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Effect of light on the rate of reaction

Answer Text:
Effect of light on the rate of reaction
-The effect of heating and illuminating substances is the same;
- In both cases the constituent particles absorb radiant energy leading to an increase in the number of particles with activation energy resulting in increased rate of reaction;
- Light energizes the particles involved in a reaction;
- This increases the chances of effective collisions per unit time thus increasing the rate of reaction.
- Light of higher frequencies give higher reaction rates e.g. UV Light;
- Examples of reactions affected by light are those
involving halogens:
Examples of reactions affected by light:
-Reaction between #Cl_2(g) # and #H_2(g)# does not take place in the dark but is explosive in bright light
#Cl_2(g)+ Br_2(l) overset(light)(to)CH_3Br + HBr(g)#
-For the reaction between methane and bromine, decolourisation of bromine only occurs in the presence of light
Experiment: Effect of light on the decomposition of silver bromide.
-Procedure:
- About 20#cm^3# of 0.1M potassium bromide is put in a glass beaker.
- 5cm3 of 0.05M silver nitrate solution is added.
- The resulting pale yellow precipitate is divided into three portions in 3 separate test tubes.
- One of the test tubes in immediately placed in a dark cupboard; the second on a bench and the third is placed in a direct source of light e.g. sunlight.
Observations.
- Formation of a pale yellow precipitate of silver bromide when silver nitrate reacts with potassium bromide.
Equation:
#KBr(aq) +AgNO(ag) to AgBr(s) +KNO_3(aq)#
pale yellow
-Test tube in light: precipitate changes colour from pale yellow to grey.
-Test tube on the bench: slight change in colour from pale yellow to
slight grey.
-Test tube in dark cupboard: no observable (noticeable) colour change in precipitate.
Explanation:

- Light decomposes silver bromide to metallic silver (hence the grey colour) and bromine.
Equation:
#2AgBr(aq) overset(light)(to) 2Ag(s)+ Br_2#
- No observable change in test tube placed in darkness due to lack of light;
- The degree of decomposition and hence change depends on the light intensity falling on the test tubes;
- The rate of decomposition of silver bromide increases with increase in light intensity.