edited by GEORGE L. GILBERT Denison University Granville, Ohio 43023
Buifer Demonstration SUBMITTED BY
Charles E. Ophardt Elmhurst college Elmhurst, IL 60126 CHECKED BY
Paul T. Ruda Cleveland Hlll U.F.S.D. at Cheektowaga Chaektowaga, NY 14225
Preparation Into separate 100-ml beakers, put 75 ml of 0.01 M HC1 and 0.01 M HC2H302(HAc). Into separate 50-ml beakers, put 1.5 g of NaCl and NaC2H302 (NaAc). Put several grams of NaAc into a separate beaker. A dropping bottle of 6 M HCI is also required. Prepare a mixed indicator by mixing equal amounts of metacresol purple or thymol blue and bromcresol green. The mixed indicator produces the following colors a t the approximate pH's indicated as measured with a pH meter: Mired Indicator Colors
As a result, H+ ion concentration decreases and pH increases. This same equilibrium shift does not take place with C1- and H+ ions since H-CI bonds are very weak and thus HCI, a strong acid, remains almost totally ionized. Testing the Buffer Capacity Add 5 drops of 6 M HCI to the beakers in the following order: HCI alone, HAc alone, HCI-NaC1 mixture, and finally the HAc-NaAc mixture. In the first three beakers, the color changes reflect the "unbuffered" increase in hydrogen ion concentration and a subsequent decrease in pH. An additional 10-15 drops of HCI may be added to the HAc-NaAc solution, which shows that i t is a "true buffer" solution which does not significantly change pH upon the addition of small amounts of acids. Explain that the pH does not change because "new" H+ ions react with excess acetate ions from the salt to form acetic acid molecules. HCI -new H+ + C1HAc molecules
new H+ + excess Ac-
-
YeliowYellow Green Green Blue Approx. pH 1.5 1.8 2.3 4.6 5.2 7.6 The demonstration may be scaled up by using multiples of the amounts given. An overhead projector could also be used to enhance visibility for large groups.
After 20-25 drops of HCI have been added to the buffer, the color does begin to change. As an added bonus, ask the students what could be added to change the indicator color back to green. If they understand the equilibrium principles, they will reply that more NaAc is needed. If you have extra NaAc, yon should add this to the solution to confirm the prediction.
Demonstrations and Remarks In all three demonstrations emphasize the difference in properties between strong and weak acids and that only a weak acid or base and its salt is a buffer.
Phase Transfer Catalysis and Carbanion Formation
Red
Orange
Strong and Weak Acids Add 2 ml of mixed indicator to the two beakers containing the HCI and HAc. Note the resulting different colors and thus the difference in pH although the acids had the same starting concentration. Based npon the pH difference explain the difference between the ionization of HC1, a strong acid, and HAc, a weak acid. (virtually 100%ionized) HCI = H+ + CIHAc* H+ + Ac(only 3-5% ionized) Since the pH of HAc is greater than that of HCI, the implication is that the concentration of free H+ ions is less in a HAc solution because the H-Ac bond is stronger than that in HCI. Preparation of the Buffer Pour one-half of each acid solution into the appropriate beaker containing its salt (HCI with NaCl and HAc with NaAc). Note the dramatic change in the mixed indicator color of the HAc-NaAc solution. Explain this change in pH (or lack of change) based npon equilibrium principles and the type of weak or strong acid present. The NaAc produces excess Aions which form strong bonds with H+ ions and shift the HAc equilibrium to the left to form the weak acid (HAc). HAc
608
% H+
+ Ae-
Journal of Chemical Education
SUBMITTEDBY
John W. Hlll Lori J. Enloe Unlverslly of Wisconsin Rlver Falls, WI 54022
CHECKEDBY
Richard R. Doyle Dsnlwn Unlrsrslfy Granville, OH
By employing phase transfer catalysis, carbanions can be made from weak acids with pK, values up to about 23. By proper selection of the acidic compounds, we can produce highly colored carbanions, some of which are stable for weeks under ambient conditions. We have devised lecture demonstrations that show the formation of such carbanions, the action of phase transfer catalysts, and the necessity of water for this catalysis. Previous methods for demonstration of carhanions involved the use of potassium amide in liquid ammonia (1) and lithium aluminum hydride in pyridine or tetrahydrofuran (2). By employing phase transfer catalysts, we are able to obtain colored carbanions using an aqueous solution of sodium hydroxide or the solid base with only a drop of water. We employed two acidic hydrocarbons, indene (pK, = 19.9) and flnorene (pK. = 22.6) (3) and a series of para-substituted phenylacetonitriles, for which no pK, values could be found in the literature. For the base, we used 10% 25% and 50% aqueous sodium hydroxide and solid NaOH with one drop of added water.
