LETTERS O n Unimoleculai a n d First Order Reactions
To the Editor: The article on unimolecular reactions in the SeptemOf JOURNAL because the author had experienced some difficulties during the course of teaching this subject. It was written with the hope that i t would provoke an exchange of opinions. The author, therefore, welcomes the criticism by W. F. Luder which in the issue of JOURNAL. The author thinks there is no ambiguity in his definition of unimolecular reactions and therefore the reaction 2Ns0,
-
2NnO4
+ On
as written by Luder, cannot be recognized as unimolecular because i t involves two molecules. But, if it is written as Glasstone' does on p. 1028, i . e., NO, N104 '/% o*
-
+
i t is a unimolecular reaction. The ambiguity may be due only to the fact that we do not know the actual mechanism of this reaction. The second point raised by Luder is that the author does not make any distinction between the order and molecularity of a reaction. The subject was considered only from the point of view of molecularity, i . e., "the number of atoms or molecules taking part in each act leading to chemical reaction," as it is defined by Glasstone (p. 1024). "Textbook of Physical Chemistry," D. Van Nostrand Company, New York. 1940.
There is no clash between this definition and the one used by the author. The order of the reaction depends upon the number of atoms or molecules whose determine the velocity or kinetics of the process (Glasstone's defi.ition), A first order reaction, as Luder states, is therefore one in which the depends only one
H~~~there is a clash with the view expressed by the author in the article. The definition as above is factory for the second and higher orders, but does not according apply to the first order if the reaction to the pattern of a radioactive chanrre - where concen. tration does not come into question a t all. In his mind the author identifies the first order reactions with the exponential law. Such reaction may be unimolecular or otherwise. A unimolecular reaction should always follow the exponential law, but there are cases kno& in which other than unimolecular reactions also follow the same law. In this case i t is natural to think of them as first order reactions. Thus unimolecular and first order reactions are not necessarily the same thing. One can therefore say that the first order
reaction i s one i n which the rate depends upon only one substance, irrespectine of concentration. The definition of the first order as used by Luder must be modified. His definition of unimolecular reactions is the same, except in wording, as that given by the author. GEORGE ANTONOW FORDHAM UNIVBRSITY NEWYo= CITY
Editor's Note: W e recently receivedfrom Dr. F. Fromm the following addition to his letter i n the January issue, describing the construction of sfereochemical mod& of carbon a t o m :
The proper valence angle is easily obtained, if the bisecting lines are drawn on each face of the tetrahedron and the three flaps of the paper tubing representing the
valence are pasted to these lines. A paper tubing of a somewhat larger diameter is used for connecting the models.
