22 terms

Photochemical Smog

Where does photochemical smog occur?
In the troposphere (lower atmosphere)
What is the appearance of photochemical smog?
yellowish haze
What are the conditions for the formation of photochemical smog? (5)
Exhaust fumes, often in built up cities and industrial areas
No wind - stationary air mass
warm > 18°C
temperature inversions (layers of different temperatures)
What are primary pollutants?
Pollutants directly emitted into the atmosphere from car exhausts and furnaces
Name 5 primary pollutants.
Carbon monoxide
carbon dioxide
sulfur dioxide
unburnt hydrocarbons
nitric oxide
What are secondary pollutants?
Pollutants produced by chemical reactions with primary pollutants.
What secondary pollutants form from nitric oxide? (draw equations) (5)
Nitrogen dioxide:
NO →(+O₂)→NO₂
NO →(+O₃)→NO₂ + O₂
NO →(+HOO·)→NO₂ + HO·
Then nitrogen dioxide can react with hv, O₂ and H₂O.
Reacting with hv (sunlight) it forms →NO + O
The O formed reacts with O₂ (and a stabilising molecule M) to form → O₃ +M* (M absorbs extra energy)
When reacting with O₂ → NO + O₃
When reacting with H₂O → HNO₃ + HNO₂
What is HOO· ?
peroxyl radical
What appearance does NO give the photochemical smog?
Yellow/brown colour
What are the effects of oxides of nitrogen?
Respiratory problems, yellowing of leaves, photosynthesis, acid rain, greenhouse gas.
What are the effects of O₃?
Attacks double bonds in polymers - cracks rubber
Severe respiratory problems
Photosynthesis, yellowing of leaves
What are the effects of CO?
Toxic, has a stronger affinity than O₂ to hemoglobin in blood therefore suffocation.
What are the effects of unburnt hydrocarbons?
React with other molecules to form secondary pollutants
What is the purpose of a catalytic converter?
To remove primary pollutants, CO, UHC and NO by catalysing their conversion into CO₂, steam and nitrogen.
What is the catalyst metal inside a catalytic converter?
Platinum, palladium or rhodium - expensive metals.
How is the structure designed?
In a honeycomb formation to increase surface area.
What is the material that the honeycomb structure is made of?
Porous cordierite - can be used at temps up to 1300°C
How is CO removed - equation?
2CO + O₂ → 2CO₂
CO + H₂O → CO₂ + H₂
How are unburnt Hydrocarbons removed? (equation)
UHC + O₂ → CO₂ + H₂O
UHC + H₂O → CO + H₂
Removal of NO - equations?
2NO + 2CO → 2CO₂ + N₂
2NO + 2H₂ → 2H₂O + N₂
When does the catalytic converter not function very well?
At low temperatures
What is the optimum air to fuel ratio in the combustion chamber in the engine?
14.6 : 1 (air to fuel)