Dissolving Iron Nails: A Kinetics Experiment The student in the General Chemistry Laboratory doing a kinetics experiment is, in many instances, confronted with studying an unfamiliar reaction, possibly using unfamiliar chemicals. In order to focus on the aspects of kinetics, it would he best for the student t o study a familiar reaction. The dissolution of iron nails by dilute hydrochloric acid is a well-known reaction, important in corrosion prohlems.'The kinetics of thereaction may he investigated by measuring the rate of appearance of Fez+. A model describing this chemical system may be proposed t o explain the rate of appearance of Fez+. Hydrogen ions are reduced to hydrogen a t impurity sites or a t dislocations or other imperfections on the iron surface. At the same time Fez+ is produced in a layer of solution a t the surface of the iron nail by local currents passing through the metaL2 Assuming that the concentration of Fez+ in the layer rapidly reaches a steady state, the rate of appearance of Fez+ in the body of the solution is due to the rate of diffusion of Fez+ out of the surface layer. The rate of appearance of Fez+ in the body of the solution, deldt, is dc - k S dt a,, (~.".fa'C) where V is the volume of the solution, c is the concentration of Fez+ in the body of the solution, S is the surface area of the nails, k is a proportionality constant, and c,,,f., is the eoneentration of Fez+ in the layer of solution a t the surface. The thickness of the layer of solution adhering t o the surface, 6, is changed by stirring, hut possibly has a limiting value. If the initial concentration of Fez+in solution is zero, and, if k is small, then the expression describing the initial rate of appearance of Fez+ in solution is Since iron dissolves slowly in a0.1 MHCI - 1.0M NaCl solution,the surface area may beconsidered constant over 15 mi", and the change in Fez+ concentration over that time may be determined. By varying the surface area (using a different number of nails), different rates are observed from which the observed rate constant, k , b , may be calculated. The experimental procedure employs 100 ml of solution and 2,3,4,5 and 6 common 6d nails. The concentration of iron in solution, after 15 min of reaction, was determined spectrophotometrically by adding bromine and NHISCN to an aliquot of the solution. A plot of the average rate of appearance of iron in solution versus the number of nails was found to he linear. Measuring the dimensions of several nails gave the average surface area of a single nail. Using the slope of the line, the volume of solution (100 ml) and the area, k,b. was calculated. From student data, k,b, is (5.91 f 1.70) X lo-? M ml min-1 mm-2 a t 23% k , b with stirring was found to be 1.65 X 10W6M ml min-' mm+ a t 23°C. Since stirring should decrease 6, hob, is larger, as expected. Since the stirring rate using a magnetic stirrer was difficult t o control, the data under these conditions was more scattered. The students were interested in the experiment because of its relation to corrosion and because they wereable tosuccessfully apply kinetics principles to a familiar reaction. 'Uhlig, H. H., (Editor) "The Corrosion Handbook," John Wiley and Sans, Inc., New York, 1948, p. 8, p. 125. zLaQue, F. L., "Marine Corrosion-Causes and Prevention," John Wiley and Sons, New York, 1975, pp. 23-28.
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Clemsan University Clemson, South Carolina 29631
400 / Journal of Chemical Education
Charles P. Monaghan J a m e s C. Fanning
