exam que~tion exchange JOHN J. ALEXANDER University of Cincinnati Cincinnati, Ohio 45221

Physical Chemistry-Thermodynamics Jack Richlin Monrnouth College West Long Branch, NJ 07764

One difficulty found in testing in physical chemistry courses is that the goal of assessment of understanding of the theo- retical material often involves evaluation of derivations and manipulations of memorized equations. Additionally, working out of problems and derivations can take time, so the scope of material the instructor can reasonablv cover in a 50-minute exam is necessarily limited. It then follo& that the point value for each auestion must he inflated in order that the student be assured there is sufficient time allotted to think through each ouestion and search for the Droner a~nroach and crucial . .. assumption or condition.

Assignment of ~ a r t i a l ~ o i n t values for each answer is dif- ficult for the instructor because if the student misses the initial thrust of the auestion. hwshe will often fill the space with false starts whichehow some understanding and-leave the in- structor with the lingering feeling that the student was on the right track but simply did not recognize the critical point.

The style of question herein presented allows the instructor to cover a wide range of material a t varying achievement levels in a reasonable period of time. I t is particularly suited for testing in thermodynamics, and the questions can be arranged to test for basic understanding, subtle points, or rote memory.

Normally, questions of this type would constitute ap- proximatefy 50 percent of an examination, the other half being given over to traditional-style problems. Below are examples of subject matter and degree of sophistication for which this format may be used profitably.

Quesilon

State whether the followine are (a) absolutelv true. (b) conditionally true, or ( c ) absol&ly fake. If your &wcr is (hl, eive the conditiun(s1 which make it true. If vour answer is (cl, substitute, if possible, a correct statementfor the situation.

1) A system gains in heat, q, when it expands isother- mally.

2) The van der Waals' constant, a, corrects for the excluded . . volume of the gas molecules. dSSrJt = dq1T. If V is a function of T and p , then ( a T l a p ) ~ = -(&'I ~ P ) T ( ~ T I ~ v ) , . At the boiling point of a solution, the chemical potential of the solute in the liquid phase is equal to the chemical notential of the solvent in the vaoor ~ h a s e . . .

6) ~~(GIT)I~T],, 7 -HIT2. 7) Positive devlat~on from Raoult's law in a binarv solution

produces a low-boiling azeotrope. 8) From d U = -pdV + TdS, we get the Maxwell relation

( a p l a ~ ) ~ = -(asiav),. 9) pliquid < paolid for a pure substance a t a constant pres-

sure. 10) AG = J - SdT.

Acceptable Solution

1) (h) The external pressure must he greater than zero. (a) is ac- ceptable at the junior level.

2) (c) The constant, b, corrects for excluded volume. 3) (b) True when the process is reversible (dq = dq,,).

4) (a) 5) (c) Change the word solute to soluent. R I l a ) ... ~ - , 7) tb) A low-hdng iurutrupc will hc prduced unly when the vapor

pressurc ofrhe sdutim is higher than the vapor pressures of huth pure components at that temperature.

8) (c) (Jp1JS)v = -(JTlJV)s. 9) (h) True when T > Tr.*,..

10) (h) True when the process is carried out at constant pressure.

Superfluous Steps in Glycolysis? Thomas P. Chirpich Memphis State University Memphis. TN 38152

Students encounter the glycolytic pathway in detail in biochemistry but may also have had previous encounters with it in a physiology or a bacteriology course. Too often they feel that there is not much more to he learned and regard the pathway only as a series of steps to be learned by rote. The followine auestion shows that there is much more to the pathwa<and a t the same time it asks the students to integrate biochemistry with other chemistry that they have encoun- tered, such as the meaning of, and need for, certain free energy changes or the effect of ring size on ring stability.

Question

Glucose + 2 Pi + 2 ADP - 2 lactate + 2 ATP + 2 HzO

The overall elvcolvsis reaction shown above occurs in eleven enzyme-catalyzed stkps. If the sole consideration is to produce ATP, four of these stem could be eliminated. Altering the remaining seven s ~ e ~ s s ~ ~ h t l ~ toaccommodate 1he ;ede lchs would still result in the same overall equation. What are the four steps which could be eliminated? Give some possible reasons why they are still included in the pathway.

Acceptable Solution

The four steos that could he omitted are the ohosohorvlation of ~. . ,~ ~ ~

glunse, the phosphorylarion of frurtoze-6-phospharr, rhr convrrsion of 3-phowhoalyrrrate to 2-phosphoglgcrratr, and the phuiphoryl transfer from phosphoenol-pyruvate. Glucwe could still be converted to lactate with the production of two ATP.

Omission of these steps is equivalent to carrying out the pathway with most intermediates in the nonphosphorylated form. There are several oossible reasons for the inclusion of these four seeminelv su- oerfluok steos: la) Phwohorvlated comoounds do not oass t<rbueh . . . . , .~~ ~ ~ .~~ ~ " memhrnnrs readily so lhal pho.;phorylatiun tend* to prevent Imr of the mtermed~.ates due to diffusion out ul the rcll or into the mltw ehondria. In this regard, it should be noted that it isanonphosphor- ylated intermediate, pyruvate, that moves into the mitochondria to he oxidized to COz in aerobic degradation. (b) Intermediates in this pathway are also involved in other pathways, such as the production of five-carbon sugars and of the hemoglobin regulator, 2,3-diphos- ohorelvcerate. Both of these oathwavs ultimatelv need ohosohnrvi- . -. . , , ated intermediates. Without phoiphorylntion. the five-carhon wyars would lurm the more stahle six-membered n n p rather than the live-membered ring* nereirary fur RS.1 and DNA structure. The phouphurylation prevents the imnatim of ria-memhered mas smee i t derivatimi the hydroxyl needed t u form rhr six-memhwed rings. (rl M'irhuut the firit two ~hoivhorvlntions. no rarlv stw i n the . . pathway would have a negative-standard free energy change. Only such stem can he used as reeulatorv steos effieientlv: the" assure that , . ,. , artiwtionol the regulated rnryme results in the reactL.ngmng in the forward dirrr~ion. td, The nrgat:ve free enerzy chance fur the first strprndhleb the cell toc<,nccntrateglucc,se if th~~r~rts~deconcentm- tion is low.

Volume 59 Number 4 April 1982 319