GEORGEL. GILBERT Denison University Granville. Ohio 43023
Another Look at a Mechanical Model of Chemical Equilibrium SUBMlTTED BY
A Visual Demonstration of Raoult's Law SUBMITTED BY
Archie S. Wilson
Wllllam Laurna
UnlversHy ol MlnnssDta Mlnneapolls, MN 55455
~utztownUnlvsrslty Kutrtown, PA 19530
CHECKED BY
CHECKED BY
James W. Long
Leonard C. Gmtz UW Center-Waukerha Waukesha, W153186
An article in this Journal' describes a mechanical model of chemical equilibrium in which water is transferred from container A to container B with a small beaker while a t the same tune water is transferred from container B ro container A with a large beaker.'l'he water level in container H reaches a level be& that of container A. The levels remain in dynamic equilibrium as the water transfer continues. A more exact analogy can be made with a model in which water is transferred between graduated cylinders by means of glass tubing. Equipment needed: two 500-mL graduated cylinders, glass tubing (about 45 cm in length) of various outside diameters up to about 10 mm. The tubing should be fire polished until a slight lip is formed from the natural contraction of the glass. About 200 mL of colored water is placed in each graduated cylinder. Two glass tubes of quite different diameters are used to transfer water simultaneously between the graduated cylinders. Tube A, which initially rests within and on the base of cylinder A, is used to transfer water from cylinder A to cylinder B by closing off the upper portion of the tube with the index fineer and carefullv maneuvering the column of w a t e r i n t ~ c ~ f n dB. e rAr the same time, t u b e n in cylinderR is used to transfer water from cylinder B to cylinder A. Equilibrium is reached when the volume of water (VA) transferred from cylinder A to cylinder B is the same as the volume of water (VB)transferred from cylinder B to cylinder A. At this point the height of water ( h d standing in cylinder A and the height of water ( h ~ standing ) in cylinder B will remain unchanged during the transfer of water. At equilibrium: ~
~
~
where r is the inside diameter of the glass tubing. In this model, h is analogous to a concentration, and r2 is analogous to a rate constant. The effect of the rate constants on the position of equilibrium may be explored by changing (or exchanging) the tubes. This demonstration may clank and splatter if done too enthusiastically. A white lab coat serves as a convenient backdrop to improve visibility and may save the clothes of the overly enthusiastic demonstrator. The demonstration cannot be scaled up and is most easily observed when shown to relatively small groups of students either in a small classroom or in a prelaboratory setting.
' Sorum, C . H. J. Chem. Educ. 1948,25,489. 598
Journal of Chemical Education
Uncer~lly01 Oregon Eugene. OR 97403
Into each of two test tubes, large enough to be seen by the class, add a few drops of bromine, and stopper the tubes. This operation can be done before the class period in a fume hood. Enough drops of bromine should have been added so that after evaporation some liquid remains. After equilibrium is attained, to one test tube add a few drops of chloroform; restopper the test tube, and mix the contents to regain equilibrium. Since the vapor pressure of bromine in the solution is decreased according to Raoult's law, the reddish brown color of the vapor phase will have diminished.
Reduction of Permanganate: A Kinetics Demonstrationfor General Chemistry SueMlmD sv
Margaret J. Steffel The Oh10 State Unlverslty Marlon Campus 1465 MI. Vernon Ave.
CHECKED BY
George Volpe and J. T. Maloy Seton Hall Unlverelty South Orange. NJ 07079
The reduction of MnOa- to MnZ+in aaueous solution can be used to demonstrate the four factors that control reaction rates in solution: the natures of the reactants, concentrations of reactants, temperature, and presence of acatalyst. I t also provides an example for discussion of reaction intermediates and reaction mechanism. The Natures of the Reactants Two reactions using different reducing agents to reduce permanganate are contrasted:
T o show the first reaction, measure 100 mL of 0.0050 M KMn04 into a beaker. Stir in 100 mL of 0.50 M FeS04 in 1.0 M HZSO,. Reaction is immediate, as shown by the disappearance of the purple color of Mn04- and the appearance of thevellow color of Fe3+:since excess F ~ S Ois Aused. the color will be greenish yellow. After combining the solurions, write the colors of'reacrants (ourole MnO;. ereen Fe2-.colorless H30t) and products (&tu& colorle&"~n~+,yeliow Fe3+) below the eouation on the blackboard. T o show the second reaction, measure 100 mL of 0.0050 M KMn04 into a beaker. Stir in 100 mL of 0.50 M C2H204 (oxalic acid) in 1.0 M HBO4. There is no immediate evi-
