Photochemical Reaction
Category : JEE Main & Advanced
Absorption of radiant energy by reactant molecules brings in photophysical as well as photochemical changes. According to Einstein's law of photochemical equivalence, the basic principle of photo processes, each reactant molecule is capable of absorbing only one photon of radiant energy. The absorption of photon by a reactant molecule may lead to any of the photo process.
The chemical reactions, which are initiated as a result of absorption of light, are known as photochemical reactions. In such cases, the absorbed energy is sufficient to activate the reactant molecules to cross the energy barrier existing between the reactants and products or in other words, energy associated with each photon supplies activation energy to reactant molecule required for the change.
(1) Characteristics of photochemical reactions
(i) Each molecule taking part in a photo process absorbs only one photon of radiant energy thereby increasing its energy level by \[hv\,\,or\,\frac{hc}{\lambda }\]
(ii) Photochemical reactions do not occur in dark.
(iii) Each photochemical reaction requires a definite amount of energy which is characteristic of a particular wavelength of photon. For example, reactions needing more energy are carried out in presence of UV light (lower \[\lambda \], more E/Photon). A reaction-taking place in UV light may not occur on exposure to yellow light (lower \[\lambda \]and lesser E/Photon)
(iv) The rate of photochemical reactions depend upon the intensity of radiation’s absorbed.
(v) The \[\Delta G\] values for light initiated reactions may or may not be negative.
(vi) The temperature does not have marked effect on the rate of light initiated reactions.
(2) Mechanism of some photochemical reactions
(i) Photochemical combination of H2 and Cl2 : A mixture of \[{{H}_{2}}\] and \[C{{l}_{2}}\] on exposure to light give rise to the formation of HCl, showing a chain reaction and thereby producing \[{{10}^{6}}\,\text{to}\,{{10}^{8}}\]molecules of \[HCl\] per photon absorbed.
\[{{H}_{2}}+C{{l}_{2}}\xrightarrow{sunlight}\,2HCl\]
The mechanism leading to very high yield of HCl as a result of chemical change can be as follows. Chlorine molecules absorb radiant energy to form an excited molecule which decomposes to chlorine free radicals (Cl) to give chain initiation step.
Light absorption step : \[C{{l}_{2}}\xrightarrow{hv}\,C{{l}_{2}}^{*}\] ........(i)
Chain initiation step : \[C{{l}_{2}}^{*}\to \,C{{l}^{\bullet }}\,+\,C{{l}^{\bullet }}\] ........(ii)
The chlorine free radical then combines with \[{{H}_{2}}\] molecule to form HCl and \[{{H}^{\bullet }}\] free radical. The \[{{H}^{\bullet }}\] free radical so formed again combines with another \[C{{l}_{2}}\] molecule to give HCl and \[C{{l}^{\bullet }}\] free radical back resulting into chain propagation step.
Chain propagation step : \[C{{l}^{\bullet }}+\,{{H}_{2}}\,\to \,HCl\,+\,{{H}^{\bullet }}\] ........(iii)
\[{{H}^{\bullet }}\,+\,C{{l}_{2}}\,\to \,HCl\,+\,C{{l}^{\bullet }}\]
The combination of two \[C{{l}^{\bullet }}\] free radicals leads to chain terminating step.
Chain terminating step : \[C{{l}^{\bullet }}\,+\,C{{l}^{\bullet }}\,\to \,C{{l}_{2}}\] ........(iv)
(ii) Photochemical combination of H2 and Br2 : The combination of \[{{H}_{2}}\] and \[B{{r}_{2}}\] to form HBr in presence of light is also an example of chain reaction like photochemical combination of \[{{H}_{2}}\] and \[C{{l}_{2}}\]. Here two \[B{{r}_{2}}\] molecules absorb photon, however, inspite of chain reaction only one molecule of HBr is formed for each 100 photon absorbed by 100 molecules of \[B{{r}_{2}}\].
\[{{H}_{2}}+B{{r}_{2}}\,\xrightarrow{light}\,2HBr\]
Mechanism
Light absorption step : \[B{{r}_{2}}\,+\,hv\,\to \,B{{r}_{2}}^{*}\] ........(i)
Chain initiation step : \[B{{r}_{2}}^{*}\,\to \,B{{r}^{\bullet }}\,+\,B{{r}^{\bullet }}\] ........(ii)
Chain propagation step : \[B{{r}^{*}}+{{H}_{2}}\,\,\to \,HBr\,+\,{{H}^{\bullet }}\] ........(iii)
\[{{H}^{*}}\,+\,B{{r}_{2}}\,\to \,HBr\,+\,B{{r}^{\bullet }}\] ........(iv)
Chain termination step : \[B{{r}^{\bullet }}+B{{r}^{\bullet }}\to \,B{{r}_{2}}\] ........(v)
The lower values of HBr formation per photon of light absorbed has been attributed to the fact that step (III) is highly endothermic and thus before step (III) can take place most of the bromine free radicals recombine as per step (V) to give \[B{{r}_{2}}\] molecule and thus providing less feasibility for step (IV) i.e. steps regenerating free radicals. Also the decomposition of HBr increases with increase in temperature.
(3) Quantum yield (or quantum efficiency) : The quantum efficiency or yield \[\left( \varphi \right)\] of a photochemical reaction may be expressed as, \[\varphi \,=\,\frac{\text{No}\text{. of molecules reacted or product formed}}{\text{No}\text{.}\ \text{of photon absorbed}}\]
(4) Application of photochemistry : Photochemistry has significant role in our daily life. Some of the photochemical reactions commonly known as cited below,
(i) Photosynthesis in plants
(ii) Photography
(iii) The formation and destruction of ozone layer
(iv) Photoetching in electronic industry
(v) Many polymerization reactions.
(vi) Modern printing technology
(vii) Free radical combinations to obtain many compounds.
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