A. F. Trotman-Dickenson
Edward Dovies Chernicol Loborotory Aberystwyth, Wales
A Simple Apparatus for Gas Reactions The decomposition o f di-t-butyl peroxide
Gas
reactions should figure prominently in elementary courses of chemical kinetics because of their basic simplicity. Unfortunately it has proved difficult to realize their full didactic value because of the obstacle~to laboratory work which helps greatly to bring a topic to life. The conventional system for the study of a gas phase reaction calls for a fairly expensive rotary pump together with a trap system cooled with liquid nitrogen to protect it. A reaction vessel and various storage vessels must be connected through taps to the vacuum line. Finally some means of following the reaction must be provided. The simplest is a mercury manometer which may have to be heated for less volatile reagents and is always fragile. The set-up will often include a mercury diffusion pump which is also expensive. Experience has shown that such a system can be satisfactorily used in elementary university laboratories where glass-blowers are a t hand to effect repairs, but it is not practicable for most schools. Apparatus
We have devised a simple apparatus which gives excellent results' for the decomposition of di-t-butyl peroxide but can be had for a fraction of the price of the conventional system. The key to its operation is the study of the reaction in an inert atmosphere rather than the usual vacuum. The apparatus is illustrated in Figure 1. A is a 500-ml round bottom flask to which are attached two inlet tubes B and C. B is closed with a
Subaseal 7.5-mm self-sealing stopper which may be of either ordinary or silicone rubber. Chemical attack on an ordinary rubber stopper is slow and its life is usually determined by the number of times it is pierced. C, which should be fairly straight, is closed with a regular stopper. Pressure differences are measured on the Pyrex spiral Bourdon gauge. The mirror, E, attached to the arm of this spiral is illuminated with an ordinary galvanometer lamp, through the plane window, F. When a scale is placed at 50 cm from the mirror, the sensitivity is approximately 1 mm deflection for 1 torr. The vessel is held inside a 2-1 beaker by the clamp, G. The beaker is filled with cooking fat near to the top so that when the liquid is a t H the mirror is above the lip. The beaker is placed in a suitable can so that 3-5 em of rock wool or other lagging surrounds it. The bath is heated by a Nichrome spiral of about 200 ohms inside a suitably shaped 10 mm od Pyrex tube. Heating is controlled by a mercury thermometer controller, but no stirring was required for a temperature that was uniform to within about. O.Z°C. The temperature is read with a mercury in glass thermometer. Operation
After the temperature has settled down, which takes about a half-hour, the vessel is flushed out with an inert gas. A short length of narrow-bore metal tube is attached to the gas supply and inserted through C to the bottom of the vessel. We have usually used nitrogen as an inert gas, but carbon dioxide or the town gas supply will serve. After a few minutes the stopper is firmly replaced in C . Reaction is started by injection with a hypodermic syringe of about 0.3 ml of di-tbutyl peroxide through B. Greater and lesser amounts can be used but this gives a convenient pressure change. Tuberculin syringes are of convenient size. Reaction begins instantaneously as the liquid vaporizes. The pressure change is followed against time. The slight tendency of the vessel to act as a gas thermometer is reduced by closing the space around the spiral with a stopper, J , so that pressure fluctuations within and without the spiral are partially balanced. Results
The decomposition of di-t-butyl peroxide is described by the reactions (CHs)lCOOC(CH3)a= 2(CHa)&O 2(CHa)aCO = 2CH8 2CHs Figure 1. The reedion vessel, A, 500-rnl Rork; B, inlet tube closed with Suboreol; C, inlet tubeclosed with stopper; D, Pyrexspiroi Bowdon gauge; E, mirmr behind plane window, F; G, damp; J, stopper thot helps to offset thermometer effect.
396
/
Journal of Chemical Educofion
=
GHe
+ 2(CHa)%CO
(1) (2) (3)
'Manufactured by: Jencons (Scientific) Ltd., Mark Road, Heme1 Hempstead, Herts., England. Price £12 5s ( ~ $ 3 0 ) .
Figure 2. Fincorder plots of first four runs ~erformedwith apparatus b y a student.
of which reaction (1) is rate-determining. Three molecules of volatile product are formed from each molecule of reactant, hence first-order rate constants, k, are found from the relation log(3Pi - P t ) = -k1/2.303
+ log ZPi
where P1 is the initial partial pressure of peroxide, and P, is the partial pressure of reactants and products a t time t . P1 can be found either by direct estimate of the initial pressure (for slow runs) or from the final pressure after 6 half-lives (for fast runs). k is best found from a linear plot of log(3Pt - P , ) against t . The first four plots obtained by the first student to use this form of the apparatus are shown in Figure 2. Figure 3 shows the plot of log k(secrl) against the reciprocal temperature; the points were obtained by six different students. The line is that recommended by Batt and Beuson2after consideration of their own experimeuts and those described in the literature. I t can he seen that the agreement is excellent for the line given by the Arrhenius equation log k(secr1) = 15.6 - (37,400/2.303RT)
The decomposition of di-t-butyl peroxide is an excellent reaction for didactic purposes for several reasons. The material is readily and cheaply available in high purity. The rate determining step is a true unimolecular homogeneous reaction. The substance and its de-
Figure 3. Comporite Arrhenius plot far the decomposition of di-t-butyl perwide obtained b y flve rtudenb. Solid line is fmm best equation in the literature. Reference in footnote 2.
composition are of importance in the initiation of polymerizations commercially. Finally, the activation energy has a simple physical significance. I t is equal to the strength of the 0-0 bond and hence can be used as a good introduction to the whole topic of strengths of bonds. Other
Applications
We are investigating application of the apparatus to other reactions. The temperature range can be readily extended by using a bath of fused tempering salts in place of the cooking fat. There should he no difficulty in injecting accurately mixtures of liquids and hence studying bimolecular reactions. We hope to publish accounts of further experiments in due course. The chief limitation would seem to be that it is difficult to study reactions which are only truly homogeneous in vessels whose surfaces have become coated with a deposit (carbonaceous products?) either deliberately or as a result of many trial runs. 'BATT,L.,
AND
BENSON, S. W., J. C h m . Phya., 36, 895 (1962).
Volume 46, Number 6, June 1969
/
397
