Page 55 - Chemistry ICSE Class IX
P. 55
Chemical Changes and Reactions 43
Reactions in Which Light Energy is Either Absorbed or
Evolved — Photochemical Reactions
The chemical reactions which take place only in the presence of light (or
visible radiation) are called photochemical reactions.
Some common photochemical reactions are
Reaction between hydrogen and chlorine. Hydrogen and chlorine
gases when exposed of diffused sunlight react together to form
hydrogen chloride.
diffused sunlight
H (g) + Cl (g) o 2HCl(g)
2
2
Photosynthesis. Photosynthesis by green plants is a photochemical
reaction. During photosynthesis, carbon dioxide (CO ) and water
2
(H O) combine in the presence of chlorophyll (present in the green
2
plants) and sunlight to form glucose.
sunlight
6CO (g) + 6H O(l) o C H O (aq) + 6O (g)
2
12
2
6
2
6
carbon dioxide water chlorophyll glucose oxygen
(from air) (from soil)
'ZRQUWTG QH RJQVQITCRJKE ſNO The chemical reaction which takes Photographic plate
place when a photographic plate/ lm is exposed to an object is a
photochemical reaction.
Photographic plate/ lm is coated with a paste containing silver
bromide (AgBr) and a small quantity of silver iodide (AgI). When such
lm is exposed to the object, silver bromide on the lm gets converted
to silver and bromine.
light
2AgBr o 2Ag + Br 2
Some other photochemical reactions are:
Fading away of the colour of the bright-coloured fabrics on long
exposure to the sunlight.
Process of vision.
Illustrative Questions
Q.1. Why is decomposition reaction called the opposite of combination reaction? Write equations for these reactions.
Ans. Decomposition reaction is called the opposite of combination reaction. This can be supported by the following
reactions:
electric spark
Combination reaction: 2H (g) + O (g) o 2H O(l)
2
2
2
hydrogen oxygen water
electrolysis
Decomposition reaction: 2H O(l) o 2H (g) + O (g)
2
2
2
acidulated water hydrogen oxygen
Q.2. What is the difference between the displacement and double displacement reactions? Write equations for these
reactions.
Ans. In a displacement reaction, only one atom or a group of atoms is displaced, whereas in a double displacement
reaction, two different atoms or groups are displaced by other atoms or groups.
Displacement reaction: Zn(s) + CuSO (aq) o ZnSO (aq) + Cu(s)
4
4
Double displacement reaction: AgNO (aq) + NaCl(aq) o AgCl(s) + NaNO (aq)
3
3
Q.3. Two test tubes A and B contain solutions of NaBr (or KBr) and NaI (or KI). How would you nd which tube
contains which salt?
Ans. To both the tubes, add 0.5 mL of carbon tetrachloride (CCl ), 5 mL of chlorine water and shake. Observe the
4
colour in the CCl layer. The solution which gives orange colour is that of NaBr (or KBr). The solution which
4
gives purple colour contains NaI (or KI).
