Question

I had an equilibrium laboratory and had to answer a few questions after the lab. Here are 2 questions I can't seem to know how to answer.

1. Explain in your own words why a saturated solution in contact with some undissolved solid is considered to be an equilibrium situation?

2. Rationalize how the addition of the silver nitrate to the Fe/SCN equilibrium system had an effect even though neither Ag nor NO3 appears in the equation shown in the text.

Attempt:

For number 2 I was going to write that the Ag ion will replace the Fe in FeSCN and the NO3 will do the same but with the Fe ion?

Answer 1

#1. In as few words as possible, the solid is continuing to dissolve to form a solution and material in the solution is continuing to ppt; therefore, the material in solution is constant and the solid is constant. That's equilibrium.

#2. You didn't write the equations in your text but I assume it is
Fe^3+ + SCN^-= ==> FeSCN^2+

The reason equilibrium is upset is because Ag^+ reacts with SCN^-. You can look up what it forms. If you pull SCN^- out then the Fe/SCN equilibrium shifts to the right.