Consider the gas-phase reaction between nitric oxide and oxygen showing the initial concentrations o — Kinetics Chemistry Question
Problem Context
Consider the gas-phase reaction between nitric oxide and oxygen showing the initial concentrations of the reactants at a certain temperature, T:
2 NO(g) + O2(g) 2 NO2(g)
Determine the order with respect to NO.
Model Answer
Comparing Expt. 1 and 3, doubling [NO] while keeping [O2] constant shows a quadrupling of the rate. So reaction is second order in NO.
Determine the order with respect to O2.
Model Answer
Comparing Expt. 3 to Expt. 2, doubling [O2] while keeping [NO] constant shows a doubling of the rate. So reaction is first order in O2.
Calculate the rate constant and give its units at this temperature.
Model Answer
rate = k[NO]2 [O2]1
0.057 M/s = k(.020 M)2 (0.020 M)1
k = 7.1 x 103 M–2 s–1
Calculate the initial rate of disappearance of O2(g) in units of M•s –1 for experiment 3.
Model Answer
Rate of disappearance of O2 is half of the rate of formation of NO, so 0.114 M s –1.
If this reaction follows a two-step mechanism with the first step being 2 NO N2O2
i. Write an equation for the second step of the mechanism.
Model Answer
Step 1 2 NO N2O2
Step 2 N2O2 + O2 2NO2
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Overall 2 NO + O2 2 NO2
ii. Identify the rate determining step of this mechanism and outline your reasoning.
Model Answer
If step 1 is rate-limiting, then the predicted rate law is k[NO]2 [O2]0 . If step 2 is rate-limiting, then the predicted rate law is k[NO]2 [O2]1 . The latter is observed experimentally.