Review Questions

The questions below, and the comparable ones in succeeding chapters, are designed to test your knowledge mainly of some of the factual material presented in the chapter.

The problems within the chapter, and the more elaborate ones given below as Additional Problems, are designed to test your problem-solving abilities.

1. Which three gases constitute most of Earth's atmosphere?

2. What range of altitudes constitutes the troposphere? The stratosphere?

3. What is the wavelength range for visible light? Does ultraviolet light have shorter or longer wavelengths than visible light?

4. Which atmospheric gas is primarily responsible for filtering sunlight in the 120-220-nm region? Which, if any, gas absorbs most of the Sun's rays in the 220-320-nm region? Which absorbs primarily in the 320-400-nm region?

5. What is the name given to the finite packets of light absorbed by matter?

6. What are the equations relating photon energy E to light's frequency v and wavelength A?

7. What is meant by the expression photochemically dissociated as applied to stratospheric 02?

8. Write the equation for the chemical reaction by which ozone is formed in the stratosphere. What are the sources for the different forms of oxygen used here as reactants?

9. Write the two reactions that, in addition to the catalyzed reactions, contribute most significantly to ozone destruction in the stratosphere.

10. What is meant by the phrase excited state as applied to an atom or molecule? Symbolically, how is an excited state signified?

11. Explain why the phrase ozone layer is a misnomer.

12. Define the term free radical, and give two examples relevant to stratospheric chemistry.

13. What are the two steps, and the overall reaction, by which a species X, such as CIO, catalyti-cally destroys ozone in the middle and upper stratosphere via Mechanism I ?

14. What is meant by the term steady state as applied to the concentration of ozone in the stratosphere?

15. Explain why, atom for atom, stratospheric bromine destroys more ozone than does chlorine.

16. Explain why ozone destruction via the reaction of 03 with atomic oxygen does not occur to a significant extent in the lower stratosphere.

Additional Problems

1. A possible additional mechanism that could exist for the creation of ozone in the high stratosphere begins with the creation of (vibrationally) excited 02 and ground-state atomic oxygen from the absorption by ozone of photons with wavelengths less than 243 nm. The 02* reacts with a ground-state 02 molecule to produce ozone and another atom of oxygen. What is the net reaction from these two steps? What do you predict is the fate of the two oxygen atoms, and what would be the overall reaction once this fate is included?

2. In the nonpolluted atmosphere, an important mechanism for ozone destruction in the lower stratosphere is:

Does this pair of steps correspond to Mechanism I ? If not, what is the overall reaction?

3. A proposed mechanism for ozone destruction in the late spring over northern latitudes in the lower stratosphere begins with the photochemical decomposition of C10N02 to CI and N03, followed by photochemical decomposition of the latter to NO and 02. Deduce a catalytic ozone destruction cycle, requiring no atomic oxygen, that incorporates these reactions. What is the overall reaction?

4. Deduce possible reaction step(s), none of which involve photolysis, for Mechanism II that follow(s)

the X + 03-» XO + 02 step, such that the sum of all the mechanism's steps does not destroy or create any ozone.

5. As will be discussed in Chapter 2, atomic chlorine is produced under ozone-hole conditions by the dissociation of diatomic chlorine, Cl7. Given that diatomic chlorine gas is the stablest form of the element and that the AH° value for atomic chlorine is +121.7 kj molcalculate the maximum wavelength of light that can dissociate diatomic chlorine into the monatomic form. Does such a wavelength correspond to light in the visible or the UV-A or the UV-B region?

6. Under conditions of low oxygen atom concentration, the radical HOO can react reversibly with NG2 to produce a molecule of H00N02:

(a) Deduce why the addition of nitrogen oxides to the lower stratosphere could lead to an increase in the steady-state ozone concentration as a consequence of this reaction.

(b) Deduce how the addition of nitrogen oxides to the middle and upper stratosphere could decrease the ozone concentration there as a consequence of other reactions.

(c) Given the information stated in parts (a) and (b), in what regions of the stratosphere should supersonic transport airplanes fly if they emit substantial amounts of nitrogen oxides in their exhaust?

7. At an altitude of about 35 kilometers, the average concentrations of O* and of CH4 are approximately 100 and 1 X 1011 molecules cm"3, respectively; the rate constant k for the reaction between them is approximately 3 X 10"lu cmJ molecules"1 s"1. Calculate the rate of destruction of methane in molecules per second per cubic centimeter and in grams per year per cubic centimeter under these conditions. [Hint; Recall that the rate law for a simple process is its rate constant k times the product of the concentrations of its reactant concentrations.]

8. The rate constants for the reactions of atomic chlorine and of hydroxy 1 radical with ozone are given by 3 X lO^11 e"250/T and 2 X 10"12 e"940/T, where T is the Kelvin temperature. Calculate the ratio of the rates of ozone destruction by these catalysts at 20 km, given that at this altitude the average concentration of OH is about 100 times that of CI and that the temperature is about

—50°C. Calculate the rate constant for ozone destruction by chlorine under conditions in the Antarctic ozone hole, when the temperature is about — 80°C and the concentration of atomic chlorine increases by a factor of 100 to about 4 X 105 molecules per cubic centimeter and that of 03 is 2 X 1012 molecules/cm3.

9. The Arrhenius equation (see your introductory chemistry textbook) relates reaction rates to temperature via the activation energy. Calculate the ratio of the rates at —30°C (a typical stratospheric temperature) for two reactions having the same Arrhenius A factor and initial concentrations, one of which is endothermic and has an activation energy of 30 kj mol 1 and the other which is exothermic with an activation energy of 3 k] mol 1. In energy units, R = 8.3 | K 1 mol

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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Responses

  • Joonas
    Which atmospheric gas is primarily responsible for filtering sunlight in the 120220nm region ?
    3 years ago
  • belisarius
    Which atmospheric gas is primarily responsible for filtering sunlight in the 120220 nm region?
    3 years ago
  • stephanie
    What is meant by the term steady state as applied to the concentration of ozone in stratosphere?
    2 years ago

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