[SPM Chemistry Kedah Paper 1 Q17] Why the answer is B?


  1. Kaca fotokromik terhasil apabila kaca digabungkan bersama-sama argentum klorida dan kuprum (I) klorida. Apabila didedahkan kepada cahaya matahari, kaca fotokromik menjadi gelap.
    Photochromic glass is produced when glass is combined with silver chloride and copper (I) chloride. When exposed to sunlight, the photochromic glass darkens.

Antara berikut yang manakah persamaan setengah yang menerangkan fenomena tersebut?
Which of the following is the half equation that describes the phenomenon?
A Cl^- β†’ Cl + e
B Ag^+ + e β†’ Ag
C Cu β†’ Cu^+ + e


Hi Farhaini,
The question is asking about the half equation that describes the phenomenon. In this case, it is the equation that has to do with the darkening of the photochromic glass.
Basically, the photochromic glass darkens because of the silver.
In the presence of UV light, the silver chloride decomposes.
The following reactions occur:
2Cl^- β†’ Cl_2 + 2e
Ag^+ + e β†’ Ag -------------(\times 2)
Overall equation: 2AgCl β†’ 2Ag + Cl_2

Silver ions, Ag^+ are colourless but silver atoms aren’t, which is the reason for the darkening of the glass.

The reason we combine it with copper (I) chloride because it helps to reverse the process. When there is little or no UV light, the following reactions occur:
Cl_2+2Cu^+ β†’ 2Cl^-+ 2Cu^{2+}
Ag+Cu^{2+} β†’ Ag^+ + Cu^+ -------------(\times 2)
Overall equation: 2Ag + Cl_2 β†’ 2AgCl
We get back silver chloride and the glass becomes clear again.

Hope this helps! :slight_smile:

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Ouhhh now I understand Thank you very much :relaxed::blush::relaxed::hugs:

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thanks for the awesome information.