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IONIC REACTIONS IN SLOW MOTION
Submitted by: Carl W. Scharf, Arlington State College, Arlington, Texas Checked by: Wayne Taylor, Michigan State IJniversity PREPARATION
REMARKS
Provide an overhead projector with a horizontal stage, 4-in. petri dishes, stirring rod, solutions of lead nitrate, potassium chromate, and sodium carbonate, and dilute sulfuric acid, With a medicine dropper place solutions as indicated in Figure 1 in the petri d16h. Merge the two solutions with the water pool by stirring rod or by adding one or two extra drops of solution near the junctions. Use caution to avoid mechanical mixing,
By diffusion the ions will slowly approach each othcr and in a short time a faint dark haze will appear on the screen and will gradually become darker and spread into various patterns as precipitate continues to form (Fig. 2 ) . By using sulfuric acid and sodium carbonate solutions as reagents, an interesting variation is accomplished which results in bubbles of carbon dioxide being evolved along the moving bonndaries (Fig. 3).
Figure
I
Figure 2
Figure 3
EQUILIBRIUM PREPARATION
Provide two glass crystallizing dishes 8 in. in diameter and 4 in, deep, one 3-in casserole, one 3Ij2-in.casserole, one 4-in. casserole, and one 1-in. casserole, and enough in. of nater to fill one crystallizing dish to within the top. Red ink or dye may be added to make the nater level more visible. DEMONSTRATION
One student is asked to take a position behind each of the crystallizing dishes xvhich are side by side on the lecture table, Th? student behind the d ~ s hfilled with \later is givcn the 3-in casserole and thr other the 3'/%-in casserole. Each 15 asked to dip from his container and pour the nater in the opposite container in turn. Students are asked to obserw
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A. The change in the amount of liquid that moves B.
from left to right and right to left as the reaction proceeds. The change In the I e d s of lquid in the two dishes as the reaction proceeds.
C. The attainment of equilibrium, that is, when liquid is moving in each direction a t an essentially constant rate TI After equilibrium has been attained, the reaction continues in both directions but with no change in rates and no change in the amounts of liuuid in the two dishes E. That time is required for the attainment of equilibrium. F. That a t equilibrium the rates of the two reactions are the same but the amounts of the liquid in the t ~ dishes o are not necessarily the same. After thc above observations have been made, the 4-in casserole is substituted for the 3-in. casserole and a new set of conditions established. Finally, the 1-in. casserolc is substituted for the 3l/A1. casserole. REMARKS This analogy to a chemical equihbrium can be very
effective in bringing the significant factors to the attention of students. For a more extended discussion see the reference given above.
