AP2 EXPERIMENT on CHEMICAL EQUILIBRIUM-for BEGINNERS LAWRENCE P. EBLIN The Ohio State University, Columbus, Ohio
Complete directions are g i m for an experiment on chemical equilibrium to be performed by students of general chemistry. A n introduction includes a statement of why such an experiment has been designed, and a summary of the purposes which it serves. The experiment has been used for two years at Ohio State i n classes which have had high-school chemistry.
A
LL TEACHERS of chemistry will probably agree that a true appreciation of the nature of chemical equilibrium is necessary for the understanding of chemical phenomena. However, the begin-
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ning student is often expected to gain this appreciation from a mere study of those portions of his text in which equilibrium is discussed. There seems to be a sad dearth of laboratory experiments designed for the express purpose of providing the student with a concrete basis on which to construct his concept of chemical equilibrium. Experience with freshmen a t this university has resulted in the belief that the exercise on equilibrium in the laboratory manual of McPherson, Henderson, and Evans' could be expanded and revised to meet the purposes which have evolved from our MCPRER~ON, HENDERSON, m o EVANS, "Laboratory manual
to accompany the fourth edition of 'A course in general chemistry,' " Ginn 81 Company, Boston, 1934, p. 87.
teaching. Two years ago such a revision was undertaken, and experience in teaching the experiment since that time has justified expectations. The purposes determining the organization of the experiment may be summarized as follows: (1) To make clear the distinction between reversible and irreversible reactions. (2) To instil an understanding of the nature of chemical equilibrium and the part that it plays in reversible reactions. (3) To show that hydrolysis is to be understood as one type of equilibrium. (4) To keep manipulative details on a simple basis. (5) To encourage extensive use of the chemistry text at the same time that the experiment is being performed. In the organization of the experiment, valuable assistance and advice have been given by fellow laboratoty instructors. Appreciation is espeaally due to Mr. Sherman E. Smith and Dr. W. C. Fernelius. The directions are furnished to the student in the following form. liQUlLIBRIUM AND HYDROLYSIS
The report for this experiment shall consist only of the answers t o the twenty-five numbered questions. These answers are to be in the form of complete sentences which are as brief as is con? , sistent with comoleteness. I. A n ~rreversibleReaclion. To 5 cc. of silver nitrate solution add a few drops of dilute hydrochloric acid. The precipitate formed is silver chloride. (1) Write the equation. (2) Define "precipitate." Allow the solid t o settle, then decant the supernatant liquid. Now add 5 cc. of dilute nitric acid to the precipitate. Is silver chloride soluble or insoluble in nitric acid? (3) What condition permitted the first reaction to go to completion? 11. .I h e r r i b l c Rmrlioa. Set up an upwaratus for preparing carbon dioxide, consisting of a test-tube generalor, a thistlc-tuk. and 3 delivery tube. l'lace 5 cc of lime-water in another resttube. Insert the delivery tube from the generator to a point about 1 cm. from the surface of the linle-water. Place a small piece of marble in the generator and cover t h water. Add dilute hydrochloric acid drop by drop throu$ the thistle-tube, and as the action proceeds in the generator, gently shake the test-tube containing the limewater. (4) Write the equation for the reaction in the generator and explain why the reaction goes t o completion. (5) What forms in the test-tube containing the lime-water? (6) Write the equation for the reaction leading t o its formation. (7) Wby does this reaction go t o completion? Continue t o generate carbon dioxide and to shake the testtube until a further change occurs. (8) Write the equation for this further reaction (consult text). Now warm the test-tube over a Bmsen %me. (9) Write the equation far what you now observe and state two conditions which permit the reaction t o go t o completion. 111. Neulralieetion. In your high-school chemistry course, you have learned that acids change (red, blue) litmus t o (red, blue) and that bases change (red, blue) litmus t o (red, blue). ( ~ n h c o r the e correct words.) (10) What name is given to the type of substances represented by litmus? You have also learned that if an acid solution is carefully added t o a solution of a base, a point is reached a t which neither red nor blue litmus changes color. (11) What is this paint called? What is the name given t o this type of reaction? (12) What condition permits this type of reaction t o go t o completion? IV. Eguili6rilmr. Obtain about 5 cc. each of solutions of
ammonium thiocyanate and of ferric chloride. What is the color of each of these solutions? Pour one cc. of each solution into a beaker containing 50 cc. of water. What do you observe? The equation for the reaction is: 3NHGNS
+ FeClr
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Fe(CNS)s
+ 3NH'Cl
What is the color of a solution of ammonium chloride? (Prepare a small portion of such a solution if necessary t o answer this question.) (13) Decide which compound is responsible for the color when ferric chloride and ammonium thiocyanate solutions are mixed, and justify your answer. Half fill four test-tubes with the colored solution obtained above. Retain one tube for comparison. To a second tube add one or two cc. of the ammonium thiocyanate solution. What is the effect upon the intensity of the color? To a third tube add a small portion of the ferric chloride solution, drop by drop. What effect does this have? (14) What do you conclude as t o the effect upon a reaction, of increasing the concentration of one of the reactants? (15) State the Law of Mass Action and aoolv it to this oarticular case. To iil
