JCE Online Journal of Chemical Education
 | Subscriptions  | Software Orders  | Support  | Contributors  | Advertisers  | 



  Home > JCE Print > Journal of Chemical Education > Issues > 2000  > November  >
Chemical Education Today
Letters
Equilibrium: A Teaching/Learning Activity
Todd P. Silverstein
Department of Chemistry, Willamette University, Salem, OR 97301-3922

Cover
November 2000
Vol. 77 No. 11
p. 1410

Full Text
Back in September of 1998, Wilson contributed an excellent exercise on equilibrium (J. Chem. Educ. 1998, 75, 1176); comments by Sadavoy and by Paiva and Gil (J. Chem. Educ. 1999, 76, 900) further refined the exercise. I would like to add two points to further improve this excellent classroom activity. Most importantly, readers should note that Wilson's equation for Keq in the instructions for "Activity #1" includes a typo in section 6; the equation should read "number of items P/number of items R", and not vice versa.

Second, Paiva and Gil suggested that rounding errors in Keq can be avoided by giving students 10 boxes of 40 matches, instead of just 40 matches. A much simpler solution to the problem of round-off errors would be to simply start with the "correct" number of matches. For example, as stated by Wilson, if the R-to-P transfer fraction (kfwd) is 3/4 and the P-to-R transfer fraction (krev) is 1/8, then Keq should be 6. If the total number of matches between the two teams is 40, then solving for (40 - x)/x = 6, at equilibrium R (= x) must have 5.7 matches and P, 34.3. Clearly, without breaking matches into pieces, an accurate value for Keq is not possible when starting with 40 matches. Starting with 42 matches, however (x= 6), will yield an accurate value of Keq = 36/6 = 6.

The trick is to choose the total number of matches (#mtot) such that each team, R and P, ends with an integral number of matches at equilibrium and the quotient P/R yields an accurate value for Keq. Solving the algebraic expression for the equilibrium constant, (#mtot - x)/x = Keq, we get #mtot = x(1+Keq). In this expression, x is the number of matches held by team R at equilibrium. If x is chosen so that the product x(1+Keq) = #mtot is an integer, then round-off errors will cancel out, and an accurate value of Keq will be obtained from the exercise.

For example, for Wilson's initial transfer rates of kfwd = 1/2 and krev = 1/4, Keq = 2; one would have to start with either 39 or 42 matches (x = 13 or 14, respectively) in order to end with an accurate value of Keq. Similarly, for transfer rates of kfwd = 0.88 and krev = 0.11, Keq = 8.0; starting with either 36 or 45 matches (x = 4 or 5) will yield an accurate value of Keq. Starting with 40 matches would yield Keq = P/R = 35/5 = 7 instead of 8. Hence using the algebraic expression #mtot = x(1 + Keq) allows one to judiciously choose the initial number of matches so that for any values of kfwd and krev, the equilibrium situation will yield an accurate value of Keq.

More Information
*  Citation
Silverstein, Todd P. J. Chem. Educ. 2000 77 1410.
*  Keywords
Demonstrations; Equilibrium; Introductory / High School Chemistry; Teaching / Learning Aids
*  History
Created:
Last Updated:
October 6, 2000
April 15, 2005
  Home > JCE Print > Journal of Chemical Education > Issues > 2000  > November  > Page 1410



Chemistry Teacher Connection

The "Chemistry Teacher Connection" (CTC) is especially for high school chemistry teachers. For only $40/year, it offers an online-only subscription to CLIC along with membership in the Division of Chemical Education, normally $65/year. CTC subscribers receive access to all articles and supplements from 1996 through the current issue.


C&EN CLICs

Through special arrangement with the ACS, JCE High School CLIC is now able to provide subscribers with online access to Chemical & Engineering News articles that have been selected specifically for secondary science instructors and their students. 


JCE Collections Available
Occasionally, collections of JCE back issues become available for donation to individual teachers, schools, or libraries. JCE matches collections with interested recipients. Recipients pay shipping costs or pick up the collection.

Contributions Welcome
JCE welcomes your submission

Subscriptions

Fishing for New Ideas
Always in the
process of
improving, CLIC
welcomes ideas and comments.

Email Us

NSF logoDivCHEDACS ACS PubsFor journals in other fields of chemistry visit ACS Publications.