Tricarboxylic Acid Cycle
Heat Capacity of Monatomic Solids
To the Editor: The schematic diagram. of the tricarboxylic acid cycle presented Dr. f i z a Nafissy [J. CHEM. EDUC., 49, 620(1972)] provides a useful demonstration of the fact that the concentrations of the various intermediates remain essentially constant as two-carbon units pass through the cycle. Unfortunately, the diagram is somewhat over-simplified in its implication that the carbon atoms lost as COz are identical to those which entered as acetyl CoA. documented and can be demThat this is untrue is onstrated by following the fate of 14C-labeled acetate through one turn of the cycle. In such an experiment, no label is found in COz (cf. Lehninger, A. L., "Biochemistry," Worth, N.Y., 1971,p. 339). In view of the experimental data, the Nafissy diagram might be modified to show a step in which the wheels of the four-carbon "carrier" are switched around (corresponding to rotation of tires on an automobile which carries two spares) before the first molecule of COz is ejected. This would place the diagram in agreement with the fact that the carbon lost as COz was originally present in oxaloacetate, and did not arise directly from acetyl CoA.
To the Editor: I should like to make two comments on Professor Dencek article on Heat Capacity [J. CHEM. EDUC. 49, 793 (1972)l. ~ i ~a monatomic ~ t l ~ solid can quite properly be regarded as a single giant molecule in the siinple classical the limiting approach given in ~i~~~~ 10 of the article. case of llnof a mole of nonlinear n-mers with heat capacity 3R - ( 3 ~ 1 as ~ ) equals the nupber of atomspresent in this case ~~~~~d~~~~ number (fi) the classical beat cap>city is identical to that of a single nonlinear molecule of Natoms
David Blackman Federal City College 425 Second Street, N.W. Washington, D.C. 20001
C, = (3@
- 6)k + 2k 3 +3 2k
where the last two terms are the contributions from motion of the center of mass and of the three degrees of rotational freedom, respectively. For any macroscopic crystal of course the classical molar Cv will he 3R for all other terms are negligible. Secondly the well known fact that certain low-melting metals (particularly the alkali metals) exhibit a Cv above 3R a t room temperature has no connection with the heat capacity of their conduction electrons. It is due to the development of imperfections and holes in the lattice which become a new means of energy uptake as the melting point is approached and less significantly there is also the role of anharmonic components of vibratory motion as the temperature is increased. Charles E. Hecht Hunter College of CUNY New York, New York 10021
Curing the Cheat Sheet Syndrome To the Editor: We would like to share with your readers two methods we have used to bridge the gap between open- and closedhook examinations. The first method permits each student to bring up to two 8% X 11 in. study or crib sheets to an examination. (The effort required to prepare these sheets almost makes their use redundant!) The demeaning detecting that proctoring frequently seems to involve is avoided. For the second method we are indebted to one of our students, Mr. Gordon Ashley. The examination period is simply divided into two portions of variable length, and the exam, itself, is in two parts. The first part is closed book and permits the testing of those aspects of learning which require memorization, e.g., the use of terminology as manifested in exact definitions. After the first part is collected, the open book portion of the exam is distributed. We have routinely used these techniques for exam periods as short as 50 min. Our students have appreciated the advantages of both methods.
Fiber Tip Chromatography Hints To the Editor: With regard t o Philip Kasimer's letter in your February 1973 issue, we have heen demonstrating chromatography principles, also using fiber tipped pens. We find an inexpensive paper to use is blotter paper and this is more readily available for the classroom teacher to acquire. An additional time saver is to stretch a ruhher band across the top of the solvent container, stand the blotter paper strip on the bottom and lean the top against the ruhher band. This method works fine as long as the ink spot is higher on the paper than the solvent. Harlan F a k i n , Supervisor
Rubin Battino Stanley Weissman Wright State University Dayton, Ohio 45431
448 /Journal of Chemical Education
Physical Science Pacific Science Center Foundation 200 Second Avenue North Seattle, Washington 98109
