AP Chemistry
Oxidation-Reduction
Corrosion of Iron Experiment Purpose Observe the corrosion of iron and investigate conditions related to corrosion. Pre-Lab Discussion Corrosion is a 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. See Chemistry The Central Science (Brown, LeMay, Bursten, Murphy) pages 874 – 876 for a detailed description of the corrosion of iron. 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. 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 Tirrill Burner or Hot Plate Graduated cylinder, 100 mL Beaker, 250 mL Ring stand Iron ring Wire gauze
Glass stirring rod Dropper pipet Petri dishes (2) Emery paper Metric ruler Safety goggles Lab apron 1 Revised: 14 March 2013
AP Chemistry
Oxidation-Reduction
Materials 0.1 M K3Fe(CN)6 [potassium hexacyanoferrate(III)] Agar powder 1% phenolphthalein solution Distilled water Copper wire, 5 cm Zinc strip, 5 cm 10d 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, put out the flame 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.
AP Chemistry
Oxidation-Reduction
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?
AP Chemistry
Oxidation-Reduction
Teaching Notes Equipment Balance Tirrill Burner or Hot Plate Graduated cylinder, 100 mL Beaker, 250 mL Ring stand Iron ring Wire gauze
Glass stirring rod Dropper pipet Petri dishes (2 per student) Emery paper Metric ruler Safety goggles Lab apron
Materials 1.1 M K3Fe(CN)6 [potassium hexacyanoferrate(III)] FW 329.26 g [use 3.2926g / 100mL] Agar powder 1% phenolphthalein solution Distilled water Copper wire, 5 cm Zinc strip, 5 cm [Use electrode strips cut] 10d Iron nails (4 per student) Pre-Lab Questions: 1. What type of reaction is the rusting of iron? Oxidation-reduction reaction. 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? 2 Fe(s) 2Fe2+(aq) + 4eEº= 0.44 V (Anode) O2(g) + 2H2O(l) + 4e 4OH (aq) Eº= 0.41 V (Cathode) 2Fe(s) + O2(g) + 2H2O(l) 2Fe(OH)2(s) Eº= 0.85 V (white, green solid) Further oxidation to red flakes: More acidic water increases corrosion. If the pH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water: 2H+(aq) + 2e- --> H2(g) The iron(II) ions also react with hydrogen ions and oxygen to produce iron(III) ions: 4Fe2+(aq) -->4Fe3+(aq) + 4e- E° = -0.77 v (Anode) 4e- + 4H+(aq) + O2(aq) -->2H2O(l)
E° = 0.815 v (Cathode)
4Fe2+(aq) + 4H+(aq) + O2(aq) -->4Fe3+(aq) + 2H2O(l)
E° = 0.145 v
The loose porous rust or Fe(OH)3 can slowly transform into a crystallized form written as Fe2O3.H2O the familiar red-brown stuff that is called "rust" forming tubercles. 4Fe(OH)2(s) + O2(g) 2Fe2O3H2O(s) + 2 H2O(l) 3. Define the term “sacrificial” metal?
A metal that is more easily oxidized. Fe is above on the activity series (Zn, Al, Mg, Na, Ca, K) 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?) Iron rust does not bond to the base metal iron and is slightly water soluble. Therefore, iron rust will rub off the metal and expose more metal to oxidation. Copper rust adheres to the copper metal and protects the copper from other oxidation. Lab Report 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?
