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) \(\left[\operatorname{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)\right]^{-}\) (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

(c) \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \)

Step 1: Identify the Dissociation Products

Silver bromide (\( \text{AgBr} \)) is slightly soluble in water and remains as a solid. However, in the presence of sodium thiosulfate (hypo), \( \text{AgBr} \) can dissolve.The dissociation of silver thiosulfate can form complexes with \( \text{S}_2\text{O}_3^{2-} \) (thiosulfate) ions, such as \( \text{Ag}\left(\text{S}_2\text{O}_3\right)_2^{3-} \). This complex ion is known to enhance the solubility of silver ions.

Step 2: Examine the Possible Complexes

Consider the complexes that can form with \( \text{Ag}^+ \) ions in thiosulfate solution:- \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)\right]^{-} \)- \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \)Among these, the complex \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \) is a well-known complex ion which increases the solubility of silver salts.

Step 3: Determine the Correct Complex

Based on the chemical knowledge that \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \) is a common silver thiosulfate complex formed in solutions like photographic fixing solutions, it will be the key complex enhancing the solubility of \( \text{AgBr} \) in hypers. Hence, the correct option should correspond to this complex.

Step 4: Select the Correct Answer

From the options given, option (c) corresponds to the complex \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \).Therefore, option (c) is the correct answer.

Key Concepts

Silver bromide

Silver bromide, often denoted as \( \text{AgBr} \), is an interesting compound primarily because of its notable lack of solubility in water. This property makes \( \text{AgBr} \) an excellent candidate for use in photographic films, as the silver ions can be reduced by light exposure, creating a visible image. However, when \( \text{AgBr} \) is introduced to a solution containing thiosulfate ions, its behavior changes dramatically. Despite silver bromide's typical insolubility in water, thiosulfate enables it to form a soluble complex. This characteristic is crucial in its role in photographic developing, where it must be removed from film without leaving any residue.

Complex ion formation

In chemistry, the formation of complex ions is a fascinating process. It involves the association between simple ions and ligands, which are typically negatively charged or neutral molecules that donate electrons to the metal ion. In the case of silver bromide interacting with thiosulfate, complex ion formation plays a critical role. When \( \text{Ag}^+ \) ions, which come from the breakdown of silver bromide, encounter thiosulfate ions, they bind together to create complex ions. These complexes dramatically enhance the solubility of silver in solutions that would typically precipitate it out, allowing the silver ions to remain dissolved instead.

Thiosulfate complex

The thiosulfate complex is crucial to enhancing the solubility of silver compounds in aqueous solutions. Specifically, the \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \) complex is one of the most stable and significant forms this interaction can take. Formation of such a complex involves a coordination bond where the thiosulfate acts as a bridging ligand, providing an electron pair to the silver ion. This particular configuration is notable because it efficiently sequesters silver ions, preventing them from recombining with bromide ions to form \( \text{AgBr} \) again. By doing so, the thiosulfate complex ensures prolonged solubilization of the silver, making it easier to process in applications like photography.

Solubility enhancement

One of the main benefits of forming complexes like \( \left[\text{Ag}\left(\text{S}_2\text{O}_3\right)_2\right]^{3-} \) is the significant enhancement of solubility for otherwise insoluble compounds. The interactions between silver ions and thiosulfate ligands stabilize the dissolved phase of silver. This effectively prevents precipitation, which is crucial in many industrial and scientific applications.

• In photography, this solubility enhancement helps in the developing process by ensuring all silver halides are removed from negatives, leaving behind only the desired image.
• In chemistry, enhanced solubility allows for reactions to proceed more efficiently by maintaining a higher concentration of the reactive species in solution.
• Environmentally, it aids in the treatment of silver waste, reducing solid waste and enabling the recycling of silver from solutions.

The ability to manipulate solubility through complex ion formation is a powerful tool, and thiosulfate complexes exemplify this by transforming insoluble silver salts into readily manageable solutions.