Question

2 NO(g) + O₂(g) -> 2 NO₂(g)

Consider the following mechanism for the reaction represented above.
Step 1: 2 NO ⇌ N₂O₂ (fast reversible)

Step 2: NO₂ + O₂ → NO₂O₂ (slow)
Which of the following statements is true?
(A) Step 1 represents a unimolecular reaction.
(B) Increasing the concentration of NO will decrease the overall rate of the reaction.
(C) Raising the temperature will have no effect on the numerical value of the rate constant.
(D) The rate law that is consistent with the mechanism is rate = k[NO][O₂]

Answer 1

Chemical reaction rate laws like rate = k[NO][O₂] are determined experimentally and reflect the step-by-step mechanism of the reaction, which may include several intermediates affecting the overall kinetics.

Step-by-step explanation:

The question pertains to the rate law of the chemical reaction 2 NO(g) + O₂(g) → 2 NO₂(g). The statement that the rate law is rate = k[NO][O₂] would imply a bimolecular reaction where the concentration of both NO and O₂ impact the rate equally, and both are involved in the rate-determining step. However, the mechanism and experimental data provided in the references reveal different rate laws involving various reactants.

For instance, the rate law for the reaction between NO and hydrogen is rate = k[NO]²[H₂] indicating that the reaction is second order with respect to NO and first order with respect to H₂, making the overall order of the reaction third order. This discrepancy highlights that rate laws must be determined experimentally and can reflect complex reaction mechanisms, which may include intermediate steps that alter the overall reaction kinetics.