Reactions with Cl^-. Test samples (about 3 drops) of each of the cations solutio

Question

Reactions with Cl^-. Test samples (about 3 drops) of each of the cations solutions with 3.0 M NaCl. Add the reagent drop wise (about 5 drops) to each cations solution. Avoid an excess of the chloride ion at this point as some of the cations tend to form complex ions with chloride. (Which ones?] Try the same reactions using 6 M HCL (CAUTION!). Are the results any different? Write equations for any reactions that occur. Cheek the K_sp values and perform some simple calculations to justify the formation of these precipitates.

Explanation / Answer

Reaction of metal cations with HCl

Group I metal cations all form a precipitate with dilute HCl solution. The metal cations are Ag+, Hg+, Pb2+ and Tl+.

Thus these metal cations would be avoided in the above experiment.

With a more concentrated 6 M HCl solution, we tend to get precipitate dissolve in solution for say CuCl2 forming CuCl4^2-, AgCl forms AgCl2-. Along with this other metal cations also tend to react with 6 M HCl solution such as FeCl3 forms.

The precipitation of metal chlorides is based on its Ksp value. So the ones whose Ksp is lower would get precipitated with low concentration of HCl in solution, whereas, the ones which have higher Ksp value would get precipitated with higher concentration of HCl in solution. Ag+ with Ksp 1.8 x 10^-10 gets precipitated much quickly over say PbCl2 with Ksp 1.6 x 10^-5.

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