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Chapter 18: Problem 70

The hydroxyl radical, \(\mathrm{OH}\), is formed at low altitudes via the reaction of excited oxygen atoms with water: $$ \mathrm{O}^{*}(g)+\mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{OH}(g) $$ (a) Write the Lewis structure for the hydroxyl radical. (Hint: It has one unpaired electron.) Once produced, the hydroxyl radical is very reactive. Explain why each of the following series of reactions affects the pollution in the troposphere: (b) \(\mathrm{OH}+\mathrm{NO}_{2} \longrightarrow \mathrm{HNO}_{3}\) (c) \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}+\mathrm{OOH}\) \(\mathrm{OOH}+\mathrm{NO} \longrightarrow \mathrm{OH}+\mathrm{NO}_{2}\) (d) \(\mathrm{OH}+\mathrm{CH}_{4} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CH}_{3}\) \(\mathrm{CH}_{3}+\mathrm{O}_{2} \longrightarrow \mathrm{OOCH}_{3}\) \(\mathrm{OOCH}_{3}+\mathrm{NO} \longrightarrow \mathrm{OCH}_{3}+\mathrm{NO}_{2}\) (e) The concentration of hydroxyl radicals in the troposphere is approximately \(2 \times 10^{6}\) radicals per \(\mathrm{cm}^{3}\). This estimate is based on a method called long path absorption spectroscopy (LPAS), similar in principle to the Beer's law measurement discussed in the Closer Look essay on p. 564 , except that the path length in the LPAS measurement is \(20 \mathrm{~km}\). Why must the path length be so large?

Short Answer

Expert verified
The hydroxyl radical, OH, has the Lewis structure O-H with an additional unpaired electron on the oxygen atom. The reactions mentioned affect troposphere pollution as follows: (b) The reaction between OH and NO₂ forms nitric acid, contributing to acid rain; (c) OH, CO, and O₂ react to form CO₂ and peroxy radicals, which then react with NO to form NO₂, converting pollutants into other species; (d) The series of reactions involving OH and CH₄ leads to the formation of CO₂ and H₂O, partially mitigating methane's effects as a greenhouse gas. Lastly, the path length for LPAS has to be 20 km because hydroxyl radicals have low concentrations in the troposphere, and a long path length increases absorption for accurate measurements.

Step by step solution

01

(a) Draw the Lewis Structure for the Hydroxyl Radical, \(\mathrm{OH}\)

To draw the Lewis structure for the hydroxyl radical, begin by counting the total number of valence electrons. Oxygen has 6 valence electrons, and hydrogen has 1. Together, the radical has 7 valence electrons. Connect the oxygen and hydrogen atoms with a single bond (representing 2 electrons) and draw three lone pairs (2 electrons each) around the oxygen atom. The remaining one electron will be unpaired in the oxygen atom, representing the radical electron. The Lewis structure for the hydroxyl radical looks as follows: O / \ H • / \ • •
02

(b) Explain the Reaction \(\mathrm{OH}+\mathrm{NO}_{2} \longrightarrow \mathrm{HNO}_{3}\) in Relation to Troposphere Pollution

This reaction represents hydroxyl radicals reacting with nitrogen dioxide to form nitric acid. Nitric acid is a significant contributor to acid rain, which leads to various environmental problems such as acidification of soil and water bodies. By forming nitric acid, the reaction removes the highly reactive hydroxyl radicals and nitrogen dioxide species from the troposphere, potentially reducing pollution.
03

(c) Explain the Reactions Involving \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2}\) and \(\mathrm{OOH}+\mathrm{NO}\) in Tropospheric Pollution

The first reaction, \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}+\mathrm{OOH}\), represents hydroxyl radicals reacting with carbon monoxide and oxygen to form carbon dioxide and peroxy radicals. Carbon dioxide is a greenhouse gas that contributes to global warming. In the second reaction, \(\mathrm{OOH}+\mathrm{NO} \longrightarrow \mathrm{OH}+\mathrm{NO}_{2}\), peroxy radicals react with nitrogen monoxide to form the hydroxyl radical and nitrogen dioxide. The sequence of these reactions ultimately converts carbon monoxide and nitrogen monoxide, both pollutants, into other species that may contribute to pollution.
04

(d) Analyze the Series of Reactions Involving \(\mathrm{OH}+\mathrm{CH}_{4}\), \(\mathrm{CH}_{3}+\mathrm{O}_{2}\), and \(\mathrm{OOCH}_{3}+\mathrm{NO}\)

These reactions represent the oxidation of methane by hydroxyl radicals. Methane is a significant greenhouse gas that contributes to global warming. The series of reactions eventually lead to the formation of carbon dioxide and water, partially mitigating the effects of methane as a greenhouse gas.
05

(e) Explain the Need for \(20\mathrm{~km}\) Path Length in LPAS for Hydroxyl Radicals Measurement

The path length in long path absorption spectroscopy (LPAS) must be large (20 km) to measure the concentration of hydroxyl radicals because hydroxyl radicals have relatively low concentrations in the troposphere (approximately \(2 \times 10^{6}\) radicals per \(\mathrm{cm}^{3}\)). A large path length increases absorption of light by the hydroxyl radicals, making the measurement more accurate and reliable.

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Most popular questions from this chapter

A first-stage recovery of magnesium from seawater is precipitation of \(\mathrm{Mg}(\mathrm{OH})_{2}\) with \(\mathrm{CaO}\) : \(\mathrm{Mg}^{2+}(a q)+\mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Mg}(\mathrm{OH})_{2}(s)+\mathrm{Ca}^{2+}(a q)\) What mass of \(\mathrm{CaO}\), in grams, is needed to precipitate \(1000 \mathrm{lb}\) of \(\mathrm{Mg}(\mathrm{OH})_{2} ?\)

Explain why increasing concentrations of \(\mathrm{CO}_{2}\) in the atmosphere affect the quantity of energy leaving Earth but do not affect the quantity of energy entering from the Sun.

The rate of solar energy striking Earth averages 168 watts per square meter. The rate of energy radiated from Earth's surface averages 390 watts per square meter. Comparing these numbers, one might expect that the planet would cool quickly, yet it does not. Why not?

In 1986 an electrical power plant in Taylorsville, Georgia, burned 8,376,726 tons of coal, a national record at that time. (a) Assuming that the coal was \(83 \%\) carbon and \(2.5 \%\) sulfur and that combustion was complete, calculate the number of tons of carbon dioxide and sulfur dioxide produced by the plant during the year. (b) If \(55 \%\) of the \(\mathrm{SO}_{2}\) could be removed by reaction with powdered \(\mathrm{CaO}\) to form \(\mathrm{CaSO}_{3},\) how many tons of \(\mathrm{CaSO}_{3}\) would be produced?

The Ogallala aquifer is the largest in the United States, covering \(450,000 \mathrm{~km}^{2}\) across eight states, from South Dakota to Texas. This aquifer provides \(82 \%\) of the drinking water for the people who live in this region, although most \((>75 \%)\) of the water that is pumped from it is for irrigation. Irrigation withdrawals are approximately 18 billion gallons per day. (a) The Ogallala aquifer might run dry, according to some estimates, in 25 years. How many cubic kilometers of water would be withdrawn in a 25 -year period? (b) Explain the processes that would recharge the aquifer.
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