Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

The solubility of \(\mathrm{AgBr}\) in hypo is due to the formation of (a) \(\mathrm{Ag}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) (b) \(\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)\right]\) (c) \(\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]^{3-}\) (d) \(\mathrm{Ag}_{2} \mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

The solubility of AgBr in hypo is due to the formation of the complex ion \(\left[\text{Ag}(\text{S}_2\text{O}_3)_2\right]^{3-}\), option (c).

Step 1: Understanding the Context

The question asks about the compound formed when silver bromide (AgBr) dissolves in hypo, often referring to sodium thiosulfate (Na2S2O3), a common photographic fixer. Silver halides typically dissolve in hypo due to complex ion formation with thiosulfate ions.

Step 2: Analyzing Complex Ion Formation

AgBr, an insoluble salt, can dissolve in a solution that forms a stable complex ion with its constituent ions. In this case, we look at how silver ions (Ag+) interact with thiosulfate ions (S2O3^2-). An important reaction involves the formation of complex ions like \[\left[\text{Ag}(\text{S}_2\text{O}_3)_2\right]^{3-}\], where the silver ion is coordinated with two thiosulfate ions to form a soluble complex.

Step 3: Identifying the Correct Option

Based on the common reaction, AgBr's solubility in a thiosulfate solution is greatly increased due to the formation of the complex ion \[\left[\text{Ag}(\text{S}_2\text{O}_3)_2\right]^{3-}\]. This stabilizes the silver ion in solution, making the correct answer option (c).

Key Concepts

Complex Ion Formation

Complex ion formation is a fascinating concept in chemistry where a central metal ion bonds with surrounding molecules or ions, called ligands. This process results in a complex.
Complex ions form when metal ions in a solution, like silver ions ( Ag^+ ), interact with ligands such as thiosulfate ions ( S_{2}O_{3}^{2-} ).
One of the primary reasons complex ion formation is crucial is because it influences the solubility of otherwise insoluble compounds.
A perfect example is when silver bromide ( AgBr ) dissolves in a solution due to the formation of a stable complex ion.
This process is driven by the interactions between the silver ions and thiosulfate ions, resulting in a soluble complex.
Such interactions are vital in many industrial applications, including photographic developing where hypo, or sodium thiosulfate, is used.
In the formation of complex ions, the central metal ion like silver can combine with various ligands, and this enhances the metal's ability to remain in the solution rather than precipitate.
Essentially, complex ion formation improves solubility, allowing certain reactions to occur more readily and efficiently.

Silver Bromide Solubility

Silver bromide (AgBr ) has very low solubility in water but its solubility can be drastically increased through the formation of complex ions.
When AgBr is added to a solution containing sodium thiosulfate (Na_{2}S_{2}O_{3} ), it undergoes a transformation that makes it more soluble.
The key to understanding this process is recognizing the role that complex ions play in altering solubility.
The silver ion (Ag^{+} ) from AgBr interacts with thiosulfate ions (S_{2}O_{3}^{2-}) to form the complex ion \[\left[ \text{Ag} \left( \text{S}_2\text{O}_3 \right)_2 \right]^{3-} \].
This specific interaction significantly increases the solubility of silver bromide.
The formation of this complex ion not only prevents AgBr from precipitating but also helps stabilize the silver ion in the solution.
In photography, this principle is applied when using hypo to dissolve and remove undeveloped silver halide crystals during the fixing stage, crucial for developing photographs effectively.

Thiosulfate Complexes

Thiosulfate complexes are a pivotal part of many chemical reactions due to their ability to form stable complexes with metal ions.
One of the most notable examples is when thiosulfate ions form complexes with silver ions, creating compounds that are more soluble than the original metal salts.
In the context of silver bromide, the interaction with thiosulfate ions is a classic demonstration of this principle.
The chemical reaction produces the complex ion \[\left[ \text{Ag} ( \text{S}_2\text{O}_3 )_2 \right]^{3-} \], which enhances the solubility of silver bromide in the solution.
This complex is key to many applications, such as in the field of photography, where it aids in the removal of undeveloped silver halide crystals.
Thiosulfate's ability to stabilize metal ions in solution showcases its utility in both analytical chemistry and industry.
Furthermore, the formation of thiosulfate complexes is not limited to silver.
Numerous metals can form similar complexes, broadening the range of possibilities in chemical synthesis and processing.
Understanding these complexes and their properties allows scientists and industries to manipulate solubility and reactivity for various practical applications.