J. F. Davies and
A. F. Trotman-Dickenson Edward Dovies Chemical Loboratory Aberystwyth
II
The Dissolution of Tin In Solutions of Iodine A kinetic experiment
Reactions that can he followed by physical methods offer considerable didactic advantages when used as an introduction to chemical kinetics. Chief among these is that the student can see the progress of the reaction with time. Unfortunately, most physical methods require expensive apparatus, and hence are unsuitable for large classes. Moreover, they usually take an hour or so to set up and adjust before measurements can begin. The dissolution of tin in solutions of iodine in benzene, however, can be set up rapidly, and the apparatus required is widely available. The progress of the reaction can he followed by the loss of weight of a tin disc. Prepare tin discs about 1.5 cm in diameter and 2 mm thick by casting a bar of tin metal in a clean glass tube. The tin bar can be cut into discs with a small hacksaw or on a lathe. Then drill fine holes through the discs. Each disc should he cleaned with fine emery paper and then handled carefully by the edges. Hang on a thread from the hook of a single pan balance. Monofilament nylon thread is suitable for this purpose. Adjust the length of the thread so that the disc hangs about 1 cm below the surface of 10 ml of iodine solution contained in an open 25 ml. weighing bottle placed on an Archimedian bridge. The disc must hang horizontally, hut thermostating is not necessary because the temperature coefficientsof diffusion controlled reactions are very low. Benzene is the most suitable, readily available solvent because of its low viscosity and the high solubility for
iodine and tin (W) iodide in benzene. It is convenient to prepare solutions of 10 g of iodine in 100 ml benzene and to obtain weaker solutions by dilution. Rote of Loss of Weight of Tin D i x with Time -Solution: 0.6 g iodine in 10 ml benzenTime (min) Weinht (n)
Readings can be taken at 2 min intervals as soon as the balance is steady. Seven or eight readings are usually taken a's is shown in the table, but reasonably accurate rate constants could he obtained in a shorter time. The loss of weight occurs fast enough for the student to see a continuous movement of the scale of the balance. Runs with a succession of iodine solutions of diierent concentrations yield the plots shown in Figure 1. These can be combined to give the plot in Figure 2. From the latter the students can clearly see that: Rate of loss of tin = k [IS].
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Thus the nature of firstrorder behavior can be established. This interpretation of the results can he recognized by students who have not yet learned to
It may he of interest to note that this experiment originated in a small piece of work undertaken in 1944 by one of the writers, then in his last year at school, and his chemistry master E. J. F. James' (now Lord James of Rusholme, Vice-Chancellor of the University of York). 'TROTM.\N-DICKENSOX, A. F. AND JIMEG,E. J. F., J. Chern. Soe., 736 (1947).
Time (min.1 Figure 1. 'fariotion of weight d dixa with time when immersed in solution= of different concentrationr. Concentrotions (g lz In 10 ml) marked on lines.
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Figure 2. Dependence of rote of logs of weight of tin d i m on concentrations of iodine solutions.
