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  Name important pollutants and their effects  Describe origin of the ozone hole

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

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

Gas: no interactions  Not rigid  Completely fills container  Compressible  Low density

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

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

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

Atmospheric pressure  Barometer is used for measuring atmospheric pressure

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

Units of pressure

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

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

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

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

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

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

Combined gas law  Fold together Boyle and Charles:  P 1 V 1 /T 1 = P 2 V 2 /T 2  Given five of the variables, find the sixth  Units must be consistent  Temperature in Kelvin

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 L

What is the atmosphere

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

Pollutants: things that shouldn’t be there  SO 2 power plant emissions  PM-10 particulate matter from agriculture and construction  CO vehicle emissions  O 3 sunlight and vehicle emissions  NO 2 vehicle emissions  Pb smelters and battery plants

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

Ozone: still a problem

Bad ozone, good ozone  Stratospheric sunscreen  O 3 + UV = O 2 + O  O 2 + O = O 3 + heat

Chlorofluorocarbons (CFCs): wonder chemicals ApplicationsProperties RefrigerationColorless Air-conditioningNontoxic PropellantsInert Foaming agents Odorless

Hidden dangers  CFCs are unstable in UV CF 2 Cl 2 + UV = Cl + CF 2 Cl  Cl radicals catalyze decomposition of ozone Cl + O 3 = ClO + O 2 O 3 + UV = O 2 + O ClO + O = Cl + O 2  One Cl atom can react again and again destroying countless O 3 molecules

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

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