Instructor Information
JCE Classroom Activity: #75
Trusty or Rusty? Oxidation Rate of Nails
In this Activity, metallic iron in the form of steel nails is oxidized by atmospheric oxygen in the presence of water. A hydrated iron(III) oxide of indefinite and variable hydration, Fe2O3 n H2O, forms. The hydrated iron(III) oxide is familiar to everyone as “rust”. 4 Fe(s) + 3 O2(g) + 6 H2O(l) → 4 Fe3+(aq) + 12 OH–(aq) → 2 Fe2O3(s) + 6 H2O(l) The oxidation is carried out in a gel, and the Fe3+ formed is detected with magnesium salicylate, Mg(C7H5O3)2 (found in some backache pain reliever tablets), which reacts with iron(III) ion Fe3+ to form a garnet-colored complex ion. Fe3+(aq) + 2 C7H5O3–(aq) → Fe(C7H5O3)2+(aq) Galvanized nails fail to produce a garnet color, illustrating the protection of the iron by the more chemically reactive zinc coating. A full discussion of the chemistry involved, experimental procedures, and notes for the successful reproduction of this reaction as a Tested Demonstration is presented in this issue and the references cited therein (1).
Integrating the Activity into Your Curriculum This Activity can be used to understand redox chemistry and the relative oxidation potentials of various metals. It illustrates one of the most common reactions of iron and approaches used to retard that reaction.
About the Activity Students prepare a mixture of gelatin, magnesium salicylate, and sodium chloride using three consumer products: unflavored gelatin, backache pain reliever tablets, and table salt. To remove oil remaining from manufacture of the nails, students wash them with hot water and liquid soap and dry them before using. This can also be accomplished by shaking the nails with mineral spirits (paint thinner) in a clean can. The hot gelatin mixture takes about 20 minutes to gel. It may have a light brown color. The gel sets more quickly if the containers are placed on a smooth stone or metal surface. The garnet color indicating the oxidation of the iron metal in the nails becomes discernible after 1 hour and is clearly visible after 3 hours. Gelatin mixtures may be discarded in the trash. Containers for the nails in gelatin should be large enough for two nails, but small enough so that the gelatin mixture covers both nails. Disposable containers made of white polypropylene plastic (recycle code 5), such as small, clean margarine or cream cheese containers, work well. Heat-resistant glass containers, such as custard cups, may be used if they are the proper size. Disposable glass jars should not be used as they may break when the hot gelatin mixture is added. Metal containers are not suitable as they will participate in the reaction. The gelatin mixture is hot and may cause burns if spilled. The instructor may choose to prepare the gelatin mixture and pour it into dishes for students. perforated
Answers to Questions 1. Color typically appears first at the nail point and head, which are the areas most stressed during the manufacture of the nail. If the nail is bent sharply, the color will often appear first at the bend due to the mechanical stress. 2. Nails are usually coated with oil to retard rust formation before purchase. The oil coating can easily be felt with the fingers or wiped on a tissue. 3. No color forms around a galvanized nail. The zinc coating is more electrochemically reactive than the iron. It oxidizes instead of the iron. 4. The color forms more rapidly when the nail is in contact with copper. The iron is electrochemically more reactive than copper, so the nail becomes a sacrificial anode and the rate of oxidation of iron is increased. This phenomenon is known as galvanic corrosion. 5. Galvanic corrosion can result when dissimilar metals are in contact with each other. Steel nails corrode rapidly if used to fasten copper roofing. Aluminum siding corrodes rapidly in contact with steel fittings or copper plumbing. Plastics are increasingly popular because they are inexpensive and resistant to both corrosion (metals) and rot (wood).
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
photo: J. J. Jacobsen, E. K. Jacobsen, J. L. Harris
Stephen W. Wright Pfizer Global Research and Development, Groton, CT 06340;
[email protected] In this Activity, students investigate the process of rusting by studying the oxidation of steel nails in a gel using supermarket chemicals. An indicator makes the presence of Fe3+ produced by the oxidation visible. Factors that accelerate or retard the rate of iron oxidation are studied.
References, Additional Related Activities, and Demonstrations 1. Wright, Stephen W; Folger, Marsha R.; Quinn, Ryan P. The Oxidation of Iron in a Gel Using Consumer Chemicals. J. Chem. Educ. 2005, 82, 1633–1635. Additional experiments involving the oxidation of iron in a gel are in (a) Tested Demonstrations in Chemistry; Alyea, Hubert N., Dutton, Frederic B., Eds.; Journal of Chemical Education: Easton, PA, 1965; p 134; (b) Summerlin, Lee R.; Borgford, Christie L.; Ealy, Julie B. Chemical Demonstrations: A Sourcebook for Teachers, Volume 2, 2nd ed.; American Chemical Society: Washington DC, 1988; p 191. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 82 No. 11 November 2005 •
Journal of Chemical Education
1648A
JCE Classroom Activity: #75
Student Activity
Trusty or Rusty? Oxidation Rate of Nails
mixture is hot and may cause burns if spilled. Safety goggles should be worn during this Activity.
Try This
You will need: 3 six-penny (6d, 2-in.) common nails, 1 six-penny (6d, 2-in.) galvanized nail, water, liquid dishwashing soap, paper towels, small pan, hot plate or stove, small glass jar, 2 sevengram packets of unflavored gelatin, a backache pain reliever tablet (containing 580 mg of magnesium salicylate tetrahydrate), sodium chloride, copper wire, aluminum foil, spoon, and 2 plastic containers (small plastic margarine or cream cheese containers work well), optional: pliers. __1. Clean 3 six-penny (6d, 2-in.) common nails and 1 six-penny galvanized nail using hot water and liquid dishwashing soap. Dry the nails immediately with a paper towel. __2. Place two nails from step 1 into each of two plastic containers: one common nail, one galvanized nail, one common nail with about 2 in. of copper wire wrapped around the shank, and one common nail with a 1 1 in. piece of aluminum foil wrapped around the shank. The copper and aluminum should be in close contact with the nail. If space permits in one container, add an additional six-penny common nail that has been bent sharply in the middle with pliers. Place the nails in the containers before the gelatin mixture is poured over them. __3. Add two, 7-g packets of unflavored gelatin to 30 mL (2 tablespoons) of water in a small glass jar. Stir with a spoon to make as evenly wetted a mixture as possible. This mixture will quickly become a stiff paste. __4. Place 90 mL (6 tablespoons) water in a small pan. Add 0.6 g (~1/4 teaspoon) of sodium chloride and 1/2 of a magnesium salicylate backache pain reliever tablet. Stir and heat the mixture to boiling on a hot plate or stove. __5. Add the gelatin–water paste to the boiling contents of the pan slowly enough to keep the mixture at a gentle boil. Stir. Once the gelatin appears to have dissolved, remove the mixture from the heat. A small quantity of the ingredients in the backache pain reliever tablet will not dissolve. __6. Gently pour half of the hot gelatin solution into each of the two containers from step 2. The gelatin mixture should just cover the nails, which should not be moved or allowed to come into contact with each other as the gelatin mixture is poured. Allow to cool undisturbed. Observe the nails every 20 minutes for the first two hours, and then at four hours and after about 24 hours, noting the time and color around each nail at each interval.
Questions 1. 2. 3. 4. 5.
Around what part(s) of the nails does the color first appear? Why? Why is it necessary to clean the nails before use? What substance seems to be on the nails? What is its purpose? What happened when a galvanized nail was used? Why? What happened when copper wire was wrapped around the nail? Why? Why are copper nails used to fasten copper roofing and aluminum nails used to fasten aluminum gutters and siding? Why might plastic fasteners be gaining popularity?
Information from the World Wide Web (accessed Aug 2005) Iron. http://www.webelements.com/webelements/elements/text/Fe/key.html C&EN: It’s elemental: The periodic table—iron. http://pubs.acs.org/cen/80th/iron.html How does rust work? http://science.howstuffworks.com/question445.htm Galvanic corrosion. http://www.corrosion-doctors.org/Aircraft/galvdefi.htm This Classroom Activity may be reproduced for use in the subscriber’s classroom.
1648B
Journal of Chemical Education •
Vol. 82 No. 11 November 2005 •
www.JCE.DivCHED.org
photo: J. J. Jacobsen, E. K. Jacobsen, J. L. Harris
The oxidation of iron by atmospheric oxygen in the presence of water to form rust (a hydrated iron(III) oxide, Fe2O3 n H2O) is a familiar phenomenon. The reaction occurs according to the following equations: 4 Fe(s) + 3 O2(g) + 6 H2O(l) → 4 Fe3+(aq) + 12 OH–(aq) → 2 Fe2O3(s) + 6 H2O(l) Rusting, and efforts to prevent it, cost billions of dollars each year. Many metals, such as aluminum, react with oxygen to form a thin oxide layer that adheres tightly to the surface and protects the underlying metal. By contrast, iron(III) oxide does not adhere well when water is present. Strategies to retard the oxidation reaction fall into three categories. The first is to coat the iron surface to exclude water and/or oxygen, for example with oil or paint. The second is to place more easily oxidized metal in contact with the iron. The more easily oxidized metal reacts with oxygen instead of the iron. The third strategy is to alloy the iron with other metals such as chromium and nickel to form stainless steel. In this Activity you will observe the oxidation of iron nails in a gel and compare the effects of factors that may speed or slow the rate of iron oxidation. The gel will contain magnesium salicylate, Mg(C7H5O3)2. The salicylate anion reacts with iron(III) ion to produce a dark red– purple iron(III) salicylate ion as follows: Fe3+(aq) [colorless] + 2 C7H5O3–(aq) [colorless] → Fe(C7H5O3)2+(aq) [red–purple] Iron(III) produced by oxidation of the iron in the nail will therefore become visible as a red– Be Safe! The gelatin purple color. As more iron(III) ion is produced, the color will intensify.
