Lab #17 - The Corrosion of Iron
Introduction: The corrosion
of iron costs industry and consumers billions of dollars annually. What
are some of the factors that contribute to corrosion? What can be done
to control corrosion?
4 Fe + 3 O2 -----> 2 Fe2O3 iron(III) oxide; ferric oxide
The rusting or corrosion of the metal can be broken down into a series of distinct stages.
a) In the first stage Fe(s) is converted to Fe2+(aq) and the solution becomes basic.
2 Fe(s) + O2(g) + 2 H2O(l) ------> 2 Fe+2(aq) + 4 OH-1(aq)
b) In the second stage the Fe2+(aq) ions undergo further reaction with oxygen from the air.
4 Fe2+(aq) + O2(g) + H2O(l) ------> 4 Fe3+(aq) + 4 OH-1(aq)
c) As the Fe3+(aq) ions are formed they immediately react with the OH-1(aq) ions to produce insoluble Fe(OH)3(s)
Fe3+(aq) + 3 OH-1(aq) --------> Fe(OH)3(s)
d) Finally, on standing, the Fe(OH)3 eventually dehydrates to produce Fe2O3.
2 Fe(OH)3(s) ----> Fe2O3(s)
+ 3 H2O(g)
Notice that both the O2(g) and H2O(l)
are needed to cause the rusting of the iron. Rusting does not take place very
quickly in regions where humidity is low. However, dissolved
salts will greatly accelerate the corrosion process, which accounts for the
rapid rusting of cars in parts of Canada where salt is used to melt ice and
snow in the winter.
Problem? What are some of the factors involved in corrosion?
Apparatus: 250 mL beaker, ring stand, iron ring, wire gauze, Bunsen burner, test tubes, petri dish
Materials: 9 iron nails, steel wool, red and blue litmus paper, copper wire, zinc strip, powdered agar, distilled water, phenolphthalein solution, 0.1 M solutions of sodium hydroxide, potassium hydroxide, sodium chloride, hydrochloric acid, potassium nitrate, potassium ferricyanide and iron(II) sulphate.
Procedure: CAUTION: Avoid allowing the solutions to come in contact with your skin. Wash your hands thoroughly after the experiment.
Part A: Reaction of Iron with Various Reagents
1. Polish 7 nails with steel wool and place one in each of 7 test tubes.
2. Cover each nail with one of the following 6 reagents: sodium hydroxide, potassium hydroxide, sodium chloride, hydrochloric acid, potassium nitrate, and sulphuric acid. Cover the last nail with distilled water.
3. Use red and blue litmus paper to determine if the solution in each of the 7 test tubes is acidic, basic or neutral.
4. Allow the nails to stand overnight in the solutions. (Go to Part B in the meantime)
5. The next day, observe and record any changes that have taken place. To each solution add two drops of potassium ferricyanide solution. Observe any changes.
6. Add one drop of the potassium ferricyanide solution to about 1 mL of iron(II) sulfate solution. The potassium ferricyanide solution reacts with Fe2+ ions to form an intense blue precipitate. Compare this result with that obtained when the potassium ferricyanide solute was added to the various solutions covering the nails.
Part B: Reactions Involving Two Metals
2. Add 5 drops of 0.1 mol/L potassium ferricyanide solution and 3 drops of phenolphthalein solution to the agar mixture. Stir thoroughly.
3. Polish two nails with the steel wool. Place one nail on one side of a petri dish. Bend the second nail sharply with a pair of pliers and place it one the other side of the petri dish.
4. Obtain a 25 cm length of bare copper wire. Tightly wrap approximately 10 cm of this wire around the shank of a polished iron nail. Leave 5 cm and tightly wrap the remaining 10 cm portion around a polished zinc strip. It is essential for all contacts to be firm. Place this system into a second shallow petri dish.
5. Pour enough lukewarm agar carefully over the nails in the petri dishes to completely cover the nails.
6. Leave the dishes overnight. Record
2. What pattern is evident? Account for the pattern.
3. When iron metal (Fe) corrodes, what ions are formed?
4. In Part B, what did you observe at the head, at the pointed end, and at the sharp bend of the nails that is different from the rest of the nail? What two colours are present? What ions are indicated by each colour? How can you account for this in terms of the way the nails are manufactured?
5. Based on the results of this experiment, what are the factors that contribute to the corrosion of iron?
6. What methods are used to reduce the corrosion of metals?