Edwin C. Steiner and Gordon E. Hartzell The Dow Chemical Company Midland, Michigan
Device for Measuring Instantaneous Rates of Gas-Evolving Reactions
conventional manometric methods for measuring the rates of gas-evolving reactions relate volume changes at a fixed temperature to concentration of reactant. A plot of some suitable function of the concentration against time yields the rate constant. Instantaneous (or differential) gas evolution rates are more rarely used, but they yield rate constants directly and the same system may be studied at several temperatures for the differential rates. For the instantaneous rates to he valid, reaction increments during a measurement must be very small (about 1%or less) and volume changes of the order of a few milliliters must be measured accurately. When ordinary reaction vessels are used, the large free space in the vessels makes the total gas volume extremely sensitive to small pressure changes. Consequently, any device for measuring small volume changes must be operated by extremely minute pressure changes. We have assembled such a device, consisting of a hypodermic syringe with an airdriven spinning plunger, for this purpose. The apparatus has been used successfully to determine both reaction rates and activation parameters for a number of reactions. Activation energies are easily and quickly determined on a single reaction mixture by measuring differential reaction rates at a number of temperatures. The small quantities of reactants required allow temperature changes to be made quickly, and the duration of each measurement is short. Therefore, the whole process may often be accomplished in less than two hours. Although the method in its simplest form lacks high precision, the short time r e quired and the low cost of equipment suggest that the technique would be useful as an experiment in a physical chemistry laboratory course.
temperature bath was made from a beaker (filled with silicone oil and equipped with a stirrer) and two infrared heat lamps (controlled by a thermoregulator). Procedure
The reaction mixture is placed in the vessel at a temperature at which the rate of reaction is negligible. The system is saturated with the gas to be evolved and then quickly heated to the desired temperature while open to the atmosphere. When the temperature becomes constant, the system is closed by means of the stopcock and the gas evolution rate is determined by observing the displacement of the spinning plunger over a short timed interval. Care must be taken to ensure that the temperature remains constant during the time of measurement. The gas evolution rate, corrected to standard conditions, is used to calculate the rate constant, taking into consideration the stoichiometry and order of the reaction. The system is again opened to the atmosphere, heated to the next higher temperature, and a new rate determined. During this process, the concentration of reactants changes. I n many cases the change can be kept to a few percent and the actual concentrations at each determination may be estimated satisfactorily in the following way: The total time lapse between determinations is recorded. The rates of gas evolution of successive determinations are averaged and multiplied by the lapsed time between them to give the total
,/-HYPODERMIC ,CELLOPHANESYRINGE
S T O P C O C ~ ~ ~
TAPE FINS 'FAN
Apparatus
A flask equipped with a magnetic stirrer, condenser, and thermocouple is mounted in a constant temperature bath of relatively low heat capacity, equipped so that the temperature can be increased rapidly when necessary to a predetermined value. To the top of the condenser is attached a hypodermic syringe mounted in a horizontal position, a thermocouple, and a stopcock leading to the atmosphere (see Fig. 1). The plunger of the syringe is made to spin rapidly by a gentle stream of air directed a t three cellophane tape fins attached to the plunger. When the plunger is spinning in this manner, it is virtually frictionless and responds faithfully to the slightest change in pressure. The syringe is easily leveled while it is open to the atmosphere by adjusting it until the spinning plunger no longer drifts in or out. A fairly satisfactory constant
TIP
CONDENSER.
THERMOCOUPLE TO CONTROLLER' HEAT LA
REACTION MIXTURE ,HEAT
E-
LAMP
u MAGNETIC STIRRER Figure 1.
Spinning syringe apparatus.
Volume 42, Number 7 0, October 1965
/
559
Figure 3.
Decomposition of u,a'-dimethoxy-a,a'-dimethylbibenryl.
Examples
Azobisisobutyronitrile (AIBN) is known to undergo first-order decomposition above 60°C to yield nitrogen: Figure 2.
Decomporition of orobirirobutyronitrile.
CN
CN
Decomposition of Azobisisobutyronitrile
Temperature Nn evolution (T) (ml see-' X 10')
AIBN present K LX 10' (mmoles) (set-')
amount of gas evolved. The concentrations are then corrected from the initial known concentrations by appropriate amounts. A more accurate method of determining the extent of reaction could also be used. A large second spinning syringe could be added to measure the total volume of gas evolved. I n this case the system would not be opened to the atmosphere between determinations, and the change in volume due to change in temperature would have to be determined by performing a control experiment with gas-saturated solvent in the absence of reactants.
-
CN
A
c H r L iN = N d - c H1 8
6Hs
6Hs
2 cH8-A.I
+ Nz
CHa
A 0.15 M solution of AIBN (0.638 g., 3.89 mmoles) in toluene (25 ml) in a 50-1111 flask was studied by the described method using a 2-ml syringe to measure gas evolution rates. Timed intervals of 1-3 min were used and a temperature range of 30' was studied. The experiment took about 2 hrs during which time approximately 37% of the AIBN had decomposed. The data are shown in the table and in Figure 2. The experimentally determined activation energy was 34.3 kcal "
,
-
first order decomposition in diphenylmethane solution at temperatures above 230°C with evolution of methane.2 This reaction was determined to have an activation energy of 61 kcal per mole using the spinning syringe method (Fig. 3). The reported activation energy, using conventional methods, is 58 kcal per mole. WALLING, C., "Free Radicals in Solution," John Wiley & Sons, Ino., New York, 1947, p. 513. H ~ T Z E LGL.,E., AND HUYSER, E. S., J. Org. Chem., 29,3341 (1964).
