A researcher is studying the gas–phase reaction X + Y + Z → products. Spectroscopic evidence suggests the reaction proceeds through the following two-step mechanism at 400 K:
Step 1 X + Y ⇌ I (fast, reversible)
Step 2 I + Z → products (slow)
The rate law derived from this mechanism successfully predicts the initial rate over a moderate range of reactant concentrations, but discrepancies appear when the concentration of Z is increased to very high values.
(a) Describe two distinct assumptions that must be satisfied for the rate law obtained from the mechanism above to remain valid when the pre-equilibrium approximation is applied.
(b) Qualitatively explain how a very large initial concentration of Z could cause the derived rate law to deviate from experimental observations. In your explanation, address the role of the intermediate I.
(c) Identify one experimental observation—other than a deviation between predicted and measured rates—that would indicate the pre-equilibrium approximation is no longer valid under the high-[Z] conditions. Justify how the observation supports your claim.