cool but do not allow the agar to
set.
3.
Place a nail covered in Cu wire and a nail covered with Zn in a Petri dish.
4.
Cover the nails in the Petri dish with the agar mixture and place a labeled lid on the
dish. Store as directed.
5.
Prepare four short finishing nails by washing them in soap to remove any protective
oil coating.
6.
Place one small nail in each of four medium sized test tubes. To one add distilled
water. To the second add 0.1 mol/L sodium hydroxide. To the third add 0.1 mol/L
hydrochloric acid. To the fourth add 0.1 mol/L sodium chloride. Set all four test
tubes in a small beaker labeled with your name. Store as directed.
7.
Do Not pour the extra agar solution down the drain – it will clog the drain. Scrape
out and throw in the garbage.
FOLLOW-UP AND OBSERVATIONS:
8.
The next lab day, make detailed diagrams on the observation sheet for the nails in the
test tube solutions. Label each observable half reaction using the numbers 1- 9 in the
Introduction.
9.
Test the solutions with pH paper, record.
10. Next add a drop or two of potassium ferricyanide to each test tube and note any
colour change. Record observations same as above.
11. Make detailed diagrams on the observation sheet for the nails in the test tubes. Label
each observable half reaction using the numbers 1- 9 in the Introduction.
OBSERVATIONS
Name:
Initial Diagram – Before addition of potassium ferricyanide
pH =
pH =
pH =
Final Diagram – After addition of potassium ferricyanide
Final Diagram (1 Petri dish)
Additional Reaction
10.
pH =
Analysis: Use the half reactions and reactions from the Introduction.
Test tubes reactions
Write the anode, cathode 1/2 reactions and the overall REDOX reaction
including cell potential for the Fe nail in the solution indicated.
Make sure to match the observation to the products obtained.
1. a) In H2O
b) Subsequent reactions that produced the observed product(s):
2. In HCl solution
3. In NaOH solution
4. a) In NaCl solution
b) Subsequent reactions that produced the observed product(s):