Heat of precipitation - Journal of Chemical Education (ACS Publications)

Heat of precipitation. H. L. Clever. J. Chem. Educ. , 1963, 40 (5), p A386. DOI: 10.1021/ed040pA386.2. Publication Date: May 1963 ...
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CHEMICAL PROJECTS Research Ideas for Young Chemists JAY A. YOUNG, King's College, Wilker-Borre, Penno. JOHN K. TAYLOR, Notional Bureau of Standards, Worhington, D. C.

An Easily Prepared Wide Range Buffer Series

See ~ A R U O Dw. Y , R., J . c h ~ mEduc., . 38, 559-60 (1961).

are required. Questions:

Professor Carmody indicates that this series of buffer solutions would be useful in several different kinds of demonstrations and experiments, though he does not mention m y speci6e examples. Can you identify one or more demonstrations or experiments for which this series of buffer solutionswould beuseful? The buffer capacity of the series of buffer solutions was measured against hydrochloric acid. Determine the buffer capacity

By including a fourth solute, s weak acid with a pIi of about 10.5, can you prepare a similar series of buffer solutions with higher buffer capacity in the pH range from a l ~ o u9.5 t t o 11.51 Would i t be possible to prepare a series of buffer solutions, similar t o those described in the article, using weak bases instead of weak acids?

Heat of Precipitation SPPCLEVER,H. L., J . Chem. Educ.. 38, 470-1 (1961). Heats of precipitation of insoluble salts can be readily determined using a simple calorimeter constructed from s. amall beaker, a n Erlenmeyer flask, some insulation, a rubber stopper, and a thermometer calibrated in O.l°C intervals. The results are precise t o within +5% when care isexercised.

Can you perform the experiments indicated above, but using a better cslorimeter than the one described in the article? I n such work, is i t still reasonable t o make the same assumptions concerning the densities and specific heate of the solutions, or should these be mare precisely evaluated?


Heats of solution are identical, except for algebraic sign, with heats of precipitation. A procedure for the determination of the heat of solution of sulfuric arid is described h,v I3. W O L T H ~ S , ET AL., J. CHEM.EDUC.,38, 4 7 2 3 (1961). With suitable modifications, this procedure can be used t o determine the he& of solution of other solutes. Select. a moderately soluble salt and determine both its heat of precipitation and its heat of solution by direct experimental measurements. Are your results cornparable, except for algebraic sign? If not, what modi6eations must be made in your procedure so that the results do agree?

Using the prooedure outlined in the article, and the same assumptions concerning the densities and specific heat capacities of the solutions, determine the heats of precipitation of several slightly soluble substances. Is there any relation between the results you have obtained and the eleetranegativities of the ionic species involved? Or, is there some other similar relationship which you can elicit from a tabulation of your results? Account far therelationship you have discovered and test your explanation by further laborat,ory work.

A Simple Demonstration of Some Precipitation and Solubility Effects See MATIJEVIC,E., el al., J. Chem. Edue., 38, 397-9 (1961). This article could be subtitled. "When Solubilitv Product is Not Constant." I t is a thorough, and dear, disc&sion of the phenomena which are observed when excessive amounts of silver nitrate are added t o solutions of sodium (or potassium) bromide and when excessive bromide are added t o solutions of silver nitrate. Due t o the formation of complex ions, d e a r solutions, turbid suspensions of silver bromide, and almost clear sols, are formed under certain conditions. These observed facts can be classified and clarified by a graphical tabulation which the authors call a. solubility diagram. I t would be interesting and instructive t o construct solubility insoluble instead f, ailver bradiagrams for mide. s u c h an investigation is strongly recommended t o the interested student.



Journal o f Chemical Education


Select any other insoluble salt which can be formed by mixing two ~olutionsand from the data you obtain when varying the amounts and ooncentrations of the reagents, construct a. diagram similar t o the upper portion of the figure in the article. If the necessary optical apparatus is available, also construct several diagrams similar t o the lower portion ,f ~i~~~~ 1, or three dimensional representation of your results. Identify each different area in your diagrams, and account for the formation of precipitate, either as a turbid suspension or almost clear 801, and account for the presence of true solutions by further laboratory studies of the systems which exhibit these phenomena.