A General Chemistry Experiment on the Identification of Reaction Products The identification of reaction products is of prime importance in many areas of chemical research. This process requires both careful observation and thoughtful interpretation of ohsenred properties. The experiment presented here attempts to focus on this problem by making use of direct observation of simple inorganic reactions. Reaction products are identified by comparison of directly observable pmperties (color, odor, physical state, solubility) with those of known substances, obtained from the chemical literature. The experiment is suitable for use in a general chemistry course, perhaps early in the Key to ldentlflcatlon ol Products course. I t can serve as a useful starting point for discussion of Gae. modern instrumental methods of Formvh Color Odor Combustibility product identification as well as combustibls giving the student practice in HI Colatlcar Odorleaa Odorless SupwMmmbustion 0. C0l0.lesa identifying products in simple Colorless Odorleas Incombusfibla co. NO, B ~ W choking systems. Since most of the reacchoking tions employed here are oxidaBr% Red-bmm InaolublsSolids tion-reduction reactions, the exFormulo Color periment may also be used as a n s p d a youow exercise in balancing oxidationMnO, brownto black reduction equations. Cu red-bmm loblack As an example, consider the re~ o l u b l e l o n s a n dMo~douloul orm mu lo color Formulo color action 6f copper metal with concentrated nitric acid. When the yellow in CCI. M~o,p d a Clr red-brown metal is added to the MnOP P O Bh redin CCI. C+* v n Ir pinkinCC1. colorless liquid, bubbling begins, y d o w to b m m caoi0m"p 18a brown gas appears in the tube, CI012yellow Cuz* blue and the solution turns green. mentoeolorleas PC'* M"'* pinktomlorlea POy~~~owtoorangc Identification of the products of C ~ I ~ . I L S ~ I ~ ~ S ~ ~ ~ M ~ I I U I I I the reaction is achieved by conn+.z~~*,N~*.K*,NH.+.cI-.BT-,I-,NO.-, C O P , SO.'-,OH-: thioseaamidc, HIS, HzO,.CCI.. sulting a table listing properties of common substances. (Or, the use of a chemical handbwk could be introduced a t this point.) Two common brown gases are B h and Nos. Smce Br is not present in the reactants, the gas is identified as NO?. There is no soluble green substance listed which could be formed as a product of this reaction. One suspects the presence of Cuz+ ion, which is blue. Diluting the solution with water, the blue color of C U ~ a+p p e a r ~ . l . ~The . ~ student writes reactants and products and proceeds to balance the equation. The nu?her and variety of reactions which can be used in this way are Limited only by the experience and imagination of the instructor. Examples of other reactions which we have used are given below, along with a table for the identification of products. Experimental 1) Add 3 ml to 6 N HC1 to a small amount of zinc in a large test tube. Test the gas with a glowing splint. 2) Add 3 ml of coned HNOa to a piece of copper shot in s test tube. Allow to stand for 30-60 s and add 5-10 ml water. (CAUTION: this reaction has a tendency to "takeoff'.) 3) Add 3 ml of 6 N HC1 to a small amount of NaaCOs. Test the gas with a glowing splint. 4) Add 5 ml CuSO, to a small amount of zinc. Shake and allow to stand for 5 min. 5) Add 2 ml thioacetamide solution ( 1 M ) to 2 ml KMnO, solution. Allow tostand for 2 min and filter. 6) Mix 2 ml KMnO, and 2 m16 NHCL. Add 2 ml thioacetamide. Filter. 7) Mix 2 ml KMnO, and 2 ml 6 N NaOH. Add 2 ml 3% Hz02, dropwise. (Watch far appearance of transient green color.) Filter. 8) Mix 2 ml KlCn07 and 1m l 6 N H C l . Add 2 ml thioacetamide. 9) Add 2 ml chlorine water (or bromine water) to 2 ml CClr. Shake. Add 1ml NaI solution andshake well. 10) Mix 2 ml Fe(NH&(SOI)= solution and 1 m16 NHC1. Add 2 ml3% H102. I wish to thank my colleagues a t Federal City College, both students and faculty, who have used this experiment and offered helpful suggestions.
l T h e green color appears t o be due to Cu2+-NO2- complexes, which are yellow-green (Sneed). Since NO* reacts with water to pmduce NO2- and NOa- (Cotton Wilkinson), some NO2- would be present in solution. Addition of water shifts the complexation equilibrium back to the left.
+
+
CU(H~O)~),'+x NO;
==== CU(NO,);-'
+ 4 H,O
Sneed, M. C., Maynard, J. L., and Brasted, R. C., "Comprehensive Inorganic Chemistry," Vol. 2, D. Van Nostrand Co., New York, 1954. Cotton, F. A,, and Wilkinson, G., "Advanced Inorganic Chemistry," 3rd. Ed., Intencience Publishers, 1972. Federal City College 42.5 Second St. N.W. Washington, D.C. 20001
James R. Preer
Volume 52, Number 6, June 1975 / 389
