Surface-Catalyzed Unimolecular Reaction Kinetics

A surface-catalyzed unimolecular gaseous reaction involves a single reactant molecule adsorbing onto a catalyst surface and then reacting to form products. The reaction rate is influenced by factors such as reactant pressure and the degree of surface coverage.

Analysis of Correct Statements

• Statement B: Zero order kinetics at high pressure
  When the pressure of the reacting gas is sufficiently high, the catalyst surface becomes saturated with adsorbed reactant molecules. In this saturation regime, the surface concentration of adsorbed species is maximal and essentially constant. The rate-limiting step, whether it's the reaction of adsorbed molecules or their desorption, no longer depends on the external gas pressure. Consequently, the reaction exhibits zero-order kinetics with respect to the reactant pressure.
  Rate $\approx k$ (constant) at high P
• Statement C: First order kinetics at low pressure
  Conversely, at very low pressures, the catalyst surface is sparsely populated with adsorbed reactant molecules (low surface coverage, $\theta$). The rate of reaction often depends on the number of adsorbed molecules and the availability of vacant surface sites. Under these dilute conditions, the rate is typically found to be directly proportional to the gas pressure ($P$), leading to first-order kinetics.
  Rate $\propto P$ at low P
• Statement D: Rate proportional to surface coverage
  The rate of a surface reaction is fundamentally linked to the amount of reactant adsorbed on the catalyst surface. If the reaction mechanism involves adsorbed species, the rate ($r$) is often directly proportional to the fraction of the surface covered by the reactant molecules ($\theta$).
  r $\propto \theta$ This relationship holds under varying conditions, reflecting the dependence on available adsorbed reactant.

Explanation for Statement A

• Statement A: Freundlich adsorption isotherm
  The behavior described by statements B and C (pressure-dependent reaction orders) is typically derived using the Langmuir adsorption isotherm model, which assumes monolayer adsorption on a homogeneous surface. While Freundlich adsorption isotherm applies to non-ideal adsorption scenarios, the Langmuir model provides the standard framework for explaining the kinetics of simple surface-catalyzed unimolecular reactions. Therefore, stating it follows Freundlich isotherm is generally not the primary or most accurate description in this context.