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).

 

[Blueshift5]

Part 1: The precipitate disappears in step c because sodium thiosulfate reacts with silver bromide to form a soluble complex, which can be easily dissolved in the solution. This reaction is known as a complexation reaction, where a complex is formed between two or more molecules.

Part 2: The reaction that occurs in step c is:
2AgBr + 2Na2S2O3 → Ag2S2O3 + 2NaBr
This reaction forms a soluble complex, silver thiosulfate, which dissolves in the solution and causes the precipitate to disappear.

Part 3:
I. There is still a precipitate at the end of part b because the addition of aqueous ammonia does not change the chemical composition of the precipitate. It only slightly changes the color due to the formation of a small amount of silver hydroxide, but the majority of the precipitate is still silver bromide.

II. The addition of sodium thiosulfate makes the precipitate disappear because it reacts with the silver bromide to form a soluble complex, as explained in part 1. This complex can be easily dissolved in the solution, causing the precipitate to disappear.