chemical queries
J. A. YOUNG Kings College Wilkel-Barre, Pennsylvanm
. . . especially
Son Diego Stole College Son Diego, Cdifornia
for
introductory chemistry teachers
Question I t would appear that there are several ways in which the following equation can be balanced, and all of them seem to give acceptable answers.
Will you kindly comment on this?
Answer
by Robert B . Smith, University of Nevada, Las Vegas, Nevada I t is not uncommon to see equations which display variable stoichiometry. When an equation can be balanced in several different "acceptable" forms it is generally true that any one of the forms is an incomplete or incorrect description of what is actually taking place in the reaction system. Probably, a number of independent reactions are taking place.' I n this instance eqns. (1) and ( 2 ) , even though balanced, make no chemical sense and are incorrect. To establish this, however, we must know more about the mechanism of this particular reaction. I n the oxidation of t-butyl alcohol the four products, shown above, have been found experimentally. Let us write equations representing plausible overall successive stages through which oxidative degradation of the t-butyl alcohol molecule might proceed, bearing in mind that each of these actually shows only the net effect of a complicated series of steps (CHs)aCOH
+ 203 +
CH3COCHa 201 CHICOOH
-+ -
CHaCOCHs
+ COB+ 2H-0
+ CO* + H1O + 2H10
CHICOOH 202 2C01
(4)
(5) (6)
With this sequence of events in mind, we can view eqn. (3) as a summation of two parallel oxidation processes, one converting a molecule of alcohol to ketone, the other converting a second molecule of alcohol to carboxylic acid
Z(CH&COH
+ 60s
-
CHaCOCHs
+ CHaCOOH + 3C01 + 5Hn0
(3)
Thus we have arrived at eqn. (3) by combining the alcohol-to-ketone process and the alcohol-to-acid process in an arbitrary 1: 1 ratio. We could just as arbitrarily have combined the two processes in varying proportions to obtain an endless list of properly balanced, chemically reasonable equations. Furthermore, by bringing eqn. (6) into consideration, we could generate additional combinations of reactants and products. On the other hand, in the light of eqns. (4) and ( 5 ) , we readily see that the stoichiometrically correct eqns. (1) and (2) are chemically unreasonable because they show more molecules of ketone and acid forming than there are alcohol molecules being consumed. Perhaps the message to be drawn from this discussion is that the proper balancing of chemical equations is much more than a mere bookkeeping procedure. First, knowledge of the actual products of a reaction, derived from experimentation, is mandatory. Second, if the reaction is at all complicated, at least the general outline of a reasonable reaction mechanism is needed. If such information is not available, there is always a risk that the balanced equation one writes will not represent what would be observed experimentally. Question
I am puzzled about the details involved in the eatdysis of the reaction of the two gases, HZand 02,to produce HZO. A widely rtv$lable instructional film suggests that oxygen is adsorbed on platinum catalyst, and some texts state that hydrogen is adsorbed; both kinds of source then further indicate the other gas reacts with the adsorbed gas to form the product. Which is correct? Answer
by Richard E. Powell, University of California, Berlcley, California Succinctly, no one knows which is correct. However, a discussion of this conclusion is in order. First, it has been factually established that both hydrogen and oxygen are adsorbed, or otherwise held, on a platinum surface. Platinum is not wholly chemically inert. Oxygen is adsorbed in the form of atoms or as a surface layer of platinum monoxide. Hydrogen is adsorbed as hydrogen atoms or a s a surface layer of platinum monohydride. Typically, when platinum is used as a catalyst in this 'Some otherwise good textbooks present composite equations without sppropriate clarification. This practice is particularly common in discussions of aromatic substit,ution reactions which lead to isomeric products.
Volume 47, Number 4, April 1970
/
281
reaction, the heat evolved in the reaction makes the platinum red hot. At this high temperature, the reaction a t the surface of the platinum (whatever the details actually are) becomes very rapid. So rapid that the effective rate is limited by the rate of arrival of hydrogen molecules, or oxygen molecules (whichever is in the smaller stochiometric quantity) at the surface of the platinum. This was established by Langmuir almost fifty years ago.% Langmuir used hot platinum wires of about 0.007 square centimeter area in a 4-1 flask with gas pressures of about 0.1 torr. The temperature of the platinum wires were 900, 1110, 1260, and 1500°1
