AS CHEMISTRY OCR SALTERS B - ATMOSPHERE

NOTES ON PPM & RADIATON

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AS CHEMISTRY OCR SALTERS B NOTES
UNIT 2 TOPIC 2 - THE ATMOSHPERE: THE ATMOSPHERE
PARTS PER MILLION (PPM) ­ USED FOR VERY SMALL QUANTITIES
Major gases ­ given as % of total volume
Gases in tiny amounts ­ given as PPM
TO WORK IT OUT: EG ­ XENON MAKES UP 0.000009% OF ATMOSPHERE.
SO THERE IS ONLY THAT MUCH XENON IN 100 PARTS OF AIR.
MULITPLYING BOTH NUMBERS BY 10, 000 GIVES A LARGE ENOUGH NO. TO WORK WITH.
SO... 0.000009 X 10, 000 = 0.09
100 X 10, 000 ­ 1000, 000 = 0.09 PPM XENON
EARTHS ATMOSPHERE ­ ABSORBS RADIATION
The sun gives out electromagnetic radiation (EMR) ­ transmitted as waves &
different frequencies.
The sun mainly gives out visible radiation (light) and infrared (IR) & a small
amount of UV radiation.
Earths atmosphere absorbs some of the suns IR radiation
Earths surface absorbs radiation from the Sun & is warmed. Radiation then
re-emitted at a much lower frequency so most of it is IR.
IR RADIATION MAKES SOME BONDS VIBRATE MORE
Some molecules absorb IR radiation ­ extra energy makes covalent bonds
vibrate more.
Only molecules made of different atoms can absorb IR ­ polarities of
bonds change as they vibrate.
Oxygen & Nitrogen don't, but water, carbon dioxide, methane & nitric
oxide do ­ they are greenhouse gases.
Gas molecules' bonds have fixed vibrational energy levels ­ quantised
levels. A bonds energy can only jump from one energy level to another.
Only frequencies of corresponding radiation are absorbed ­ different
molecules absorb different frequencies of radiation.
GREENHOUSE GASES ­ MOLECULES THAT ABSORB RADIATION AND PREVENT IT
FROM LEAVING THE EARTHS ATMOSPHERE.
UV & VISIBLE LIGHT GIVE ELECTRONS MORE ENERGY
When UV or visible light hit molecules of gas electrons absorb energy and
jump up an energy level.
If enough energy is absorbed bonds break ­ THIS FORMS FREE RADICALS.
FREE RADICALS ­ PARTICLES WITH AN UNPAIRED ELECTRON.
ENERGY FROM RADIATION CAN BE CALCULATED ­ ENERGY DEPENDS ON FREQUENCY
E = Hv
E = ENERGY IN JOULES H = PLANCK'S CONSTANT V = FREQUENCY IN Hz
PLANCK'S CONSTANT = 6.63 X 10

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