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 thoseinvolving halogens:Examples of reactions affected by light:-Reaction between Cl2(g) and H2(g) does not take place in the dark but is explosive in bright lightCl2(g)+Br2(l)light→CH3Br+HBr(g)-For the reaction between methane and bromine, decolourisation of bromine only occurs in the presence of lightExperiment: Effect of light on the decomposition of silver bromide.-Procedure:- About 20cm3 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)→AgBr(s)+KNO3(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 toslight 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)light→2Ag(s)+Br2- 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.