Question

Which of the following is true about Hypo (sodium thiosulphate) ${\text{N}}{{\text{a}}_2}{{\text{S}}_2}{{\text{O}}_3}$
A) dissolves in ${\text{AgBr}}$ in photographic plate
B) gives a white precipitate in ${\text{AgN}}{{\text{O}}_3}$ ; white precipitate change to black on dilution
C) both reactions occur
D) none of the above reaction

Answer 1

Ionic compounds dissociate into their respective ions on dissolution into water. Sodium $\left( {Na} \right)$ is more reactive than silver $\left( {Ag} \right)$ can replace it in its compounds during ionic reactions.

Complete step by step answer: We can write both the reaction as follows:

1. The reaction of hypo with silver bromide gives sodium argento thiosulphate and sodium bromide.
   ${\text{4N}}{{\text{a}}_2}{{\text{S}}_2}{{\text{O}}_3}{\text{ + 2AgBr}} \to 2{\text{N}}{{\text{a}}_3}\left[ {{\text{Ag}}{{\left( {{{\text{S}}_2}{{\text{O}}_3}} \right)}_2}} \right] + 2{\text{NaBr}}$
   Thus, the hypo reacts with silver bromide in the photography plate.

2. The reaction of hypo with silver nitrate gives silver thiosulphate and sodium nitrate.
   ${\text{N}}{{\text{a}}_2}{{\text{S}}_2}{{\text{O}}_3}{\text{ + 2AgN}}{{\text{O}}_3} \to {\text{A}}{{\text{g}}_2}{{\text{S}}_2}{{\text{O}}_3}{\text{ + 2NaN}}{{\text{O}}_3}$
   Silver thiosulphate is white in color and forms precipitates. But on dilution, the following reaction occurs:
   ${\text{AgN}}{{\text{O}}_3}{\text{ + }}{{\text{H}}_2}{\text{O}} \to {\text{A}}{{\text{g}}_2}{\text{S + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}$
   Thus, silver nitrate reacts with water to give silver sulfide (which is black in color) and sulphuric acid. Due to the presence of silver sulfide, the solution turns black in color on dilution.

3. Hence, hypo reacts with both silver nitrate and silver bromide. So, our final answer is ‘C’ i.e. both reactions occur.

Additional information: It is interesting to know that hypo or sodium thiosulphate is used in the classic photographic image development process. It dissolves the unreacted silver ions from the film by the formation of a complex and this process is called fixation.

Note: The reactions which yield insoluble products are called precipitation reactions. Sodium argentothio sulphate is soluble in water whereas silver thiosulphate is insoluble in water and gives precipitates. It should always be noticed that a more reactive ion can replace a less reactive ion from its salt and such types of reactions are called replacement reactions. But it should also be noted that the reverse reactions in the above case do not occur as bromide ions and nitrate ions are more reactive than thiosulphate ions.