A research team is studying the gaseous reaction sequence below at 450 K.
Step 1 P + Q ⇌ X (fast)
Step 2 X + Q ⇌ Y (fast)
Step 3 Y → Z (slow)
The overall reaction is P + 2 Q → Z.
Assume that no other elementary steps occur and that all species are in the same homogeneous phase.
(a) Describe the experimental evidence a kineticist would seek to justify classifying Steps 1 and 2 as elementary reactions that are not rate-limiting.
(b) Explain, without using any mathematical derivation, why the rate law for the overall reaction is expected to depend only on the concentrations of P and Q, even though the slow step does not contain P explicitly.
(c) Experimental rate data at 450 K show that doubling [Q] while holding [P] constant leads to a fourfold increase in the initial rate, whereas doubling [P] with [Q] constant only doubles the initial rate. Relate these observations qualitatively to the elementary sequence provided.