Photochromic glass, used for sunglasses darkens when exposed to bright light and becomes more transparent again when the light is less bright. The depth of color of the glass is related to the concentration of silver atoms.

The following reactions are involved:

reaction $1$: $\ce{Ag+ + Cl- <=> Ag + Cl}$

reaction $2$: $\ce{Cu+ + Cl → Cu^2+ + Cl-}$

reaction $3$: $\ce{Cu^2+ + Ag → Cu+ + Ag+}$

Which statement about these reactions is correct?

A) $\ce{Cu+}$ and $\ce{Cu^2+}$ ions act as catalysts.

B) $\ce{Cu+}$ ions act as an oxidising agent in reaction 2.

C) Reaction $2$ is the one in which light is absorbed.

D) $\ce{Ag+}$ ions are oxidised in reaction $1$.

I understand why B, C, D are incorrect. I also understand why $\ce{Cu+}$ is a catalyst; however, I do not understand why $\ce{Cu^2+}$ is a catalyst and not an intermediate. It appears as a product first and then as a reactant later, unlike $\ce{Cu+}$, so shouldn't it be an intermediate instead of a catalyst?

The question is question $11$ in the October November $2009$ A-level chemistry Cambridge variant $11$ paper.

  • 4
    $\begingroup$ Rather Cu is the catalyst, switching between 2 oxidation states, catalysing the clearing up the glass in low light. $\endgroup$
    – Poutnik
    Aug 14 '20 at 16:42

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