After black and white photographic film has been developed, unreacted silver bromide is removed by reaction with sodium thiosulfate.

Question 19 [Group 7 > reactions of halide ions]

After black and white photographic film has been developed, unreacted silver bromide is removed by reaction with sodium thiosulfate.

AgBr + 2Na2S2O3 → 4Na⁺ + Br ^– + [Ag(S2O3)2]^3–

What is the function of the thiosulfate ion?

A to make the silver ions soluble

B to oxidise the silver ions

C to reduce the bromine

D to reduce the silver ions

Reference: Past Exam Paper – Chemistry (9701) March 2016 Paper 12 Q19

Solution:

     Answer:A
 

     Firstly, if confused work out oxidation changes for both silver and bromine to see if oxidation or reduction has taken place. Neither oxidation or reduction takes place and so automatically the only feasible answer is A.

   Secondly, AgBr is a precipitate, which you should be able to recall from qualitative tests and other chapters from inorganic chemistry. We see that Ag is then incorporated into this complex on the products side of the equation. The formula is enclosed in square brackets and we also observe a charge of -3. Automatically we see that this complex is in ionic form and therefore silver has gone from an insoluble solid to a soluble ion.