Goal-Oriented Teaching of
G. W. Rayner Canham University of Victoria Victoria, KC., Canada
Thermodynamics in General
Students who are nonchemistry majors often find it difficult to appreciate thermodynamics. The subject can be made more interesting if it is taught in a goal-oriented manner interrelating with other fields of chemistry. A subject of current interest, nitrogen fixation, provides an excellent case. The biological nitrogen cycle provides a background (I). The estimated annual world-wide biological production of ammonia is 1M) million tons, and the additional requirement for artificially produced ammonia isestimated to reach 88 million tons p.a. by 1975. The point can be made that application of thermodynamics and gas phase equilibria enables one to find a route for the ammonia production, and to predict the most favorable conditions. The reaction N,
+ 3H,
z==
ZNH,
Preliminary Reactions The second law of thermodynamics is used in the cryogenic fractionation of air to obtain the nitrogen supply. The majority of the air cooling is accomplished by workproducing expansion, and reversible conditions are approached by using heat exchanges with the fractionated nitrogen gas. 1 The usual catalysts are as follows: Iron for reaction (1); pyro~horiePeron for reaction (2); NifNiO for reaction (3), (from ref.
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Journal of Chemical Education
Bond enerw (keal mole-? H% N, NHI
Entropy (kcal mole-' dag-?
104 227 93 (average)
0.0312
0.0458 0.04M)
(1)
is suggested as a possible synthetic route. As each aspect is discussed, the students can decide, without recourse to laboratory experiment, whether the reaction is feasible and can themselves design a plant for the commercial production of ammonia. Main Reaction Use of bond energies (see table) to determine the enthalpy of reaction shows the reaction to be exothermic (accurate AHI(NH3) = -11.04 kcal mole-I ( 2 ) ) .Provision of entropy values (see table) enables calculations of AG, and K , to be undertaken. These lead to the conclusion that this reaction route is practical. According to Le Chatelier's Principle, maximum yields would be expected a t high pressures and low temperatures. Turning to the kinetics, this reaction would he expected to have a high activation energy considering the strength of the N=N bond, and therefore to be very slow. As an increased rate of reaction is favored by rise in temperature, a "trade-off" between rate and yield has to he considered. On both the simplistic collision theory, and a molecular orbital symmetry approach (3), a complex mechanism must he involved. The use of a catalyst is an obvious suggestion to increase the rate of reaction (a probable mechanism for the catalysis has been described f4), as has the nature of the catalyst itself (5)). Unfortunately, temperatures of above 350°C are required for contemporary catalysts, but by cooling the exothermic reaction one can prevent the ternperature from rising above the minimum required. Even with a temperature of 400°C, it can be seen from Figure 1 (adapted from ref (6)),that a respectable yield can be obtained with pressures of 200 to 300 atm.
(51).
Thermodynamic Data (from Ref. (2))
Figure 1. Variations in emmonia yield with pressure and temperature (adapted from ref (61).
Figure 2, Basic design of plant which can be designed by students from chemical principles.
Natural gas is the usual source of the hydrogen CH,
+ H20 3 CO + 3HJ
(2)
AH, = +49.3 kcal mole-1, a reaction clearly favored by low pressure and high temperature. The carbon monoxide can be used to produce further hydrogen
AHi = -9.8 kcal mole-'. Catalysts are of course used to increase the rate.* A cycle for removal of the carbon dioxide can be designed using Henry's Law. The ammonia produced at the culmination of the sequence can also he removed in aqueous solution or by condensation.
Conclusion
Compiling all the aspects discussed above, the students can put together a design for a plant. This should lwk similar to Figure 2. The personality clash of Habe, with Nernst which lead to the actual development of the nitrogen fixation process illustrates the point that advances in science are not netessarily made in a logical manner (7). As well as appreciating the importance of thermody. namics, it is hoped the students will understand the interest in a synthetic route more closely approaching the bio-
logical system in efficiency. A review of recent chemical advances in this fie'd is available Literature Cited 111 Ddiviche. C. C..Sci.Amer.. 223.1361Sept. 1970). 121 "Handbook of Chemistry and Physics," (Editor: Weaaf, R. C.1 The Chemical Rub. b,,co.. cieueland. mio. 131 Pe=r~on.R. G..C h m Eng N e w 48,s (Sept. 2s. 19701. 141 Haonsel. V., andBurdrcll. R. L., Sei. Amer., 225.46 (Dm.19711. 151 ~ o r ~ a r~s . ~ . . a n d ~ ~ i dw.. ~ ec rh .r ~ m .h d I L O ~ ~ O~426(19~1. ~I. 161 Harding, A. J.. -Ammonia. Manufaefure and Uses;, Oxford Universiw ~ r e s s .London. 1951. 171 Wolfonden,J,H,,J,CHEM,EDUC ,,44, 299119671. (81 s
~ ~ ~ ~ I I ~J .~C,HsE.Mw. E. D. U C . , ~ S , ~ R ~ ~ I ~ ~ ~ I .
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