Question

Photogray lenses contain small embedded crystals of solid silver chloride. Silver chloride is light-sensitive because of the reaction $$ \mathrm{AgCl}(s) \stackrel{\hbar v}{\longrightarrow} \mathrm{Ag}(s)+\mathrm{Cl} $$ 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. Photogray lenses do become permanently dark in time. How do you account for this?

Short answer

In photogray lenses, the reversible reaction of silver chloride to light-darkening silver particles occurs due to the presence of additional components that facilitate the reverse reaction, which doesn't happen in pure white silver chloride. Photogray lenses gradually become permanently dark over time due to the breakdown of these facilitating substances or lens material, making the reverse reaction less efficient and causing the accumulation of metallic silver particles. Thus, the lens no longer returns to its original state even in the absence of light.

Step-by-step solution

a. Explaining the difference between reactions in photogray lenses and pure white silver chloride

In photogray lenses, the reversible reaction of silver chloride takes place: \[ \mathrm{AgCl}(s) \stackrel{\hbar v}{\longrightarrow} \mathrm{Ag}(s)+\mathrm{Cl} \] This reaction occurs when the lenses are exposed to light and involves the formation of small particles of metallic silver, which cause the lenses to darken. When light is removed, the reverse reaction occurs, and the lenses return to their original, lighter state. In contrast, pure white silver chloride exposed to sunlight undergoes the same reaction as in photogray lenses, but the reverse reaction does not happen in the dark. The key difference between the two systems is the presence of other components in photogray lenses that facilitate the reverse reaction. These embedded substances can interact with the metallic silver formed during the reaction and help to re-oxidize it, allowing the lenses to return to their lighter state when no longer exposed to light.

b. Explaining why photogray lenses become permanently dark over time

Although the reaction between silver chloride and light is reversible in photogray lenses, these lenses still tend to become permanently dark over time. This can be attributed to the gradual breakdown of the facilitating substances and/or the lens material itself that allows the reverse reaction to take place. When the facilitating substances or the lens material breakdown, the reverse reaction becomes less efficient, and the lens starts to accumulate more metallic silver particles, which leads to the darkening of the lenses. Eventually, the lens may reach a point where even in the absence of light, it no longer completely returns to its original, lighter state. This results in the permanent darkening of photogray lenses over time.

Key concepts

Silver Chloride Photo-sensitivity

Silver chloride (\text{AgCl}) is a chemical compound that exhibits remarkable photo-sensitivity, an intriguing property that lies at the heart of photochromic lenses, such as Photogray lenses. The phenomenon is based on a photochemical reaction where energy from light, or photons (\text{hv}), triggers the dissociation of silver chloride into silver (\text{Ag}) and chlorine atoms (\text{Cl}).

When exposed to ultraviolet (UV) light, silver chloride reacts as follows:
\[\begin{equation}\mathrm{AgCl}(s) \stackrel{\overline{hv}}{\longrightarrow}\mathrm{Ag}(s) + \mathrm{Cl}\end{equation}\]
Small particles of metallic silver are formed in this process, and these are responsible for the darkening of the lenses. Unlike pure white \text{AgCl}, which does not revert to its original state in the dark, photochromic lenses contain additional substances that facilitate the recombination of silver and chlorine, allowing the lenses to lighten again when removed from light sources. This selective sensitivity to light is a key property that enables photochromic lenses to adapt to varying lighting conditions.

Reversible Photochemical Reactions

Reversible photochemical reactions are foundational to the functioning of photochromic lenses. These type of reactions can proceed in both directions: the forward reaction takes place under the influence of light, and the reverse reaction occurs when the light source is removed.

The underlying reversible reaction in photochromic lenses can be demonstrated as:\[\begin{equation}\mathrm{AgCl}(s) \stackrel{\overline{hv}}{\longrightarrow} \mathrm{Ag}(s) + \mathrm{Cl}\end{equation}\]
In the forward direction, the light prompts the separation of silver ions and chloride ions, creating the visible darkening associated with the silver particles. Conversely, when the lenses are removed from a light source, facilitating substances within the lens assist in the 'healing' process, where the silver reverts back to \text{AgCl}, causing the lenses to return to a clear state.

This dynamic ability to change with the light environment is not only convenient for the wearer but also provides protection from UV radiation. However, the longevity of the reversible reaction is contingent upon the integrity of the lens's facilitating components.

Photochromic Material Degradation

Even though photochromic lenses are designed to endure numerous cycles of darkening and lightening, they are not impervious to wear and tear. Over time, photochromic material degradation can occur, affecting the efficiency of the reversible photochemical reactions.

Factors contributing to this degradation include prolonged UV exposure, the presence of harsh chemicals, and even the breakdown of the lens material itself. As the lens undergoes more cycles of darkening and lightening, the embedded substances that facilitate the reverse reaction gradually lose their effectiveness. This means that the tiny silver particles formed during the forward reaction fail to recombine fully into silver chloride when the light is removed. As a result, the lenses retain a residual darkened state, which becomes more pronounced with each cycle, leading to a permanent darkening over time.

To ensure the longevity of photochromic lenses, proper care and maintenance are essential. This includes storing the lenses away from direct sunlight when not in use and cleaning them with suitable products to prevent the accumulation of residues that may further hasten material degradation.