The chemical reaction: 2AgCl (s) → 2Ag (s) + Cl₂ (g) takes place
- (a)in dark
- (b)in sunlight
- (c)on heating
- (d)under high pressure
Correct — B, in sunlight. Silver chloride is light-sensitive: sunlight supplies the energy that breaks it into grey metallic silver and chlorine gas. This is a photodecomposition (photochemical) reaction, and it is the reason white AgCl turns grey/violet on exposure to light.
- (a)in dark — In the dark silver chloride is stable and does not decompose — the reaction needs light energy, so it simply will not proceed without sunlight.
- (c)on heating — This is a light-driven (photochemical) decomposition, not a thermal one. The standard equation is written for sunlight; heat is not the trigger for the decomposition of AgCl.
- (d)under high pressure — Pressure is irrelevant to this reaction. The energy that splits AgCl comes from light, not from compressing the solid.
A decomposition reaction in which the energy is supplied by light is called photolysis or photodecomposition. Silver halides such as AgCl and AgBr break down when sunlight falls on them, releasing metallic silver (which darkens to grey/violet) and the halogen. This light-sensitivity is the chemical basis of black-and-white photography.
Sort decomposition reactions by their energy source: heat drives thermal decomposition (e.g., CaCO3 → CaO + CO2), electricity drives electrolytic decomposition, and light drives photodecomposition. Because silver chloride is famous for darkening in light, 'in sunlight' is the answer.
- 2AgCl → 2Ag + Cl2 is a photodecomposition reaction; the energy source is sunlight.
- White silver chloride turns grey/violet in sunlight as metallic silver forms.
- Silver-halide photodecomposition underlies black-and-white photography.
- Contrast with thermal decomposition (heat) and electrolytic decomposition (electricity).
- Choosing 'on heating' by confusing this with thermal decomposition.
- Choosing 'in dark' — silver chloride is stable in the dark and needs light to decompose.
Asked as 'the reaction ... takes place in ___' or as 'give an example of a photodecomposition reaction'.
No directly related past PYQ was found.
- practice — not a real PYQ
The reaction 2AgBr → 2Ag + Br2 is an example of:
- (a)Thermal decomposition
- (b)Photodecomposition
- (c)Electrolytic decomposition
- (d)Combination reaction
Answer(b) Photodecomposition — silver bromide, like silver chloride, decomposes in light.
- practice — not a real PYQ
White silver chloride turns grey/violet on standing in sunlight because:
- (a)It absorbs water vapour
- (b)Metallic silver is formed by photodecomposition
- (c)It reacts with atmospheric oxygen
- (d)It sublimes
Answer(b) Metallic silver is formed by photodecomposition — the freed silver gives the dark colour.