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In the article “Steel Wool and Oxygen: A Look at Kinetics”, James Gordon and Katherine Chancey describe an experiment about the kinetics between iron and oxygen (1). Like all good experiments this one is easy to carry out and produces good results. Moreover, it teaches chemistry with a relation to everyday life (2). In my opinion the experiment can even have a greater impact with only a small addition. The authors prefer not to discuss the magnitude of the rate constant for this experiment because it will vary with the surface area of the steel wool. This statement justifies further reflection on the possible significance of the rate constant. This constant is meant to make general statements about the rate of a reaction possible, in order to be able to compare the results of various experiments. One does not define the rate of a reaction in solution as being proportional to the decrease or increase of the amount of a certain substance but as being proportional to the decrease or increase of the concentration of that particular substance. With this definition the rate is independent of the size of the specific reactor.
Similarly, the rate of a reaction should also be independent of the surface area of a reacting solid, which can be achieved by defining the rate per unit of area. The experiment proposed offers an opportunity to measure the rate of reaction with respect to the iron surface: first by taking different pieces of steel with a well-defined surface and see how the oxygen pressure falls down. This leads to the calibration with respect to the surface area. Afterwards, with an experiment like the one described by the authors, one can compute the surface of the steel wool used on the basis of the rate constant calibrated in this way.
Literature Cited
- Gordon, J.; Chancey, K. J. Chem. Educ. 2005, 82, 1065–1067.
- Sanger, M. J.; Wiley Jr., R. A.; Richter E. W. J.
Chem. Educ. 2002, 79, 989–991.
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