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The Air Around Us

The Air Around Us. Air is more than one removed from nothing. Learning objectives. Describe what is meant by pressure Identify units of gas pressure Describe the basic gas laws Apply gas laws to simple problems Describe composition of the atmosphere

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The Air Around Us

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  1. The Air Around Us Air is more than one removed from nothing

  2. Learning objectives • Describe what is meant by pressure • Identify units of gas pressure • Describe the basic gas laws • Apply gas laws to simple problems • Describe composition of the atmosphere • Name important pollutants and their effects • Describe origin of the ozone hole

  3. Gas laws and crash safety • The airbag is chemistry applied in a very practical area • Airbags have reduced serious injuries and fatalities by significant margin • Chemistry plays a crucial role in airbag performance

  4. Timing is everything • The airbag must deploy within about 40 ms of the impact • The airbag must not deploy unless there is an impact • Inflation depends upon a rapid chemical reaction generating a quantity of gas • The bag, once inflated, must then deflate at the point of impact with the driver to prevent injury

  5. Gas: no interactions • Not rigid • Completely fills container • Compressible • Low density

  6. Atom motion and temperature • Atoms in molecules have three types of motion • Rotation – moving about the centre of mass • Vibration – vibrating about the centre of mass • Translation – movement of the centre of mass • As temperature increases, the energies of all types of motion increase • In gases molecular energies exceeds all interactions

  7. Kinetic molecular theory and pressure – a case for atoms • Pressure is caused by the energetic molecules striking the tire wall • Pumping up a tire increases the number of molecules • More molecules – higher pressure • Higher temperature – higher pressure

  8. Under pressure • Gases exert pressure by virtue of motion • Gravity makes the air density higher near the earth’s surface • Pressure decreases with elevation

  9. Atmospheric pressure • Barometer is used for measuring atmospheric pressure

  10. Atmospheric pressure • The weight of the air supports a column of mercury 760 mm high • Nominally atmospheric pressure is 760 mm Hg • Atmospheric pressure changes with the weather

  11. Units of pressure

  12. Gas laws: experience in math form • The properties of gases can be described by a number of simple laws • The laws establish quantitative relationships between different variables • They are largely intuitively obvious and familiar

  13. The four variables • Pressure (P) • Volume (V) • Temperature (T in Kelvin) • Number of molecules (n in moles)

  14. Variables and constants • In the elementary gas laws two of the four variables are kept constant • Each law describes how one variable reacts to changes in another variable • All the simple laws can be integrated into one combined gas law

  15. Boyle’s law • The first experimental gas law • Pressure increases, volume decreases (T, n constant)

  16. Charles’ Law • As temperature increases, volume increases (P, n constant) • Temperature must be measured in Kelvin

  17. Absolute zero • Gay-Lussac observed V changed by 1/273 of value at 0ºC • Plotted as V = kT (T = ºC + 273): • V = 0 at T = 0 K • Does the gas actually occupy zero volume? • No, at lower T the law is not followed

  18. Combined gas law • Fold together Boyle and Charles: • P1V1/T1 = P2V2/T2 • Given five of the variables, find the sixth • Units must be consistent • Temperature in Kelvin

  19. Standard temperature and pressure (STP) • Standard conditions allow direct comparison of properties of different substances • Standard temperature is 273 K (0ºC) • Standard pressure is 760 mm Hg or 1 atmosphere • At STP, 1 mole of any ideal gas occupies 22.414 L

  20. What is the atmosphere

  21. The atmosphere is layered • Troposphere • Where the weather happens • Stratosphere • Where the ozone is • Mesosphere • Ionosphere • The brutal strength of solar radiation ionizes all the components – permits transmission of radio signals around the earth without need of mirrors

  22. Pollutants: things that shouldn’t be there • SO2 power plant emissions • PM-10 particulate matter from agriculture and construction • CO vehicle emissions • O3 sunlight and vehicle emissions • NO2 vehicle emissions • Pb smelters and battery plants

  23. Cleaning up our act: successes • Clean Air Act of 1970 • Amended in 1977 • Top six pollutants have all decreased • Problems remain

  24. Ozone: still a problem

  25. Bad ozone, good ozone • Stratospheric sunscreen • O3 + UV = O2 + O • O2 + O = O3 + heat

  26. Chlorofluorocarbons (CFCs): wonder chemicals

  27. Hidden dangers • CFCs are unstable in UV CF2Cl2 + UV = Cl + CF2Cl • Cl radicals catalyze decomposition of ozone Cl + O3 = ClO + O2 O3 + UV = O2 + O ClO + O = Cl + O2 • One Cl atom can react again and again destroying countless O3 molecules

  28. The Antarctic ozone hole • Ozone depletion in the Antarctic is severe • Stratosphere isolated by polar vortex • Polar stratospheric clouds (PSCs) form • Clouds produce chlorine gas

  29. Montreal Protocol and CFCs • Phase-out of CFCs began 1978 • Montreal Protocol (1987) called for 50 % decrease by 2000 • Amended to complete phaseout • Chemical ingenuity required for alternatives like HCFCs

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