Consider a typical photochemical gas-phase chain reaction, such as the classical textbook example . — Chemical Kinetics Chemistry Question
Question
Consider a typical photochemical gas-phase chain reaction, such as the classical textbook example $H_2(g) + Cl_2(g) \xrightarrow{h u} 2HCl(g)$. Which of the following statements are kinetically and mechanistically valid for this specific process?
💡 Solution & Explanation
Photochemical reactions like the formation of $HCl$ from $H_2$ and $Cl_2$ are classically zero-order kinetics because the rate relies on the intensity of the incident light photons absorbed, not the concentration of the reactants (A and B are correct). The required initiation step involves absorbing a photon to break a specific bond, e.g., $Cl_2 + h u \rightarrow 2Cl^\bullet$ (C is correct). Because the driving activation energy is provided by photons and not ambient thermal collisions, the rate is practically independent of temperature, strongly violating the standard Arrhenius dependence (D is false).