Why does the white precipitation turns dark when adding Na2S2O3 drop wise to AgNO3 sollution
When adding Na2S2O3 drop wise to a solution of AgNO3, a white precipitate of silver chloride (AgCl) initially forms. However, as more Na2S2O3 is added, this white precipitate turns dark. This color change happens due to the formation of silver sulfide (Ag2S).
The reaction between AgNO3 and Na2S2O3 takes place via a series of chemical reactions:
1. AgNO3 reacts with Na2S2O3 to form AgCl initially:
2 AgNO3 + Na2S2O3 → AgCl + NaNO3 + Na2S4O6
2. The white precipitate of AgCl is formed as a result of the reaction in step 1.
3. Additional Na2S2O3 reacts with AgCl to form a soluble complex, Ag(S2O3)2^3-:
AgCl + 2 Na2S2O3 → Ag(S2O3)2^3- + 2 NaCl
4. As more Na2S2O3 is added, silver sulfide (Ag2S) begins to form due to the reaction between Ag(S2O3)2^3- and Na2S:
2 Ag(S2O3)2^3- + 3 Na2S → Ag2S + 6 NaS2O3^2-
This formation of Ag2S, which is a dark-colored compound, is responsible for the color change from white to dark when adding Na2S2O3 drop wise to the AgNO3 solution.
When you add Na2S2O3 dropwise to AgNO3 solution, the white precipitation (AgCl) turns dark because AgCl is reacting with Na2S2O3 to form a different compound called Ag2S. This reaction is commonly known as the "silver mirror reaction."
To understand why the precipitation turns dark, you need to know the reactions taking place.
1. In the original solution, AgNO3 dissociates into Ag+ and NO3- ions.
AgNO3 (aq) -> Ag+ (aq) + NO3- (aq)
2. When Na2S2O3 is added dropwise to the solution, the thiosulfate ions (S2O3^2-) react with Ag+ ions to form Ag2S.
2Ag+ (aq) + S2O3^2- (aq) -> Ag2S (s) + 2SO3^2- (aq)
Ag2S is a dark-colored compound, which gives the precipitate its dark appearance.
To perform this experiment and observe the darkening of the precipitate, one would need to prepare a solution of AgNO3 by dissolving AgNO3 in water. Then, slowly add Na2S2O3 dropwise to the AgNO3 solution while observing the changes in color. Make sure to conduct this experiment in a controlled environment and follow appropriate safety protocols, as both AgNO3 and Na2S2O3 can be hazardous if mishandled.