edited by: DAVIDA. PHILLIPS Wabash College Crawfwdsviile. IN 47933
PRUDENCE PHILLIPS Crawford~villeHigh Schwl Crawfordsvilie. IN 47933
Introductionto "Goals" In planning a chemistry wurse. some of the most important decisions a teacher must make involve the selenian of material to be covered and time to be devoted to each topic. For each column in this new series, a high schwl and a college teacher have been invited to discuss why they feel a palticular topic is important and haw it connibUtes to the students' understanding of chemistry.
me
Abouf the Edifors
...
David A. Phillios received his BS in chemistry from the University of Redlands and his PhO fromihe University of Washington. AnerdirectingWGenerai Chemistry program at Middle East Technical University in Ankara. Turkey for two years, he came to Wabash College, where his primary teaching interests are in general and inorganic chemistry. Prudence K. Phillips received her AB in ehemisby from Brown University and her MS from the University of Washington. She has been teaching high school chemistry for 11 years and is presently at Crawfordsvilie High S~hool. Both the Phillipses have participatedin a variety of Division of Chemical Educationand ACS activities.
Why Teach Kinetics to High - School Students?
Kinetics-Rates and Mechanisms
High school srudmts oftrn nvuid chemi,rrv Ix!(:nuie it demand., mnthrmatit-al reasoning. I:ur that reason, high srhail tt:a(:hers generally deemphitrize math in first-year courses except where it is necessary for understanding. Nevertheless. I stress kinrrici., a tooic . that demands nt least a bit of math. Consequently, I must justify annually the time spent on kinetics a t the exnense of other tooics in the curriculum. We chemists-do a great deal bf theoretical hand-waving, and, after a while, chemical theory seems like fantasyland t o most high school students. I stress kinetics because collision theory seems fairly easy for high school students to understand. In my course, they understand kinetic-molecular theory pretty well because it has been supported clearly by student experiments and instructor demonstrations. we have also thoroughly covered the behavior of solid spheres prior to the introduction of kinetics, which gives them all they need for a nrimitive understanding of the tooic ' ~ r o mtheir knowledge theory, ihe students practice scientific reasoning bv pndicting the efferts of concentration and temperature on reaction rate. They then have the exciting exprrience of confirniing their predictions in lah. With this information and their prior grounding in theory, they (an undersmnd reaction mwhanisms, rate laws, activation rnergy and catalysts. They have gone into the lab to answer real questions rather than to demonstrate some principle explained in advance. Is the acid a catalyst? If so, an extrapolation of the rate versus concentration graph to [H+] = 0 will show a finite reaction rate. Does the fast s t e in ~ a reaction involve one molecule or two? Altering concentrations of reacting species will clearly indicate which is the case. The lab work also helps the students to develop a host of important skills. Careful measurements of volumes, times, and
Probably Yor; of chemistry teaching denls with chemical equarions using an nm)w (81 equal sip11to connect the tormulas of products and reactants. \Vithout such highlv infcrrmative (at least to some) tquarions, trentmetits ol stoichiometry. equilibrium, thermodynamics, synthesis, and analysis would be quite impossihlr. However, there is one subject in which thr muation ior the net reaction hasonlv a limited use. Thar subject is kinetics and its main sub-areas-rates and mechanisms of reaction. Using only the equation for the net reaction and the formulas i t contains, there is no way of confidently predicting either a reaction rate or a reaction mechanism. Even the effect of temperature on the rate cannot be told with certainty. Intelligent control of a reaction requires knowledge of the variables which affect its rate. A knowledge of reaction mechanisms is in turn essential to the understanding of reactions at the atomic-molecular level. Most introductory teaching of kinetics starts (and often stops) with a determination of the factors affecting the rate. Solet's start there (hut also let'snot stop a t the standard places). Rates are most commonly discussed in terms of three variables: the nature of the system, its temperature, and the concentrations of the reactants. Even changes resulting from stirrine. -. altering surface areas. or addine" catalvsts can be handled in terms of variations in systems, temperatures, and concentrations. Reasons cited for studvine . " these effects inelude providing a deeper understanding of the nature of the reactions and an enhanced ability to control their rates. Yet few standard treatments really do either successfully.
(Continued in col. 1 , page 4 1 )
(Continued in col. 2, page 4 1 )
of
40
Journal of Chemical Education
-
Concentration Effects Almost all chemical systems studied are found t o involve first-and second-order reactants, most often with orders equal
temperatures must be made. Correct calculations of concentrations, logs, etc. must be made and the results accurately plotted. This plethora of calculations seems much more palatable to mv students when thev are usine calculations to help solvea realiab prohlem, than when they ;re just manipulatini math formulas. When the data from different labrroups from all classes are pooled, the necessity for replication is dhvious; although individual runs often don't work well, the combined data always yield clear relationships. The labs help the students to discover for themselves the idea that a reaition can have several plausible mechanisms. Refure they do the experiments, I discuss ntlcleophilic suhstitution reactions and physically model first- and secondorder mechanisms, being careful not to tip off the results to them. Consequently, the students develop a feelingfor the role of physical models in predicting reactions. Once a mechanism has been determined in lab. I introduce several reactions for which the mechanisms are'unknown. I stress that this is a maior field of chemistrv in which the students might make codtributions. In fact, many of them end un doine research nroiects in kinetics. 'I alsokress kineiicsbecause I've found clear understanding of the effects of concentration on reaction rates helm . mv" students understand dynamic equilibrium. T o summarize. mv course includes an emphasis on kinetics because: (1)the the& is qualitatively simpie; (2) students are able to practice their scientific reasoning in a meanineful wav: (3) the kxperiments make palatable the repetitiouspracti& of important lab, calculation, and graphing skills; (4) students develop a feel for the importance of molecular structure and action a t the molecular level; (5) I can introduce the idea that there are still questions in chemistry which the students may be able to answer; and (6)the ideas developed in studying kinetics are important for understanding equilibrium. Finally, I stress kinetics because the students and I enjoy clock reactions immensely. Teaching kinetics gives me lots of excuses for demonstrating these reactions while I lecture. William G. Lamb Oregon Episcopal School 6300 SW Nical Road Portland, OR 97223
About the Authors J. Mhur Campbell is Prafessw of C h e m i w at Harvey Mdd Callege. where he teaches freshman and physical chemistry. He has an A6 in Chemistry from Oberiin College, an M S from Purdue University. and a PhD from the University of California. Berkeley. He has had a long and illuJbim w e e r teaching and consuiiing at instiions m n d We wald. He is a past chairman of the Division of Chemical Education and hss sewed on many national and intwnationai committees.He has numemus honors including the Manufacturing Chemists' Award farthe Teaching of Chemistry.
Wllliam G. Lamb holds the Winningstad Chair of Physical Sciences at the Oregon Episcopal Schwl, where he teaches chemistry and ~hvsics . . and Serves as demnrnem head. He earned his MA and PhD in science edmtion ham We -nrvem ty 01 Texas at Aust n and part cipaled in the forstDreyfuslnstitbta on rl gh Schwl Chemtslry. Hss recent activnies include dirminga state science fair, development of a descriptive physics course which will serve as a prerequisite fa chemistly,and the initiation of a high school student research program in chemistry (for example, same of his students are doing proiects on semiconductor photochemistry).
to the coefficients in the net equation. No matter how many admonitions (even in bold face) there mav be aeainst deducine orders from coefficients, the ktudents note &e correlations and remember them. Why not use examples where this apparent, but untrue, correlation does not exist? Few treatments discuss the im~ortanceof zero-order r most physireactants, which commonly occur. ~ b example, ological reactions are zero-order with respect to the substrate, e.g., glucose. Consider the problems humans would have with their eating rates if this were not true. Anv variation in local glucose concentration would cause a variation in physiological rates. Many living system reactions circumvent this problem by operating with a simple first-order kinetics involving an enzyme concentration which they can control to give an appropriate rate of reaction. Similarly, many environmental problems have rates which are zero-order in certain possible reactants. Some bodies of water have deal erowths which are first-order in nitrate, zero-order in ph&phYate. Other bodies have the orders reversed. Control is nossible hv reeulatine iust one of the concentrations, but first it must be determined which concentration affects the rate. Seattle had a problem with this some years ago and initially tried controlling the zero-order reactant. Unsuccessful. One of the most useful definitions of a catalyst (by R. Bell) is a reactant whose concentration amears in the rate eauation .. with nn exponent higher than its coefficient in the chemical equation. This is a hichlv definition which eives - -operational . insight into why a catalyst affects a rate. A most important, hut seldom developed, reason for studying concentration effects is that the order of a reaction often gives direct insiaht into the formula of the activated complex formed in the rate-determining step. I know of no more powerful method for gaining perspective on how molecules actually interact than writing probable structural formulai of the activated complexes. Yet few treatments provide guidance here. The formula of the activated complex is usually just the sum of the formulas of the suhstances in the rate equation. If the rate equation is:
.
rate
-
U "
= k[XIm[Y]"
then the formula of the activated complex is X,Y,. I t is often possible to write a reasonable structural formula for the complex and thus predict which molecules collide in the rate determining step, what bonds form, what bonds break, and what products result. This is a great deal of information and it provides excellent instruction as to what is actually concealed in that eaual sien or arrow in the eauation for the net reaction. The fo;mula'of the activated coiplex also gives insight into catalytic behavior. Temperature Effects Almost every treatment presents the Arrhenius equation and discusses activation energies, but there are often three serious deficiencies, all easy to remedy with a little additional theorv. The most obvious is the failure to mention the minimum; but appreciable, effect due to the increased rate of molecular collisions with temperature, which the student learned about earlier in the cou;se. 'hue, this is a small effect compared to the exponential Arrhenius effect, hut a clear discussion of their relative sizes clarifies the issue. More serious is the error of telling the student that a catalyst operates by lowering the activation energy. This is just not true. A catalyst provides a new mechanism. The old mechanism continues with its activation energy unchanged. There must be an increase in rate simply because there are now a t least two mechanisms operating, not just the one of the original reaction. The most fascinating problem here is that, for many systems (especially those in&&ing enzymes), the catalyzed reaction may actually have a higher activation energy (Continued in col. I , page 42) Volume 61
Number 1 January 1984
41
than the uncatalyzed one, yet its rate may he thousands of times faster at comparable concentrations of substrate. It seems a shame to include such an important point so late in an essay, hut better late than never. Of course, this problem of an increased rate despite the greater activation energy is solved by considering the free energy of activation, a decrease in which does increase the rate. Thus we make a marvelous tie between thermodynamics and kinetics and a most useful extension of the entropy principle to reaction mechanisms. If the activated state is less organized than reactants and products, the entropy of activation is high. This lowers the free energy of activation, even though the energy of activation may be larger. AG.
= AE. - TAS.
--
with AE, AHH, for most systems. Thus, the rate is increased. I t is interesting that some organic texts are ahead of many physical chemistry texts in this area.
equal sign or arrow of the net equation conceals much of the most interesting part of chemistry, (2) control of a reacting system necessitates determination of reaction orders, (3) knowledge of reaction orders allows a good guess at the formula of the activated complex, (4) knowledge of this formula reveals a great deal about the rate determining step and the role of any catalyst involved, and (5) study of the temperature effect may be coupled with entropy of activation effects to provide deep insight into the energetics of reactions and a correlation of thermodynamic ideas with kinetics. Dorsn't it seem short-sighted to stop with simple rate equations and leave mechanisms as dark secrets? A knowledne of the factors affecting rates (as in the Arrhenius race equation) allows engineering control of a chemical system. Knowledne of mechanisms (such as formulas of activated complexes and free energies of activation) allows insights into the chemistrv at the molecular level. Shouldn't chemistry students aehieie such insights?
Why Teach Kinetics?
There are so many reasons for teaching kinetics early and often. If it is done iust a bit more thorouehlv than is now common, the students (with little extraeffo;) see that (1)the
42
Journal of Chemical Education
J. Arthur Campbell Harvey Mudd College Claremont,CA 91711
