Further Observations on the Cobalt(ll)-Chloride Equilibrium: Effect of Changing the Chloride Ion Concentration Luis J. A. Martins and J. Barbosa da Costa lnstituto Superior de Engenharia de Coimbra, 3000 Coimbra, Portugal As previously noted's2, the ligand substitution reaction
+
[CO(H,O),]~+4Cl-
t
[CoC1,j2- + 6H,O
provides a good general chemistry experiment for testing the predictive value of the Le Chatelier Principle. By means of distinctive color changes, students can readily~. perceive the effects of runrentratih, temperature, and other factors on the equilibrium. Recently the standard explanation of the effert of adding concentrawd HCI to the system in terms of increusing thc chloride ion cuncentrstion and so shifting the eauilibrium to the right has been criticizedq. and the main effect has been suggested to be a dehydratidn effect of the acid? We feel that this view is incorrect, and in this article we supplement the previous experimentson this equilibrium by confirmina the effect of changes in chloride ion concentration. Experlmen(a1 The following solutions are prepared: 0.10 M CoC1r6H20 in absolute ethanol (Co(I1)-EtOH), 0.01 M AgN03 in absolute ethanol, 1.00 M HgC12 in absolute ethanol (care must be exercised i n handling HgC12), and 0.15 M CoCl26H20 in distilled water (Co(I1)-H20). All chemicals were used without further purification. Results and Dlscusslon Addition uf Ag(l) solution (10 ml) to one made of I ml of soluuon Co(11j-EtOHand 14 ml of absolute ethanol leads to precipitation of AgCly) and a change in the color of the solution from blue to pink, as expected, owing to reduction in chloride ion concentration. An identical color change is observed if 15 ml of the Hg(I1) solution is added to 10 cm3 of the Co(I1)-EtOH solution. Considering the much higher stepwise ligand addition constants4 for HgC12, i.e., HgC1, + C1-
K1
t
[HgCIJ
log K, = 1.1
K4
[HgClJ
+ CI- e [HgC1,j2-
log K , = 1.0
as compared to the formation constant, K1, for the complex [CoCl]', for which log KI = -2.4, we may conclude that HgC12 competes successfully with Co(I1) for C1-, thus favoring formation of the pink complex [ C O ( H ~ O ) ~(both ]~+ [HgCl~l-and [HgC14]2- are colorless). Addition of 4 . 5 g of solid CaC12.2H20or NaCl to a violet solution obtained by mixing 10 ml of Co(I1)-EtOH and 1 ml of distilled water makes the solution distinctly blue, as expected for an increase in chloride ion concentration. The effect is more marked for the former., nossihlv because of higher solubility. The formation of the blue comnlex ICoCL12- in aaueous solution may be achieved1 by adding i 0 ml ii conce&ated HCI solution to 5 ml of Co(I1)-H20 solution. The predominant effect of adding concentrated HC1 to the latter has long been attributed5 to the increase in chloride ion concentration. The alternative suggestion that the effect is a result-of dehydration by strong acids, based on similar observations3 on addition of concentrated HzSOd can be very simply disproved. Thus addition of UP to 30 ml of concentrated nitric acid has no effect on the eq;ilibrium. Moreover, on addition of solid HgC12 to the blue solution formed with HCI the solution hecomes faint blue or even pink, although the effect on the hydrogen ion concentration is small. We thus favor the accepted explanation of the effect of concentrated HCI. With concentrated HsS04, however, its strong dehydrating effect may be important.
.
Acknowledgment We are grateful for the very helpful comments from the reviewer.
' Ophardt, C. E. J. Chem. Educ. 1980, 57,453.
Spears, L. G.. Jr.; Spears, L. G. J. Chern. Educ. 1984, 61, 252. Grant, A. W., Jr. J. Chem. Educ. 1984, 61,466. Meites, L. "Handbook of Analytical Chemistry"; McGraw-Hill: New York, 1963; table 1-17, pp 1-39. Moeller, T. 'Qualitative Analysis"; McGraw-Hill: New York. 1958; chap 8.
'
Volume 63 Number 11 November 1986
880
