Chemistry in the Atmosphere - Flashcards

The Nitrogen Cycle and Nitrogen Fixation

• The nitrogen cycle is a complex biogeochemical process involving the movement of nitrogen between the atmosphere, biosphere, and lithosphere through various chemical forms.

• Key Components of the Nitrogen Cycle:

  • Atmospheric nitrogen: The primary reservoir of nitrogen ($N_2$).

  • Atmospheric fixation: Conversion of atmospheric nitrogen into nitrogen oxides via electrical energy (lightning).

  • Industrial fixation: Artificial conversion of nitrogen to ammonia using catalysts.

  • Biological fixation: Conversion of nitrogen into organic forms by organisms.

  • Key Nitrogen Species: Ammonium ($NH_4^+$), Nitrite ($NO_2^-$), Nitrate ($NO_3^-$), Nitrous oxide ($N_2O$), and Protein.

  • Processes involved: Nitrate reduction, Denitrification, and decay of plant and animal wastes/dead organisms.

  • Geologic involvement: Igneous rocks.

• Atmospheric Nitrogen Fixation Reactions:

  • $N_{2(g)} + O_{2(g)} \xrightarrow{\text{electrical energy}} 2NO_{(g)}$

  • $2NO_{(g)} + O_{2(g)} \rightarrow 2NO_{2(g)}$

  • $2NO_{2(g)} + H_2O_{(g)} \rightarrow HNO_{2(aq)} + HNO_{3(aq)}$

• Industrial Nitrogen Fixation (Haber Process):

  • $N_{2(g)} + 3H_{2(g)} \xrightarrow{\text{catalyst}} 2NH_{3(g)}$

• Additional Nitrogen-Related Global Chemistry:

  • Production of nitric acid: $NH_3 + 2O_2 \rightarrow HNO_3 + H_2O$

  • Production of fertilizers: $NH_3 + HNO_3 \rightarrow NH_4NO_3$

  • Source of hydrogen: $CH_4 + 2H_2O \rightarrow 4H_2 + CO_2$

The Oxygen Cycle and Upper Atmosphere Chemistry

• The oxygen cycle involves the movement of oxygen through the atmosphere, hydrosphere (photic zone), and lithosphere.

• Key Processes in the Oxygen Cycle:

  • Photosynthesis by phytoplankton in the photic zone produces $O_2$.

  • Carbonate equilibrium reaction in water:

  • $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+ \rightleftharpoons CO_3^{2-}$

  • Formation of sediments: $Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3$

  • Volcanism releases $CO_2$ back into the atmosphere.

  • Oxidative weathering of minerals: $4FeO + O_2 \rightarrow 2Fe_2O_3$

• Photodissociation and Photoionization:

  • $h\nu + N_2 \rightarrow 2N$

  • $h\nu + N \rightarrow N^+ + e^-$

  • $h\nu + O_2 \rightarrow O_2^+ + e^-$

• Chemical Reactions in the Thermosphere:

  • Auroras and glows are caused by energy transitions in electrons ($e^-$) and protons ($p^+$).

  • Oxygen transition producing green and red light:

  • $e^- + p^+ + O \rightarrow O^* + e^-$

  • $O^* \rightarrow O + h\nu$

  • Nitrogen transition producing blue and violet light:

  • $e^- + p^+ + N_2 \rightarrow N_2^* + p^+ + e^- + e^-$

  • $N_2^* \rightarrow N_2 + h\nu$

  • Space shuttle tail section orange glow: Result of interaction between atomic oxygen and nitric oxide:

  • $O + NO \rightarrow NO_2^*$

  • $NO_2^* \rightarrow NO_2 + h\nu$

Photodissociation of Oxygen Calculation

• Problem 20.1: Calculate the maximum wavelength ($nm$) of a photon that can cause the dissociation of an $O_2$ molecule given a bond enthalpy of $498.7\,kJ/mol$.

• Strategy for Solution:

  • Units provided: $kJ/mol$.

  • Units needed for one bond: $J/molecule$.

  • Path: $kJ/mol \rightarrow J/molecule \rightarrow \text{frequency of photon} \rightarrow \text{wavelength of photon}$.

• Solution Steps:

  • Energy for one bond:

  • $E = \frac{498.7 \times 10^3\,J}{1\,mol} \times \frac{1\,mol}{6.022 \times 10^{23}\,molecules} = 8.281 \times 10^{-19}\,J/molecule$

  • Frequency ($\nu$):

  • $E = h\nu$

  • $\nu = \frac{E}{h} = \frac{8.281 \times 10^{-19}\,J}{6.63 \times 10^{-34}\,J\,s} = 1.25 \times 10^{15}\,s^{-1}$

  • Wavelength ($\lambda$):

  • $\lambda = \frac{c}{\nu} = \frac{3.00 \times 10^8\,m/s}{1.25 \times 10^{15}\,s^{-1}} = 2.40 \times 10^{-7}\,m$

  • $\lambda = 240\,nm$

• Conclusion: Any photon with a wavelength of $240\,nm$ or shorter ($\lambda \le 240\,nm$) can dissociate an $O_2$ molecule.

Ozone production and Stratospheric Depletion

• Ozone Layer Spectrum and Units:

  • UVC: $100-280\,nm$ (absorbed by the ozone screen).

  • UVB: $280-315\,nm$.

  • UVA: $315-400\,nm$.

  • Visible Light: $400-700\,nm$.

  • Ozone concentration is measured in Dobson Units.

• Ozone Formation and Natural Destruction:

  • O_2 \xrightarrow{UV < 240\,nm} O + O

  • $O + O_2 + M \rightarrow O_3 + M$ (Production, where $M$ is a third body for energy dissipation).

  • $O_3 \xrightarrow{UV} O + O_2$ (Destruction via UV absorption).

  • $O + O_3 \rightarrow 2O_2$

  • These processes maintain a dynamic equilibrium in the stratosphere.

• Anthropogenic Ozone Destruction:

  • Chlorofluorocarbons (CFCs) dissociate under UV radiation to release chlorine atoms ($Cl$).

  • $CFCl_3 \xrightarrow{UV} CFCl_2 + Cl$

  • $CF_2Cl_2 \xrightarrow{UV} CF_2Cl + Cl$

  • Catalytic cycle:

  • $Cl + O_3 \rightarrow ClO + O_2$

  • $ClO + O \rightarrow Cl + O_2$

  • Net Reaction: $O_3 + O \rightarrow 2O_2$

• Polar Stratospheric Clouds (PSCs):

  • PSCs provide a surface for reactions that release active chlorine.

  • $HCl + ClONO_2 \rightarrow Cl_2 + HNO_3$

  • In spring, sunlight causes: $Cl_2 + h\nu \rightarrow 2Cl$.

  • This triggered release leads to rapid ozone depletion over the poles.

Sulfur Chemistry and Acid Rain

• Sulfur Oxidation Reactions:

  • $2H_2S_{(g)} + 3O_{2(g)} \rightarrow 2SO_{2(g)} + 2H_2O_{(g)}$

  • $SO_{2(g)} + OH_{(g)} \rightarrow HOSO_{2(g)}$

  • $HOSO_{2(g)} + O_{2(g)} \rightarrow HO_{2(g)} + SO_{3(g)}$

  • $SO_{3(g)} + H_2O_{(g)} \rightarrow H_2SO_{4(g)}$

  • Sulfuric acid ($H_2SO_4$) aerosols have a local cooling effect by reflecting radiation.

• Acid Rain and pH:

  • Pure rainwater has a natural pH of approximately $5.66$ due to dissolved $CO_2$.

  • Acid rain forms when sulfur and nitrogen oxides lower this pH significantly.

• Environmental Effects and Mitigation of Acid Rain:

  • Damage to marble (calcium carbonate) statues:

  • $CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(s) + H_2O(l) + CO_2(g)$

  • $2CaCO_3(s) + 2SO_2(g) + O_2(g) \rightarrow 2CaSO_4(s) + CO_2(g)$

  • Industrial scrubbing (Purification Chamber) for coal plants:

  • Carbonate decomposition: $CaCO_3 \rightarrow CaO + CO_2$

  • Reaction with sulfur dioxide: $CaO + SO_2 \rightarrow CaSO_3$

Greenhouse Effect and Global Warming

• Definition: The trapping of heat near Earth's surface by specific atmospheric gases.

• Greenhouse Gases (GHGs) and IR Absorption:

  • Gases absorb and remit infrared (IR) radiation via vibrational modes.

  • $H_2O$ has $3$ vibration modes.

  • $CO_2$ has $2$ vibration modes.

  • $N_2$ and $O_2$ do not contribute to the greenhouse effect because they cannot absorb IR radiation.

• Global Warming Contributions:

  • $CO_2$: $55\%$

  • CFCs: $24\%$

  • $CH_4$ (Methane): $15\%$

  • $N_2O$: $6\%$

• Carbon Dioxide Trends:

  • Measurements at Mauna Loa, Hawaii, show a steady increase in concentration from roughly $315\,ppm$ in 1960 to over $350\,ppm$ by the late 1990s.

  • Sources of $CO_2$: Electricity production ($35\%$), Cars/trucks ($30\%$), Industry ($24\%$), and Residential heating ($11\%$).

• Temperature Rise:

  • Earth's surface temperature rose by approximately $0.6\,^{\circ}C$ between 1880 and 1996.

Photochemical Smog and Indoor Pollutants

• Photochemical Smog: Formed by the reaction of automobile exhaust in the presence of sunlight.

  • Primary pollutants: Nitric oxide ($NO$), carbon monoxide ($CO$), and unburned hydrocarbons.

  • Secondary pollutants: Nitrogen dioxide ($NO_2$) and Ozone ($O_3$).

• Smog Chemistry:

  • $N_{2(g)} + O_{2(g)} \rightarrow 2NO_{(g)}$

  • $2NO_{(g)} + O_{2(g)} \rightarrow 2NO_{2(g)}$

  • $NO_{2(g)} + h\nu \rightarrow NO_{(g)} + O_{(g)}$

  • $O_{(g)} + O_{2(g)} + M \rightarrow O_{3(g)} + M$

• Diurnal Variations: Pollutant levels typically peak relative to traffic and sunlight hours. Hydrocarbons and $NO$ peak during morning rush hour, followed by a mid-day peak in $NO_2$ and $O_3$.

• Indoor Radon Exposure:

  • Radon ($Rn$) is a radioactive gas that can accumulate in basements from the decay of uranium and radium in soil.

  • Decay chain:

  • $^{238}_{92}U \xrightarrow{4.51 \times 10^9\,yr} ^{234}_{90}Th \xrightarrow{24.1\,d} ^{234}_{91}Pa \xrightarrow{1.17\,min} ^{234}_{92}U \xrightarrow{2.47 \times 10^5\,yr} ^{230}_{90}Th \xrightarrow{7.5 \times 10^4\,yr} ^{226}_{88}Ra \xrightarrow{1.6 \times 10^3\,yr} ^{222}_{86}Rn$

  • The half-life of $^{222}_{86}Rn$ is $3.82\,d$.