Chemistry

Name: ___________________________

Corrosion of Iron Experiment Purpose 1. To observe the regions of oxidation and reduction in a corroding iron nail 2. To determine the effects of physical stress on the corrosion process 3. To observe the effects of a more reactive metal in contact with corroding iron Pre-Lab Discussion Corrosion is an oxidation-reduction (redox) reaction in which a free metal is oxidized, or “corroded,” by some oxidizing agent. In nature, the oxidizing agent is frequently atmospheric oxygen dissolved in water. Perhaps the most familiar example of corrosion is the rusting of iron. In this reaction, free iron, Fe, is oxidized to Fe2+ and Fe3+ ions. In the reduction half-reaction, oxygen, O2, reacts with water to form OH- ions. The half-cell reactions may be represented as follows: Anode: Fe (s)  Fe2+ (aq) + 2e- and Fe2+(aq)  Fe3+ (aq) + eCathode: 2H2O (l) + O2 (g) + 4e-  4OH- (aq) Since the redox products are in physical contact, they will react to form insoluble iron (II) hydroxide and then to hydrated iron (III) oxide. 4Fe(OH)2 (s) + O2 (g) + xH2O (l)  2Fe2O3∙nH2O (s) The rusting of iron usually is considered to be a destructive change, and considerable time and money are expended to prevent it. One method which you will observe in this experiment, make use of a “sacrificial” metal. If iron is placed in contact with a more active metal (one that is more easily oxidized) the more active metal will be corroded instead of the iron. Metals situated below iron on the table of standard electrode potentials can function as sacrificial metals. In this experiment, indicators will be used to signal the formation of Fe2+ ions and OHions as products of the corrosion of iron. One such indicator, the hexacyanoferrate(III) ion, turns blue in the presence of Fe2+ ions. The other, phenolphthalein, turns pink in the presence of OH- ions. This experiment should help promote a better understanding of corrosion and of oxidation-reduction reactions.

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Chemistry

Name: ___________________________

Pre-Lab Questions: 1. What type of reaction is the rusting of iron?

2. Write out the half-reactions, complete reaction, half-reaction voltages, and reaction voltage for the production of rust in this experiment? Which is the anode and which is the cathode?

3. Define the term “sacrificial” metal?

4. Why is iron rust considered destructive and copper rust is not destructive, but beneficial and attractive? (What is the difference in the properties of each?)

Equipment Balance Hot Plate Graduated cylinder, 100 mL Beaker, 250 mL Ring stand Metal ring Wire gauze Glass stirring rod

Dropper pipet Petri dishes (2) Emery paper Metric ruler Safety goggles Lab apron Beaker tongs Stirring rod

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Chemistry

Name: ___________________________

Materials 0.1 M K3Fe(CN)6 [potassium hexacyanoferrate(III) or potassium ferricyanide] Agar powder 1% phenolphthalein solution Distilled water Copper wire, 5 cm Zinc strip, 5 cm 10d polished iron nails (4) Safety Observe all general lab safety precautions. Always wear safety goggles and a lab apron when working in the lab. Procedure 1. Set up a ring stand, ring, and wire gauze. Add 200 mL of distilled water to the 250 mL beaker and heat to boiling. While the water is being heated, clean four nails with steel wool or emery paper. 2. Using the balance, measure out 2.00 g of agar powder. When the water is boiling vigorously, turn off the hot plate and slowly add the agar to the water, stirring constantly. When the agar has dissolved, add 10 drops of 0.1 M K3Fe(CN)6. Rinse the dropper and pipet and add five (5) drops of phenolphthalein solution. 3. Place one bent iron nail and one straight iron nail into one of the Petri dishes. Be sure the nails do not touch and are positioned as shown in Figure 1.

Figure 1 4. Tightly wrap one of the remaining nails with copper wire and the other with the zinc strip. Place these nails in the other Petri dish, as shown in Figure 1. 5. When the agar solution is cool enough, pour it into each Petri dish to a depth of 0.5 cm (make sure the nails are completely covered). 6. Make and record observations in the time remaining. 7. At the end of the lab period, place the dishes in your drawer to be observed on the following day.

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Chemistry

Name: ___________________________

Observations and Data (Write a complete description of what you see in each Petri dish and for each nail) Petri Dish (a):

Petri Dish (b):

Lab Report A brief, but complete report is expected for this lab. Please include the following: The Introduction should be a discussion of the corrosion of iron, including the halfreactions that occur at the anode and cathode. Explain how a “sacrificial” metal works to prevent corrosion of iron. Describe the experimental set-up, and explain the purpose of the two indicators. The Results should be a drawing or picture and a complete description of the qualitative results. (Where are the pink and blue regions?) The Conclusion should be a complete interpretation of the results  What is the significance of the pink and blue regions around each nail?  What conclusions can you draw from each nail? Compare and contrast the two nails in the two dishes.  Why did corrosion occur at the blue sites in Dish (a)?  What is the whitish substance surrounding the zinc-wrapped nail in Dish (b)?  Explain the difference in corrosion between the two nails in Dish (b).  What effect does stressing the nail have on the corrosion of the nail?  What other metals could be used as sacrificial metals?  Did the experiment produce expected results?  Are there any suggestions for improvements?

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