Use of Concentration-Time Curves in the Teaching of Chemical Equilibrium V. R. D4MERELL Western Reserve University, Cleveland, Ohio
I Ta
T,
Time
Time
FIGURE '
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ONCENTRATION-time curves are not generally used as teaching aids in the explanation of chemical equilibrium a t the general chemistry level, to the writer's best knowledge. The writer uses such curves in lectures and classroom discussions, and has found them to be of great help in clearing up student difficulties. Using a general equation such as A
+ B-+
C
+ heat
consider the effect of increasing the concentration of
Time
TI RGrmE
Tz 2
reactant A . This can he shown with a curve of the type shown in Figure 1, using any values for A , B, and C. At time TI more A is introduced, leading to the increase in concentration shown. From time T I to Tz the system is momentarily not a t equilibrium, then a t time T2 equilibrium is reCstablished, as shown by the concentration lines' again becoming parallel with the X axis. Such curves are of advantage in a number of ways. They can very quickly he put on the blackboard, since ordinarily only a qualitative effect is being described. They help the student to overcome the common error of thinking that the amounts of substances present a t equilibrium are the same as those shown by a balanced equation. They show that the reestablishment of equilibrium is not instantaneous, and they show in a graphical way how the application of a stress is followed by the partial removal of the stress, as predicted by Le Chatelier's principle. Figure 2 shows how a change in temperature influences an equilibrium; where for the above reaction the temperature is decreased a t time T I , and equilibrium is reestablished a t Tza t the lower temperature. Figure 3 illustrates an ionic equilibrium, where the salt of a strong base and a weak acid is introduced a t time TI and equilibrium is reestablished a t time T2.
