Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

In photography, hypo is used in the reduction of (a) \(\mathrm{Ag}\) from \(\mathrm{AgBr}\) (b) \(\mathrm{AgBr}\) into \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\) (c) AgBr into soluble thiosulphate complex (d) both (b) and (c)

Step-by-Step Solution

Answer

(c) AgBr into soluble thiosulphate complex.

Step 1: Identify Hypo in Photography

In photography, 'hypo' refers to sodium thiosulphate (\( ext{Na}_2 ext{S}_2 ext{O}_3 \)). It is commonly used as a photographic fixer to remove unreacted silver bromide (\( ext{AgBr} \)) from photographic film or paper.

Step 2: Understand the Role of Hypo

The purpose of hypo in the photographic process is to dissolve the remaining silver halide (such as \( ext{AgBr} \)) that has not been exposed to light. This process converts the unexposed \( ext{AgBr} \) into a soluble silver thiosulphate complex.

Step 3: Analyze the Given Options

- Option (a): This suggests reducing silver (\( ext{Ag} \)) from the bromide, which is incorrect since hypo dissolves \( ext{AgBr} \) instead of converting it to silver. - Option (b): Incorrect, because \( ext{AgBr} \) is not converted into \( ext{Ag}_2 ext{SO}_4 \). - Option (c): This is correct, as hypo creates a soluble thiosulphate complex from \( ext{AgBr} \). - Option (d): Incorrect, since option (b) is wrong.

Step 4: Choose the Correct Option

Based on the understanding that hypo forms a soluble thiosulphate complex with \( ext{AgBr} \), the correct response is option (c). Hypo does not convert \( ext{AgBr} \) into \( ext{Ag}_2 ext{SO}_4 \), ruling out both options (b) and (d).

Key Concepts

Silver Bromide (AgBr) in Photography

Silver bromide (2AgBr2) is a crucial component in the world of analog photography. It is a silver halide compound that is sensitive to light.
When photographic film or paper coated with silver bromide is exposed to light, a chemical reaction occurs. This reaction causes silver bromide to decompose, forming metallic silver and bromine.

Here's why silver bromide is so important in photographic processes:

• It's light-sensitive, which makes it useful for capturing images.
• The resulting metallic silver forms an image on the film or photo paper.
• Once exposed to light, areas with high light exposure turn into darker regions on the film, creating a visible image.

Without silver bromide, traditional photography as we know it would not work, because the image formation relies on this light-sensitive property.

The Role of Photographic Fixer

A photographic fixer is a chemical solution primarily used to dissolve and remove unexposed silver halide crystals from photographic film or paper after developing.
Sodium thiosulfate, commonly referred to as "hypo," serves as a primary photographic fixer. This step is crucial because it stabilizes the image.

Key roles of the photographic fixer include:

• Removing unexposed silver halide to prevent further reaction and image degradation.
• Ensuring the longevity of the developed image by removing components that could react with light.
• Providing a stable, long-lasting photographic image that remains unaffected by exposure to normal lighting conditions.

Without the use of a fixer, any remaining silver halide would continue to react when exposed to light, causing the image to fade or disappear completely.

Understanding the Soluble Thiosulfate Complex

The formation of a soluble thiosulfate complex is a pivotal part of the photographic fixing process. When sodium thiosulfate (2Na_2S_2O_32) is introduced to the treated photographic material, it interacts with unexposed silver bromide (AgBr).
This reaction converts the insoluble silver bromide into a soluble complex.

This reaction can be summarized as follows:
When sodium thiosulfate meets silver bromide, it forms a soluble silver thiosulfate complex, allowing it to be washed away easily.

The benefits of this chemical conversion include:

• Removing unwanted silver compounds, which would otherwise affect image quality.
• Ensuring that no undeveloped or unfixed remnants remain on the photograph.
• Making the photograph resistant to changes when exposed again to light.

This process ensures that only the developed image remains permanently on the film or paper, preventing ongoing chemical reactions with light that could alter the photograph.