The gas NO is released to the stratosphere by jet engines. Because it can contribute to the destruction of stratospheric ozone, its concentration is monitored closely. One monitoring technique measures the chemiluminescence given off by the reaction NO(g) + O₃(g) → NO₂*(g) + O₂(g). The asterisk indicates that NO₂ is produced in an excited state. The excited NO₂ molecule gives off red light and infrared radiation as it returns to the ground state. Because both NO and NO₂ may be present in the air sample, part of the sample is exposed to a reducing agent, which converts all the NO₂ to NO. Two samples are then analyzed, first air from the unreduced original sample, which gives the concentration of NO, then air from the reduced sample. The concentration of NO₂ in the air is the difference between the two measurements. In one experiment, two samples were collected, one from a cloud and one from clear air. The following measurements were made at 9 km above the Pacific Ocean (ppt denotes parts per trillion, 1 part per 10¹², by molecules):

(a) Determine the concentrations (in ppt) of NO and NO₂ in each air sample.

(b) In which part of the atmosphere is the concentration of NO greater?

(c) Explain the difference, taking into account other sources of NO in the atmosphere besides airplanes.

(d) Some of the NO molecules released by jet engines react with the hydroxyl radical, ·OH. Draw the Lewis structure of the most likely product and state its name.

(e) Is the product of the reaction in part (d) likely to be more stable than NO? Explain your reasoning.

(f) In the stratosphere, NO catalyzes the conversion of O₃ to O₂ in a two-step reaction with the intermediate NO₂. The overall equation for the reaction is O₃(g) + O(g) → 2 O₂(g) and the rate law for the reaction is Rate = k_r[NO][O₃]. Write an acceptable two-step mechanism for the reaction, indicating which step is the slow step.

(g) At 230°C, the temperature of the air from which the samples were taken, the rate constant for the reaction in part (f) is 6 * 10^–15 cm³ · molecule^–1 · s^–1. If the concentration of ozone in the air from which the NO clear air sample was taken was 480 ppt and the total pressure of the air was 220 mbar, what was the rate at which ozone was being destroyed in the air at 230°C?

Transcribed Image Text:

Cloud Clear air Unreduced sample 860 ppt 480 ppt Reduced sample 1110 ppt 740 ppt