Nitrogen monoxide reacts with oxygen to form nitrogen dioxide according to the overall balanced equation
2NO(g)+O2(g)→2NO2(g).
A chemist suggests three alternative possibilities for the *first* elementary step in a multistep mechanism that would lead to the overall reaction.
• Step A: NO(g)+NO(g)+O2(g)→2NO2(g)
• Step B: NO(g)+O2(g)→NO3(g)
• Step C: NO(g)+O2(g)→NO2(g)+O(g)
Assume that only one of these alternatives actually occurs as the initial collision event.
Answer the following questions. Use collision theory and kinetic reasoning—not thermodynamic data—in all justifications.
(a) For each proposed elementary step, state its molecularity and briefly describe what that molecularity implies about the number of particles that must collide simultaneously.
(b) Rank the three steps from *most* to *least* probable as the initial elementary process in the reaction mechanism. Justify your ranking with explicit reference to collision frequency, orientation requirements, and the rarity of higher-order simultaneous collisions.
(c) Experimental studies show that the observed rate law for the overall reaction is
rate=k[NO]2[O2].
Explain how this rate law supports or contradicts the ranking you provided in part (b).