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Photography lenses contain small embedded crystals of solid silver chloride. Silver chloride is light-sensitive because of the reaction:

AgCl(s)hνAg(s)+Cl(s)

Small particles of metallic silver cause the lenses to darken. In the lenses, this process is reversible. When the light is removed, the reverse reaction occurs. However, when pure white silver chloride is exposed to sunlight, it darkens; the reverse reaction does not occur in the dark. (a). How do you explain this difference? (b). Photography lenses do become permanently dark in time. How do you account for this?


Short Answer

Expert verified
  1. In photography lenses, the chlorine atom is trapped in the crystal.
  2. The reverse reaction is not possible as the chlorine is lost.

Step by step solution

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01

Define Reversible reaction

It is defined as a chemical reaction in which the reactants and the products react together to give the reactants back.

02

(a): Explanation of the difference

Photochromic glasses are often made up of embedding silver chloride (AgCl) or another silver halide in microcrystalline form into some glass.

When the silver chloride is taken out to ultraviolet light in the UV spectral region, it forms tiny silver particles that absorb the light.

The chemical equation is then shown below as:

AgCl(s)+hvAg(s)+Cl(s)

When the above reaction occurs, the reactive chlorine atom is trapped in the form of a crystal.

The process is then reversed by thermally activated processes when the exposure to ultraviolet light ends by the reaction of trapped Cl atoms and silver atoms.

In pure AgCl, the Cl atoms escape. So, the reverse reaction is not possible.

03

(b): Explanation of why the photography lenses permanently become dark during daytime

As discussed in the first part, photochromic glasses are often made by embedding silver chloride (AgCl) or another silver halide in microcrystalline form into some glass.

In these crystals, the Chlorine is trapped during the reaction shown in the Solution of the first part, and then a reverse reaction is possible.

However, the chlorine is lost over time, so the remaining dark colour comes from elemental silver.

Therefore, we get:

  1. In photography lenses, the chlorine atom is said to be trapped in the form of a crystal.
  2. The reverse reaction is not at all possible as the chlorine is lost.

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