Bill S. Yamanashi and Andrew V. Nowak Massachusetts Institute of Technology Cambridge, 02139
II
Recombination of Iodine via Flash Photolysis A chemical kinetics experiment in
I
physical chemistry
The well-known experiment of iodine atom recombination in the gas phase is chosen for college students to acquire some understanding of the kinetics of chemical recombination and the techniques for investigating fast chemical reactions. The recombination proceeds via a pair of collisions involving a third body M. Phatolysis: Recombination: I
+ I ++ hvM 12
-- + 21 In
M
It is hoped that the student will learn from this experiment the following points: (1) In order for photodissocit~tedatoms with internal and thermal energy to recombine, a third body M, the chaperone, is necessary to remove the excess internal energy of the molecule. (2) The rate of recombination depends on the nature of the third body. (3) During the recombination the atom and the third body forms transitory complex. The complex reacts with another atom to finally yield a, stable recombined molecule and an activated third body. (4) The rate expression associated with the recombination is second order with respect to iodine atom wncentration and first order with respect to third body concentrntion. (5) The iodine molecule itself acts as a very efficient chaperone.
The experimental techniques employed follow those of R. G. W. Norrish and G. Porter who were awarded the 1967 Nobel Prize in Chemistry for their work in fast reaction kinetics. The apparatus, however, has been modified for (1) facilitating the use of standard, commercially available components, (2) ruggedness for repeated use by non-experts, and (3) compactness. Background
The experiment is designed for use in a junior physical chemistry laboratory. Three or four periods of 4 hr duration are needed for completing the experiment. The first period is used for an introductory lecture on flash photolysis, the theory of termolecular kinetics, and use of the equipment. The last two or three periods are used to obtain recomhination data. Alternatively, the apparatus can he built and operated by students as part of a term project or a senior thesis. During the first week a bound set of photocopies of pertinent literature articles arranged chronologically and the operational instmctions are given to each student. From these articles the students are expected to become aware of various theoretical models from which one can determine the rate constant. Included in the hackground articles is a series originating from Porter's laboratories. The first of these (1) is the most convenient place for the student to
begin his study of gas phase iodine recombination. The authors summarize the various theories of atomic recombination in the presence of a third hody and also present data on the recomhination of iodine in the presence of all five noble gases. In their second paper (2) they show that a simple termolecular rate law is not obeyed due to the effects of iodine as a third body. The student is also urged to read a series of papers by Davidson. In the first of these (3) the authors propose the theory that the recombination most probably involves a "sticky" collision between an I atom and the third body 1LI to form a complex IM which reacts with a second I atom. By this time the student is aware that despite any earlier thoughts on the subject, the recomhination does not proceed via the association of two iodine atoms followed by third hody deactivation. In the next three papers (4-6) Davidson, et al., discuss in quantitative terms various theories for the recombination. Some other papers in the field (7-13)round out the journal material suggested for the student. Experimental
The apparatus is illustrated in Figure 1. The mea-
Figure l . Monitoring scheme. l , Monitor light; 2, Rash tuber; 3, reoction vessel; 4, interference filter; 5, photomultiplier; 6, orcillorcope. l o and I ore the monitor light intensities.
sured quantity associated with the concentration of the recombining species a t a given time is the photographed deflection of the oscilloscope trace. An external triggering unit causes the scope to scan once in synchronization with the beginning of the photolysis flash. Iodine molecules have a characteristic absorption in the wavelength region centering about 500 mp (green) where iodine atoms are transparent. Assuming Beer's law is obeyed, the concentration of iodine molecules can be measured by monitoring the intensity of a spectrally narrow beam of light centered about 500 mp passing through the reaction vessel
Volume 45, Number 1 1 , November 1968
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where lo@) = It(A) = r(A) = 1=
intensity of incident light intensity of transmitted light rut time t molar extinction coefficient length of reaction vessel (4)
