Solving Solubility Problem 1: Precipitate Disappearance

[cncbmb]

1. The Problem

The Following Steps Occur in Order:
a. Aqueous silver nitrate is added to a sodium bromide solution to form a white precipitate.
b. Aqeuous ammonia is added to the above. The contents of the container change color slightly and there is still a precipitate.
c. After step b, sodium thiosulfate is added and all of the precipitate disappears.

Part 1: Explain why the precipitate disappears in step c.
Part 2: Find the reaction that occurs in part c.

Homework Equations

none

3. Attempt to Solve the Problem

After step a, we have silver bromide, which is the initial precipitate.

After step b, I thought that we had $Ag(NH_3)_2^{+}$ and some bromide and nitrate anions, so I predicted that there wouldn't be a precipitate. My prediction was wrong and I realized that I made an error in tracing the reactions.

I. Why is there still a precipitate at the end of part b?

II. Why does the addition of sodium thiosulfate make the precipitates disappear?

 

[Borek]

It all depends on the equilibrium between precipitate and the complexing agent. Ammonia complex is not stable enough to dissolve AgBr (although it is stable enough to dissolve AgCl).