Arrow Lab

Química Ambiental · Air

Química atmosférica

Uma lição resolvida gratuita e 40 problemas de prática em que você mesmo desenha cada seta curva.

Travou em química atmosférica? Escaneie o problema.

Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.

Escaneie sua lição de graça

O que você vai aprender

  • Explain how stratospheric ozone forms and is destroyed, including the Chapman reactions and chlorine catalytic cycles.
  • Draw fishhook mechanisms for the radical steps that oxidise methane, convert NO to NO₂ and make nitric and sulfuric acids.
  • Explain how sunlight, NOx and volatile organic compounds produce tropospheric ozone and photochemical smog.
  • Trace sulfur and nitrogen oxides to the acids in acid rain and calculate the pH of unpolluted rain.
  • Decide which gases absorb infrared radiation from the vibrational modes that change their dipole moment.

Lição gratuita

O texto da lição resolvida. No Arrow Lab, cada etapa vem com as estruturas e as setas desenhadas.

Ler a aula gratuita (7 partes)

Ozone in the stratosphere

In the stratosphere, ultraviolet light below about 240 nm splits O₂ into two oxygen atoms. Each O atom adds to O₂ in a three-body collision (O + O₂ + M → O₃ + M); the third body M, usually N₂, carries away the excess energy. Ozone absorbs UV-B and UV-C and falls apart to O₂ + O, and O + O₃ → 2 O₂ removes odd oxygen. These four Chapman reactions explain why an ozone layer exists, but they predict more ozone than is measured: catalytic cycles also destroy it.

Chlorine catalytic cycles

CFCs such as CF₂Cl₂ are inert in the lower atmosphere, so they reach the stratosphere, where short-wavelength UV breaks a C–Cl bond and releases a chlorine atom. Cl• + O₃ → ClO• + O₂, then ClO• + O → Cl• + O₂: the net change is O₃ + O → 2 O₂ and the chlorine atom is regenerated, so one atom destroys thousands of ozone molecules. Most chlorine is stored in reservoirs, HCl and chlorine nitrate (ClONO₂). In the polar winter these react on polar stratospheric cloud particles to give Cl₂, which spring sunlight splits into chlorine atoms; the ClO dimer cycle then destroys ozone without needing O atoms.

The hydroxyl radical and fishhook arrows

In the lower atmosphere HO• is the main oxidant. It forms when UV photolysis of ozone gives an excited oxygen atom that reacts with water. HO• removes a hydrogen atom from methane: one electron of the C–H bond pairs with the radical's electron to make the O–H bond of water, and the other stays on carbon as CH₃•. Fishhook arrows show these single-electron moves. CH₃• then combines with O₂, drawn as the diradical •O–O•, to give the methylperoxy radical CH₃OO•.

NOx and photochemical smog

Engines make NO from N₂ and O₂ at high temperature. Sunlight splits NO₂ into NO and an O atom that makes ozone, but NO + O₃ → NO₂ + O₂ removes it again, so NOx alone cannot build up ozone. Peroxy radicals (HO₂•, CH₃OO•) made from volatile organic compounds convert NO to NO₂ without using ozone, so ozone accumulates on sunny afternoons. Peroxyacetyl nitrate (PAN) is a typical eye irritant formed in smog.

Acid rain

Pure rain in equilibrium with atmospheric CO₂ is already slightly acidic, pH about 5.6. Acid rain contains the strong acids H₂SO₄ and HNO₃. SO₂ is oxidised by HO•, giving HOSO₂•, which O₂ turns into SO₃; SO₃ then adds water to give sulfuric acid. NO₂ combines with HO• to give nitric acid. Limestone (CaCO₃) neutralises the acid, so lakes on granite are the most vulnerable, and power-station scrubbers use limestone to remove SO₂.

Greenhouse gases

A molecule absorbs infrared radiation only through vibrations that change its dipole moment. N₂, O₂ and Ar have no such vibration, so they are transparent to the infrared light Earth emits (peak near 10 μm). CO₂ has four vibrational modes (3N − 5 for a linear molecule); its symmetric stretch is IR-inactive, while the asymmetric stretch and the two bends absorb, the bend near 667 cm⁻¹ (15 μm). CH₄, N₂O, H₂O and halocarbons also absorb. The centre of CO₂'s 15 μm band is already nearly saturated, so each added CO₂ molecule has a small effect; trace gases whose bands are far from saturated, especially those absorbing in or near the 8–13 μm window where CO₂ and H₂O absorb little, have much larger effects per molecule.

What to carry forward

Atmospheric chemistry is radical chemistry driven by sunlight: photolysis makes atoms and radicals, catalytic cycles (Cl, NO, HO) regenerate their carriers, HO• oxidises methane, SO₂ and NO₂, and vibrations that change a dipole decide which gases trap infrared radiation.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 9

Stratospheric ozone

  • The ClO dimer cycle of the polar ozone holeDraw the radical steps by which two ClO radicals dimerise, the dimer is photolysed, and both chlorine atoms are regenerated with release of O₂.
  • ClO• and NO₂ form the chlorine nitrate reservoirDraw the radical combination of chlorine monoxide with NO₂ that stores active chlorine as chlorine nitrate.

Tropospheric radicals and smog

  • HO₂• converts NO to NO₂ and regenerates HO•Draw the fishhook mechanism by which the hydroperoxyl radical oxidises NO to NO₂ and regenerates the hydroxyl radical.
  • Hydroxyl radical oxidation of methane to the methylperoxy radicalDraw the fishhook mechanism by which the hydroxyl radical removes a hydrogen atom from methane and the methyl radical then combines with O₂.
  • Methoxy radical and O₂ give formaldehyde and HO₂•Draw the fishhook mechanism by which O₂ removes a hydrogen atom from the methoxy radical to give formaldehyde and the hydroperoxyl radical.
  • Methylperoxy radical converts NO to NO₂Draw the fishhook mechanism by which the methylperoxy radical oxidises NO to NO₂ and becomes the methoxy radical.

Acid rain

  • Hydroxyl radical and NO₂ combine to nitric acidDraw the radical combination of the hydroxyl radical with NO₂ that forms nitric acid in polluted air.
  • O₂ converts the HOSO₂• radical into SO₃Draw the fishhook mechanism by which O₂ removes the hydrogen atom of HOSO₂•, formed from SO₂ and the hydroxyl radical, to give SO₃ and HO₂•.
  • Hydration of sulfur trioxide to sulfuric acidDraw the polar mechanism by which water adds to sulfur trioxide to give sulfuric acid.

Preveja o produto 5

Stratospheric ozone

  • What does UV light do to CF₂Cl₂?Predict the products of stratospheric photolysis of a CFC from relative bond strengths.
  • Net reaction of the chlorine cycleAdd the steps of the Cl/ClO catalytic cycle to find its net reaction and the catalyst.

Acid rain

  • The main acid from high-sulfur coalPredict the strong acid formed in rain downwind of plants that burn high-sulfur coal.
  • Which lake suffers most from acid rain?Predict how bedrock carbonate controls a lake's sensitivity to acid rain.

Tropospheric radicals and smog

  • Where does the hydroxyl radical come from?Identify the reaction sequence that produces the hydroxyl radical in the troposphere.

Classificar 5

Greenhouse gases

  • Which CO₂ vibration is IR-inactive?Classify the vibrational modes of CO₂ as IR-active or IR-inactive from dipole changes.

Stratospheric ozone

  • Which Chapman reaction makes ozone?Identify the Chapman reaction that forms ozone and the role of the third body.
  • The nitrogen-containing chlorine reservoirDistinguish a chlorine reservoir species from chlorine and nitrogen source gases.

Tropospheric radicals and smog

  • Identify peroxyacetyl nitrateRecognise the structure of peroxyacetyl nitrate (PAN) from its formation from the acetylperoxy radical and NO₂.
  • Photochemical or sulfurous smog?Classify a smog episode as photochemical or sulfurous from its conditions and pollutants.

Comparar 4

Greenhouse gases

  • Rank the gases of dry airOrder the main gases of dry air by abundance and relate abundance to greenhouse importance.
  • Rank by atmospheric lifetimeOrder atmospheric species by lifetime from how fast the hydroxyl radical or UV light removes them.

Stratospheric ozone

  • Which replacement depletes the least ozone?Explain why a C–H bond makes an HCFC less ozone-depleting than a CFC.

Tropospheric radicals and smog

  • Which gas does HO• attack fastest?Predict relative rates of hydrogen abstraction by HO• from C–H bond strengths.

Calcular 7

Greenhouse gases

  • Wavelength of the CO₂ bending bandConvert the wavenumber of CO₂'s bending band into a wavelength in micrometres.
  • Vibrational modes of CO₂Count the vibrational modes of a linear triatomic molecule with the 3N − 5 rule.
  • CO₂-equivalent of a methane leakConvert a methane emission to CO₂-equivalent using its global warming potential.

Acid rain

  • pH of unpolluted rainCalculate the pH of rain in equilibrium with atmospheric CO₂ from Henry's law and Ka1.
  • Oxidation state of nitrogen in nitric acidAssign the oxidation state of nitrogen in nitric acid formed from NOx.
  • Electrons lost when SO₂ becomes sulfuric acidUse oxidation states to count the electrons lost when SO₂ is oxidised to sulfuric acid.

Stratospheric ozone

  • Longest wavelength that can split O₂Calculate the longest photon wavelength able to break the O=O bond from its bond energy.

Encontre o erro 2

Stratospheric ozone

  • Why the Chapman model predicts too much ozoneDiagnose the missing ozone loss processes when the Chapman reactions alone overpredict ozone.

Acid rain

  • Find the error in a student's equationDiagnose why hydration of SO₂ alone cannot give sulfuric acid.

Interprete as evidências 2

Greenhouse gases

  • Where Earth's infrared emission peaksUse Wien's law to locate the peak of Earth's thermal emission and relate it to greenhouse absorption bands.

Tropospheric radicals and smog

  • Reading a day of smog dataInterpret the time sequence of NO, NO₂ and ozone peaks during a smog day.

Planeje uma síntese 1

  • Removing SO₂ from a power-station stackChoose a chemical method to remove SO₂ from power-station flue gas.

Explique por quê 5

Greenhouse gases

  • Why is N₂ not a greenhouse gas?Explain why a homonuclear diatomic gas cannot absorb infrared radiation.
  • Why an extra methane molecule matters moreExplain why adding one molecule of methane warms more than adding one molecule of CO₂.

Stratospheric ozone

  • Why the ozone hole is over AntarcticaExplain how polar stratospheric clouds convert reservoir chlorine into active chlorine.

Tropospheric radicals and smog

  • Where the oxygen atom comes from near the groundExplain why ozone near the ground is made from NO₂ photolysis rather than O₂ photolysis.
  • Why smog needs organic compoundsExplain how peroxy radicals from volatile organic compounds let ozone accumulate.

Travou em química atmosférica? Escaneie o problema.

Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.

Escaneie sua lição de graça

Antes deste tópico