David Dingledy
and Walther M. Barnard State University College, Fredonia, New York
I I
The Stoichiometry of Copper Sulfide Formed in an Introductory Laboratory Exercise
The preparation of copper sulfide is used as an introductory chemistry laboratory exercise to demonstrate the law of constant proportions. I n this exercise a weighed quantity of copper wire is heated in a closed crucible with an exeess of sulfur until all exeess sulfur is driven off. The weight of the copper sulfide product is then used in the calculation of the percentage by weight of copper in the product and in the assignment of an empirical chemical formula. It is assumed that this formula must have a copper to sulfur ratio of a small whole number, and indeed the ratio must be a small whole number for the student to calculate the ratio with the limited evidence from the experiment. I n the table are listed the copper contents and eorresponding empirical formulas obtained from a statistical treatment of results reported by 266 general chemistry students a t the State University College at F r e donia in the fall semester of 1966. These are compared Comparison of Experimental Copper Sulfide Compositions with Theoretical
General Chemistry Students Outside the statistical distribution 18 values Median, %Cu 79.2%. Average, % Cu 78.6 f 2.6Formula CU,.,~ a
Research Student
78.3 1 0 . 3 Cu,.&
Cavellite
Chalcoeite
613.5~ CuS
For 248 values. Theoretical.
with the results obtained by an undergraduate research student and with the values calculated for the copper sulfides listed in the "Handbook of Chemistry and Physics."' Student results are usually graded by considering the deviation from CnzS (chaleocite) theoretical composition as a measure of analytical accuracy.
242
/
Journal o f Chemical Education
I n fact, the stable copper sulfide phase formed above 400°C is digenite, Cu& (= C U ~ . ~ S ) . ~ Furthermore, it has been demonstrated that complete solid solution exists between CU& and Cu2Sa t 500".3 These temperatures correspond to those attained in the laboratory exercise: the reaction is a t first thermostatted a t the boiling point of sulfur (444.6') and later heated to higher temperatures after the sulfur has boiled off. X-ray diffraction analysis of a copper sulfide sample containing 78.3y0Cu (formula Cu1.&3) showed that the product actually consisted of two phases: digenite (Cu& = Cul8S) and chaleocite (Cu2S) in appropriate proportions to account for the composite CulmS. This corresponds to the observation of Kullerud that phase relations existing in the Curs-Cu& composition range a t elevated temperatures cannot be quenched; instead the components of the solid solution exsolve on cooling and a t room temperature occur as a mixture of the two phases, digenite and ~halcocite.~ No eovellite (CuS) was found in the X-ray diffraction analysis of the student sample. It appears that this laboratory exercise is not a suitable one to demonstrate the law of constant proportions to introductory chemistry students, since the product of reaction does not have a small vhole number ratio of the elements and furthermore deviates from apparent stoiehiometry because of ihe presence of small amounts of another compound. We wish to thank Miss Bik-Lam Yeung for performing the research experiments, Dr. Robert Boenig of the Fredonia Biology Department for initiating this work, and the entire Fredonia Chemistry Department for gathering the considerable amount of data necessary for the statistical treatment. 1 WEAST, ROBERTE., (Editor) "Handbook of Chemistry and Physics," 45th ed., Chemical Rubber Co., Cleveland, 1964. DONNAY, G., DONNAY, J. D. H., AND KULLERUD, G., Amer. Mineralooist. 43.230 11958). RUE;, W. &D SAIJR,'E., Be?. Oberhess, Ges. Natur-Herlk. Gemsen, Naturw. Abt., 28,35 (1957). 'KULLERUD, G., Carnegie Ins. Wwh. Year Book, 57, 215
(1958).
