T. p r y c e - J O ~ ~ S The Hatfield Polytechnic Hatfield, England
Heterogeneous Kinetics in the Laboratory The decomposition of ammonia on a tungsten surface
A study of the rate of decomposition of ammonia on a tungsten surface has been suggested as a suitable undergraduate experiment (1,Z). Details provided in practical textbooks suggest that a suitable tungsten filament can be made either from wire of the correct dimension (1) or that a 32-V Osram lamp can be used as the reaction vessel (I), as was used by Hinshelwood and Burlc in their original experiments (3). In the teaching laboratory a t Hatfield, we have found that a projector bulb, with its large filament, proves to be a very effective reaction vessel. A 500-W G.E.C. T 1 bulb is particularly suitable. Such a bulb is a little smaller than a 0.5 dma round-bottomed flask, and can be adapted by glass-blowing a B 14 Pyrex socket onto the bulb, which is then connected to a cone using picien wax. The remainder of the apparatus, also in Pyrex, is arranged in a similar way to that described in James' textbook (1). The apparatus can be evacuated via a large bore manifold, and this has the advantage that the same pumping system can be used for a number of experiments, e.g., a homogeneous kinetics experiment such as the decomposition of di-tert-butylperoxide (4) and aB.E.T. study (5). Using such a reaction vessel, the change from zero order to first order kinetics bas been observed (6),and an estimate of the zero order rate constant made. A typical value of this rate constant obtained by students at Hatfield is 0.390 mm Hg sec-' or 5.20 X 10-l4 N m-2 sec-'. An estimate of the temperature a t which this rate constant has been measured can be made by using the temperature dependence of the specific resistance of tungsten, cr. This has a value of 4.80 X K-'
848 / lourrml o f Chemical Education
(7). The resistance of the filament is measured at room temperature, a t Hatfield, using a h'larconi Universal Bridge, and the operating resistance of the filament is obtained from the ratio of voltage to currmt. In the experiment reported, the room temperature resistance was 8.45 ohm and the working resistance 40 ohm. The specific resistance, p, can then be ohtained from the dimensions of the filament. In our case, these values are 0.118 mm2 and 1600 mm. Thus the temperature found from the relation PL
= po(1
+
uL)
is t = 770°C, which is the rise in temperature above that of the room. Hence the temperature of the reaction is 1070°K. By estimating the volume available to the reactant gas, and by knowing the surface area of the tungsten, i t is possible to calculate the rate constants in the form concentration per unit area per unit time. The volume of the bulb and connecting tube was estimated a t 0.480 dm3, and the surface area of the tungsten filament 593.3 mm2. Thus, the value of the rate constant becomes 1.117 X loz2 molecule m-2 sec-'. Hinshelwood's results (3) together with data by Icunsman (8) have been presented in this form by Topley (9) and appear in several textbooks (10, 11). They are listed below. ko = 4 X 1OZlmolecule m-l sec-I
at 904°K ( E = 158.1 kJ mole-L) (.?)
ko= 2 X 10" molecule m-l seerL at 1316°K (E = 173 6 kJ molF1) (8)
Thus, the rate constant obtained at Hatfield can be
regarded as having an acceptable value, when it is recognized that (1) no lead corrections are made in measuring the filament resistance, (2) not all the tungsten filament is a t the same temperature, (3) the volume to the ammonia gas may be inaccurate as much as 15%. The experiment can be used to demonstrate the change in Order as the reaction to that the zero order region can he extended if the amount of reactant initially available is increased, and can he used to obtain a zero order rate constant which proves to be of the correct order of magnitude. Acknowledgment
The author wishes to express his thanks to Osram (G.E.C.) Ltd., East Lane, Wembly, Middlesex for kindly supplying the dimensions of the tungsten fila-
ment; suitable projector bulbs can be obtained from their company. Literature Cited (1) J A W B ~A. , M., "Praotloai Phyaioal Chemistry" (2nd ed.), J. and A. churchill ~ t d . L. O ~ ~ O 1967, ~ . D. 242. (2) ROSE. J.. "Advanoed Physico-Chemical Experimente," Pitman Publishing CO., L O ~ ~ 1964, O ~ . P. 158. (3) HINSHE~VOOD. C. N..AND BORK.R.E.. J . Chem.Soc.. 127, 1105 (1928). (4) P m c ~ A. . H.. A N D BAHER.R. T. K., J. cam^ Enuc.. 4 2 , 6 1 4 (1965). (5) SALZBEB(I. H. MORROW. J. I.. COHEN, R., AND G R ~ E NM. , E.,
w..
s.
''Physical Chemistry. A Modern Laboratory Course." Academic Press Ino., New York, 1969, p. 429. (6) T u o ~ s o r r S. , J., AND WEBB,G . . ' . H ~ ~ ~ cstalysia," ~ ~ ~ ~oliyer ~ ~mod u I Boyd. Edinburgh. 1968, p. 85. (7) S u l r x r ; ~ ~S. s , I.. "Tungsten" (3rd ed.), Chapman and Hail Ltd.. London, 1952, p. 181. (8) K n a s ~ * w ,C. H.. L * M * ~ ,E. s.. AND D E M I N ~W. , E., phi[. MW.. 10,
1015 (1930). (8) TOPLET,B., N a t w e , 128, 115 (1931). (10) G L ~ ~ S T O N . . LAIDLEB.K. J., AND E I I R I N H.. ~ , - T ~ ~ O T Yof ~~t~ P ~ ~M O ~G ~ ~~W - BOO^ H ~ ~ I CO.. ~ ~ ha., ~ N~~ W YWL. , "1941, P. 377. (11) E M ~ E T TP.. H.. (Editor), "Catalysis," Reinhold Publishing Co.. New y o r k , 1954, I , P. 214.
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Volume 49, Number 12, December 1972
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