R. J. Kokes, M. K. Dorfman, and T. Mathia The Johns Hopkins University Baltimore. Maryland
I
I
Experiments for general chemistry IV
Chemical Equilibrium: the Hydrogenation of Benzene
The reversible reaction between benzene and hydrogen, forming cyclohexane, in the presence of metal catalysts has been the subject of many investigations since the original work of Sabatier and Senderens' in 1901. The first quantitative study
was published in 1923 by Dougherty and Taylor,=who used nickel as a catalyst, and a more extensive investigation by Burrows and Lucarini3 followed in 1927. Experimental results from these and other sources have been summarized in an article by Janz: who compares
The new freshman laboratory course at Johns Hopkins has been 38, 16 (1962). Many of the experidescribed in THIS JOURNAL, ments are innovations in an introductory course. This series of articles describes the experimental procedures in some detail. S A B A T ~P., R , AND SENDERENB, J. B., Compt. rad., 132,210
119231. ~ - ~ - . , ~
(1901).
DOUDAERTY, G., AND TAYLOR, H. S., J . Phys. C h . ,27,353
B u ~ ~ o wG. s , H.,
AND
LUCARINI, C., J . Am. Chem. Soc., 49,
1157 (1927).
ZHARKOV, V. R., AND FROST,A. V., J. Gem. Chem. U.S.S.R., 2, 534 (1932).
JANZ,G. J., J . Chem. Phw., 22, 751 (1954).
Volume 39, Number 2, February 1962
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91
them with equilibrium constants calculated from thermodynamic data. This platinum-catalyzed reaction is used for a student experiment. Equilibrium is approached with both benzene-hydrogen and cyclohexane as reactants. The apparatus is the simple vacuum system (see Fig. 1) that was described in a preceding paper.% The reaction vessel D is a cylindrical glass bulb of known volume indented a t the bottom by a finger-shaped well into which the junction of a thermocouple is inserted. This bulb, containing about 100 mg of catalyst (5% P t on charcoal), is connected to the system via a standard taper joint and a stopcock; a Variac-controlled oven is raised to enclose the bulb.
back diiusion of benzene out of B is trivial and the amount and composition of reactants in B is known. At the start of the experiment proper, the reactants are allowed to expand into the evacuated manifold and the reaction vessel containing the activated platinum catalyst and the stopcock to bulb B is closed. The pressure is monitored until there is no further preesure change in a 5-min period. It is then assumed that equilibrium has been attained and the temperature is recorded. Equilibrium pressures and temperatures are measured a t 270, 240, and 225'C. The available data, including calibration data, are sufficient to allow the student to calculate the equilibrium partial pressures of each component. I n order to calculate equilibrium constants ideal behavior is assumed. With essentially the same procedure it is possible to study the catalytic dehydrogenation of cyclohexane at 240, 220, and 200°C. I n this experiment a reaction mixture containing hydrogen is used. Apparently, the presence of hydrogen prolon~s . . - the lifetime of the catalyst. 3 The instructions given to the students wit,h this experiment include an example of the calculation of the equilibrium constant for the reaction CsHi
Figure 1. Sketch of vacuum system. 39, ryslem in use, see THIS
JOURNAL,
For a photograph of the octval
21 (1962).
At the start of the experiment, the catalyst is degassed with the oven set a t 270°C. The detachable bulb A (equipped with a three-way stop-cock and a standard taper joint) is evacuated and filled with tank hydrogen, and the catalyst is activated by exposure to air-free hydrogen for about 30 min at 270°C. (This procedure could conceivably result in an explosion if there were sizeable quantities of oxygen mixed with the hydrogen. Therefore, safety regulations are rigorously enforced. To date, this procedure has been used over a thousand times without a n explosion.) After activation, the catalyst is evacuated and the stopcock connecting it to the system is closed. First, it is necessary to prepare the air-free reactants a t a known composition. To do so, the student degasses a sample of liquid benzene, allows the air-free vapor to expand into the evacuated bulb B, and measures the pressure. When this is done, the stopcock connecting B to the vacuum system is closed. The bulb containing the liquid benzene is replaced by bulb A that has been filled with air-free hydrogen at a pressure about ten times the pressure of benzene in B. After the manifold has been degassed, the stopcocks on A and B are opened: the total pressure is noted and the stopcock on B is closed. Since the hydrogen pressure is substantially greater than the benzene pressure, KOKES,R. J., DORPMAN, M. K., EDUC., 39, 20 (1962). 1
AND
MATEIA,T.,J. CHEM.
+ 3Hs
-
CsHln
and a discussion of the relation between the equilibrium constant and thermodynamic quantities. I n the light of this discussion, they are expected to represent their results graphically as log K , us 1 / T and obtain a value for the heat of the reaction. Results from the laboratory reports of one freshman section are shown in Figure 2, together with some of the experimental values-of Zharkov and Frost4and Burrows and Lucariniaand the calculated curve given by J a n ~ . ~ This is a particularly complex experiment for students at this level; consequently, in some cases, the errors are extremely large. However, the data represented in Figure 1 show that many of the students obtained values as close to the calculated values as those appearing in
L Figure 2. Lucarini"
1
4
4
.
Hydrogenation of benzene. 0,student dot.; if. Zharkov and Frost'; d i d line. Janzs.
i
0,Burrows and
the literature. The quality of these results is apparent when it is considered that they were obtained both by the hydrogenation of benzene and by the dehydrogenation of cyclohexane. Sample Calculations
The following assumptions are made: (a) The composition of the benzene-cyclohexanehydrogen mixture is uniform throughout bulb D and the manifold. ( b ) All of bulb D that is within the oven is a t the temperature registered by the thermocouple. The manifold and that part of the stem of bulb D that extends out of the oven are assumed t o he a t r w m temperature. Ezperimnlal dala: Data obtained by calibration: Volume B = 201 cc Volume of Manifold = 5.1 ce Volume of D = 207 cc Data, obtained by estimation: Volume of that portion of D a t room temperature = 5 cc Data for dehydrogenation of CaHa: Initial pressure of CsHInin bulb B = 45 mm Initial pressure of CaHn Ha in bulb B = 418 mm Room temperature = 25-C Pressure of reactants and produots a t equilibrium in maniD = 262 mm fold Temperature of D a t equilibrium = 200°C
+
+
At the start of the experiment the gas in bulb B expands into the manifold and bulb D. At this point 201 ec 5.1 ec 5 cc = 211 cc of the mixture is a t room temperature and 207 cc - 5 cc = 202 cc i~ a t 2 0 0 T . We can compute Pi, the pressure of gas in the manifold plus D, before reaction occurs as follows:
+
+
From the initial pressures in bulb B the male fraction of cyclohexane in the reactant mixture must be 45/418 = ,108. By application of Dalton's law, the initial pressures of hydrogen and cyclohexsne in the manifold and D a r e found to be: initial prcssure of hydrogen = 222 mm initial pressure of cyelohexane = 26.8 mrn
-
In accord with the chemical equation we can write
CsH,, CsHa P a r t i d Pregsures a t Start (CaHdi 0 Partial Pressures a t Equilibrium (CeH12)i- (CsHs)+(CsHa)
+
3H2 (Hdi
+
(Hdi f 3(CsHe)
wherein ( ) refer to partial pressure and the subscript i stands for initial d u e s . According to Dalton's law, the equilibrium tntal pressure 262 mm is related to (C6H8)as follows
+ (HZ)
