Question

For a photochemical reaction, if $I$ be the intensity of absorbed light and C is Concentration of Reactant ($x$). Then the photochemical reaction $x+h\nu \to {{x}^{0}}(\operatorname{Product})$. The rate of formation of ${{x}^{0}}$ is directly proportional to:
(A) C
(B) $I$
(C) ${{I}^{2}}$
(D) C$I$

Answer 1

Photochemical reaction is chemical reaction which takes place when light is absorbed. Photo means light. Photochemical reactions are independent of concentration of reactions. The molecules absorb energy and reach an excited state.
Complete step by step answer:
- The rate of photochemical reaction is independent of concentration of reaction, as concentration of reaction decreases with time, it depends on experimental conditions.
- The concentration of reaction is not an intrinsic characteristic of a photochemical reaction.
- The rate of photochemical reaction depends on intensity of absorbed light
- To initiate a photochemical reaction, photons must have sufficient energy to initiate the reaction.
- The light must have energy greater than required to initiate the reaction, remaining energy determines the kinetic energy of electrons.
$\begin{aligned}& E=h\nu_o+\text{ }K.E. \\\ \end{aligned}$
The light should have frequency greater than ${{\nu }_{o}}$ in order to initiate reaction, remaining energy determines the kinetic energy of electrons.

So, the rate of formation of ${{x}^{o}}$ is directly proportional to $I$, so option B is correct.

Note: If the frequency of light is less than ${{\nu }_{o}}$the reaction will not take place.
Often law of mass action is applied which states that Rate of reaction is directly proportional to product of concentration of reaction their power raised to stoichiometric coefficient in balanced equation.in photochemical reaction, light is required to initiate a reaction. In absence of light, reaction will take place. So, photochemical reaction is dependent on light energy and not on concentration of reaction.
Photosynthesis is an example of photochemical reaction.