Chapter 18

Molecules in the upper atmosphere tend to contain double and triple bonds rather than single bonds. Suggest an explanation. [Section 18.2]

You are working with an artist who has been commissioned to make a sculpture for a big city in the eastern United States. The artist is wondering what material to use to make her sculpture, because she has heard that acid rain in the eastern U.S. might destroy it over time. You take samples of granite, marble, bronze, and other materials, and place them outdoors for a long time in the big city. You periodically examine the appearance and measure the mass of the samples. (a) What observations would lead you to conclude that one, or more, of the materials were well-suited for the sculpture? (b) What chemical process (or processes) is (are) the most likely responsible for any observed changes in the materials? [Section 18.4]

How does carbon dioxide interact with the world ocean? [Section 18.5]

Describe the basic goals of green chemistry. [Section 18.7]

(a) What is the primary basis for the division of the atmosphere into different regions? (b) Name the regions of the atmosphere, indicating the altitude interval for each one.

(a) How are the boundaries between the regions of the atmosphere determined? (b) Explain why the stratosphere, which is more than 20 miles thick, has a smaller total mass than the troposphere, which is less than 10 miles thick.

Air pollution in the Mexico City metropolitan area is among the worst in the world. The concentration of ozone in Mexico City has been measured at 441 ppb (0.441 ppm). Mexico City sits at an altitude of 7400 feet, which means its atmospheric pressure is only \(0.67\) atm. Calculate the partial pressure of ozone at 441 ppb if the atmospheric pressure is \(0.67\) atm.

The dissociation energy of a carbon-bromine bond is typically about \(210 \mathrm{~kJ} / \mathrm{mol}\). What is the maximum wavelength of photons that can cause \(\mathrm{C}-\mathrm{Br}\) bond dissociation?

In \(\mathrm{CF}_{3} \mathrm{Cl}\) the \(\mathrm{C}-\mathrm{Cl}\) bond- dissociation energy is \(339 \mathrm{~kJ} / \mathrm{mol} .\) In \(\mathrm{CCl}_{4}\) the \(\mathrm{C}-\mathrm{Cl}\) bond-dissociation energy is \(293 \mathrm{~kJ} / \mathrm{mol}\). What is the range of wavelengths of photons that can cause \(\mathrm{C}-\mathrm{Cl}\) bond rupture in one molecule but not in the other?

(a) Distinguish between photodissociation and photoionization. (b) Use the energy requirements of these two processes to explain why photodissociation of oxygen is more important than photoionization of oxygen at altitudes below about \(90 \mathrm{~km}\).

Why is the photodissociation of \(\mathrm{N}_{2}\) in the atmosphere relatively unimportant compared with the photodissociation of \(\mathrm{O}_{2} ?\)

What is a hydrofluorocarbon? Why are these compounds potentially less harmful to the ozone layer than CFCs?

Draw the Lewis structure for the chlorofluorocarbon CFC-11, CFCl \(_{3}\). What chemical characteristics of this substance allow it to effectively deplete stratospheric ozone?

(a) Why is the fluorine present in chlorofluorocarbons not a major contributor to depletion of the ozone layer? (b) What are the chemical forms in which chlorine exists in the stratosphere following cleavage of the carbonchlorine bond?

Would you expect the substance \(\mathrm{CFBr}_{3}\) to be effective in depleting the ozone layer, assuming that it is present in the stratosphere? Explain.

For each of the following gases, make a list of known or possible naturally occurring sources: (a) \(\mathrm{CH}_{4}\), (b) \(\mathrm{SO}_{2}\), (c) \(\mathrm{NO}\), (d) \(\mathrm{CO}\).

Why is rainwater naturally acidic, even in the absence of polluting gases such as \(\mathrm{SO}_{2} ?\)

(a) Write a chemical equation that describes the attack of acid rain on limestone, \(\mathrm{CaCO}_{3}\), (b) If a limestone sculpture were treated to form a surface layer of calcium sulfate, would this help to slow down the effects of acid rain? Explain.

The first stage in corrosion of iron upon exposure to air is oxidation to \(\mathrm{Fe}^{2+} .\) (a) Write a balanced chemical equation to show the reaction of iron with oxygen and protons from acid rain. (b) Would you expect the same sort of reaction to occur with a silver surface? Explain.

Alcohol-based fuels for automobiles lead to the production of formaldehyde \(\left(\mathrm{CH}_{2} \mathrm{O}\right)\) in exhaust gases. Formaldehyde undergoes photodissociation, which contributes to photochemical smog: $$\mathrm{CH}_{2} \mathrm{O}+h \nu \longrightarrow \mathrm{CHO}+\mathrm{H}$$ The maximum wavelength of light that can cause this reaction is \(335 \mathrm{~nm} .\) (a) In what part of the electromagnetic spectrum is light with this wavelength found? (b) What is the maximum strength of a bond, in \(\mathrm{kJ} / \mathrm{mol}\), that can be broken by absorption of a photon of \(335-\mathrm{nm}\) light? (c) Compare your answer from part (b) to the appropriate value from Table \(8.4\). What do you conclude about the \(\mathrm{C}-\mathrm{H}\) bond energy in formaldehyde? (d) Write out the formaldehyde photodissociation reaction, showing Lewis-dot structures.

An important reaction in the formation of photochemical smog is the photodissociation of \(\mathrm{NO}_{2}\) : $$\mathrm{NO}_{2}+h \nu \longrightarrow \mathrm{NO}(g)+\mathrm{O}(g)$$ The maximum wavelength of light that can cause this reaction is \(420 \mathrm{~nm} .(\mathrm{a})\) In what part of the electromagnetic spectrum is light with this wavelength found? (b) What is the maximum strength of a bond, in \(\mathrm{kJ} / \mathrm{mol}\), that can be broken by absorption of a photon of \(420-\mathrm{nm}\) light? (c) Write out the photodissociation reaction showing Lewis-dot structures.

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

(a) With respect to absorption of radiant energy, what distinguishes a greenhouse gas from a nongreenhouse gas? (b) \(\mathrm{CH}_{4}\) is a greenhouse gas, but \(\mathrm{Ar}\) is not. How might the molecular structure of \(\mathrm{CH}_{4}\) explain why it is a greenhouse gas?

What is the molarity of \(\mathrm{Na}^{+}\) in a solution of \(\mathrm{NaCl}\) whose salinity is \(5.6\) if the solution has a density of \(1.03 \mathrm{~g} / \mathrm{mL}\) ?

Phosphorus is present in seawater to the extent of \(0.07 \mathrm{ppm}\) by mass. If the phosphorus is present as phosphate, \(\mathrm{PO}_{4}{\underline{\phantom{xx}}}^{3-}\), calculate the corresponding molar concentration of phosphate in seawater.

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 \(\mathrm{Mg}(\mathrm{OH})_{2} ?\)

Gold is found in seawater at very low levels, about \(0.05\) ppb by mass. Assuming that gold is worth about $$\$ 800$$ per troy ounce, how many liters of seawater would you have to process to obtain $$\$ 1,000,000$$ worth of gold? Assume the density of seawater is \(1.03 \mathrm{~g} / \mathrm{mL}\) and that your gold recovery process is \(50 \%\) efficient.

List the common products formed when an organic material containing the elements carbon, hydrogen, oxygen, sulfur, and nitrogen decomposes (a) under aerobic conditions, (b) under anaerobic conditions.

(a) Explain why the concentration of dissolved oxygen in freshwater is an important indicator of the quality of the water. (b) How is the solubility of oxygen in water affected by increasing temperature?

The organic anion is found in most detergents. Assume that the anion undergoes aerobic decomposition in the following manner: $$ \begin{array}{r} 2 \mathrm{C}_{18} \mathrm{H}_{29} \mathrm{SO}_{3}^{-}(a q)+51 \mathrm{O}_{2}(a q) \longrightarrow \\ 36 \mathrm{CO}_{2}(a q)+28 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{H}^{+}(a q)+2 \mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}(a q) \end{array} $$ What is the total mass of \(\mathrm{O}_{2}\) required to biodegrade \(1.0 \mathrm{~g}\) of this substance?

The average daily mass of \(\mathrm{O}_{2}\) taken up by sewage discharged in the United States is \(59 \mathrm{~g}\) per person. How many liters of water at \(9 \mathrm{ppm} \mathrm{O}_{2}\) are totally depleted of oxygen in 1 day by a population of 120,000 people?

Write a balanced chemical equation to describe how magnesium ions are removed in water treatment by the addition of slaked lime, \(\mathrm{Ca}(\mathrm{OH})_{2}\).

(a) Which of the following ionic species could be, responsible for hardness in a water supply: \(\mathrm{Ca}^{2+}, \mathrm{K}^{+}\), \(\mathrm{Mg}^{2+}, \mathrm{Fe}^{2+}, \mathrm{Na}^{+} ?(\mathrm{~b})\) What properties of an ion determine whether it will contribute to water hardness?

How many moles of \(\mathrm{Ca}(\mathrm{OH})_{2}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) should be added to soften \(1200 \mathrm{~L}\) of water in which \(\left[\mathrm{Ca}^{2+}\right]=5.0 \times 10^{-4} \mathrm{M}\) and \(\left[\mathrm{HCO}_{3}^{-}\right]=7.0 \times 10^{-4} \mathrm{M} ?\)

The concentration of \(\mathrm{Ca}^{2+}\) in a particular water supply is \(5.7 \times 10^{-3} \mathrm{M} .\) The concentration of bicarbonate ion, \(\mathrm{HCO}_{3}^{-}\), in the same water is \(1.7 \times 10^{-3} \mathrm{M}\). What masses of \(\mathrm{Ca}(\mathrm{OH})_{2}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) must be added to \(5.0 \times 10^{7} \mathrm{~L}\) of this water to reduce the level of \(\mathrm{Ca}^{2+}\) to \(20 \%\) of its original level?

Ferrous sulfate \(\left(\mathrm{FeSO}_{4}\right)\) is often used as a coagulant in water purification. The iron(II) salt is dissolved in the water to be purified, then oxidized to the iron(III) state by dissolved oxygen, at which time gelatinous \(\mathrm{Fe}(\mathrm{OH})_{3}\) forms, assuming the \(\mathrm{pH}\) is above approximately \(6 .\) Write balanced chemical equations for the oxidation of \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\) by dissolved oxygen, and for the formation of \(\mathrm{Fe}(\mathrm{OH})_{3}(\mathrm{~s})\) by reaction of \(\mathrm{Fe}^{3+}(a q)\) with \(\mathrm{HCO}_{3}^{-}(a q)\)

What properties make a substance a good coagulant for water purification?

One of the principles of green chemistry is that it is better to use as few steps as possible in making new chemicals. How does this principle relate to energy efficiency?

Discuss how catalysts can make processes more energy efficient.

The Baeyer-Villiger reaction is a classic organic oxidation reaction for converting ketones to lactones, as in this reaction: The reaction is used in the manufacture of plastics and pharmaceuticals. The reactant 3 -chloroperbenzoic acid is somewhat shock sensitive, however, and prone to explode. Also, 3 -chlorobenzoic acid is a waste product. An alternative process being developed uses hydrogen peroxide and a catalyst consisting of tin deposited within a solid support. The catalyst is readily recovered from the reaction mixture. (a) What would you expect to be the other product of oxidation of the ketone to lactone by hydrogen peroxide? (b) What principles of green chemistry are addressed by use of the proposed process?

The reaction shown here was performed with an iridium catalyst, both in supercritical \(\mathrm{CO}_{2}\left(\mathrm{scCO}_{2}\right)\) and in the chlorinated solvent \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\). The kinetic data for the reaction in both solvents are plotted in the graph. Why is this a good example of a green chemical reaction?

A friend of yours has seen each of the following items in newspaper articles and would like an explanation: (a) acid rain, (b) greenhouse gas, (c) photochemical smog, (d) ozone depletion. Give a brief explanation of each term, and identify one or two of the chemicals associated with each.

Supposethat on another planet the atmosphere consists of \(17 \% \mathrm{Kr}, 38 \% \mathrm{CH}_{4}\), and \(45 \% \mathrm{O}_{2}\). What is theaverage molar mass at the surface? What is the average molar mass at an altitude at which all the \(\mathrm{O}_{2}\) is photodissociated?

If an average \(\mathrm{O}_{3}\) molecule "lives" only \(100-200\) seconds in the stratosphere before undergoing dissociation, how can \(\mathrm{O}_{3}\) offer any protection from ultraviolet radiation?

What properties of CFCs make them ideal for various commercial applications but also make them a longterm problem in the stratosphere?

It is estimated that the lifetime for HFCs in the stratosphere is \(2-7\) years. If HFCs have such long lifetimes, why are they being used to replace CFCs?

The hydroxyl radical, \(\mathrm{OH}\), is formed at low altitudes via thereaction 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) \(\mathrm{OH}+\mathrm{CH}_{4} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CH}_{3}\) \(\mathrm{CH}_{3}+\mathrm{O}_{2} \longrightarrow \mathrm{OOCH}_{3}\) \(\mathrm{OOCH}_{3}+\mathrm{NO} \longrightarrow \mathrm{OCH}_{3}+\mathrm{NO}_{2}\)

Explain, using Le Châtelier's principle, why the equilibrium constant for the formation of \(\mathrm{NO}\) from \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) increases with increasing temperature, whereas the equilibrium constant for the formation of \(\mathrm{NO}_{2}\) from NO and \(\mathrm{O}_{2}\) decreases with increasing temperature.

Natural gas consists primarily of methane, \(\mathrm{CH}_{4}(\mathrm{~g})\). (a) Write a balanced chemical equation for the complete combustion of methane to produce \(\mathrm{CO}_{2}(g)\) as the only carbon-containing product. (b) Write a balanced chemical equation for the incomplete combustion of methane to produce \(\mathrm{CO}(g)\) as the only carbon-containing product. (c) At \(25^{\circ} \mathrm{C}\) and \(1.0\) atm pressure, what is the minimum quantity of dry air needed to combust \(1.0 \mathrm{~L}\) of \(\mathrm{CH}_{4}(\mathrm{~g})\) completely to \(\mathrm{CO}_{2}(\mathrm{~g})\) ?

One of the possible consequences of global warming is an increase in the temperature of ocean water. The oceans serve as a "sink" for \(\mathrm{CO}_{2}\) by dissolving large amounts of it. (a) How would the solubility of \(\mathrm{CO}_{2}\) in the oceans be affected by an increase in the temperature of the water? (b) Discuss the implications of your answer to part (a) for the problem of global warming.