When the rate of the reaction 2NO+O2=2NO2 was studied, the rate was found to double when the O2 concentration alone was doubled but to quadruple when the NO concentration alone was doubled. Which of the following mechanisms accounts for these observations?
a.) Step 1: NO + O2=NO3 (and its reverse)
Step 2 (slow): NO+NO3=NO2+NO2
b.) Step 1 (slow): NO+NO=N2O2
Step 2: O2+N2O2=N2O4
Step 3: N2O4=NO2+NO2
To determine which mechanism accounts for the observations, we need to compare the proposed mechanisms with the experimental results.
In the first mechanism (a), the rate-determining step is the second step, which is slow. The rate-determining step involves the reaction between NO and NO3 to form NO2 and NO2. When the O2 concentration alone is doubled, it does not affect the rate-determining step, as there is no direct interaction between O2 and NO3 in this mechanism. Therefore, the rate doubling when O2 concentration is doubled does not support mechanism (a).
In the second mechanism (b), the rate-determining step is the first step, which is slow. The rate-determining step involves the reaction between NO and NO to form N2O2. When the NO concentration alone is doubled, it would directly affect the rate-determining step by increasing the concentration of the reactant NO. This would lead to a quadruple increase in the rate, as observed in the experiment. Therefore, the quadrupling of the rate when the NO concentration is doubled supports mechanism (b).
Based on the observations, mechanism (b) is the most likely explanation for the experimental results.