Question

Explain why photochemical reactions are generally zero order?

Answer 1

Reaction orders for elementary (one-step) reactions and reaction steps are equal to the stoichiometric coefficients for each reactant. The molecularity of the elementary reaction is always equal to the overall reaction order, which is the sum of stoichiometric coefficients of reactants. Complex (multi-step) reactions, on the other hand, could or might not have reaction orders that are equal to their stoichiometric coefficients. Because an unknown reaction mechanism might be either elementary or complicated, the order and rate equation of a particular reaction cannot be safely derived from the stoichiometry and must be established empirically.

Complete answer:
The reaction rate of zero-order reactions is independent of the concentration of a reactant, therefore increasing its concentration has no influence on the reaction's pace. As a result, concentration varies in a linear fashion over time. This can happen when there's a bottleneck that restricts the number of reactant molecules that can react at once, such as when the reaction necessitates contact with an enzyme or a catalytic surface.
Many enzyme-catalyzed processes are zero order if the reactant concentration is significantly higher than the enzyme concentration that regulates the rate, causing the enzyme to become saturated.
${{M}^{o}}+hv\rightleftharpoons {{M}^{+}}$
Collisions must occur often and with enough energy to surpass an activation barrier for a conventional collision-based response to occur. A photochemical reaction is a light-induced reaction in which every molecule on which the light is shone conducts the reaction via absorption of light of a certain wavelength. Due to the quantization of energy levels, only the precise correct wavelength will allow the photochemical reaction to occur. As a result, the sole question is whether the molecules conduct the reaction or not, not whether they are close to doing so.
As a result, it doesn't matter what concentration your molecules are at, because photochemical reactions only require either breaking over the activation barrier or not.
In reality, if concentration is irrelevant, the rate law must be zero-order dependent on concentration.

Note:
Photochemistry is crucial in nature since it is the foundation of photosynthesis, vision, and the production of vitamin D from sunshine. Temperature-driven reactions are not the same as photochemical reactions. Photochemical pathways get access to high-energy intermediates that cannot be produced thermally, allowing reactions that would normally be inaccessible via thermal processes to overcome significant activation barriers in a short amount of time. The photodegradation of polymers exemplifies how damaging photochemistry can be.