The photochemical decomposition of chlorine dioxide, ClO2, in the gas phase is proposed to occur by the following mechanism.
Step 1 ClO2+hν→Cl+O2
Step 2 Cl+ClO2→Cl2+O2 (fast equilibrium)
Step 3 Cl2+hν→2Cl
Step 4 Cl+O2+M→ClO2+M (termolecular, slow)
The symbol hν designates a photon of appropriate energy and M represents any third-body collision partner.
(a) Identify all reaction intermediates in the mechanism and justify each choice based on its production and subsequent consumption.
(b) Using the elementary steps, derive an algebraic expression for the net rate of appearance of O2 in terms of the elementary-step rate constants and the concentrations of the reacting species. Do not eliminate intermediates at this stage.
(c) Apply an appropriate steady-state approximation to substitute for the concentration of any intermediate(s) and obtain a rate law expressed solely in terms of the concentration of ClO2, irradiance (photon flux), and any other quantities directly measurable in an experiment. Clearly state and justify any assumptions you make.
(d) Explain how the presence of the Cl atom as an intermediate provides a mechanistic rationale for the observed increase in the quantum yield of O2 at high light intensities.
(a) Identification of intermediates with justification
(b) Derivation of a rate expression containing intermediates
(c) Application of the steady-state approximation to obtain an experimentally testable rate law
(d) Explanation of the quantum-yield behavior in terms of the intermediate