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Xavier




Chemistry Revision
The Atmosphere



Storylines

A1 ­ What's in the air?
A2 ­ Screening the Sun
A3 ­ Ozone: A vital sunscreen
A4 ­ The CFC story
A5 ­ What is the state of the ozone layer now?
A6 ­ The greenhouse effect
A7 ­ Trouble in the troposphere…

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Xavier




The atmosphere is a thin layer of gas extending 100km above Earth's surface







Chemically important regions are the troposhphere and stratosphere.
As altitude increases, density decreases.

90% of molecules in the atmosphere lie in the lower region called the troposphere.

The concentrations of substances are either measured as percentages…

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Xavier




Mixing is different in the reverse temperature gradient of the stratosphere: mixing rarely occurs
in the vertical direction, but does occur in horizontal circulation.

6.2 ­ What happens when radiation interacts with matter?

EM radiation transfers energy to chemicals.
These chemicals absorb energy and changes occur.
The changes depend…

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Xavier







Vibrational energy: corrresponds to IR
(we sense IR as heat, bonds in chemicals in our skin vibrate more energetically, the energy is
converted to KE which warms us up).

Rotational energy: corresponds to a lower frequency radiation ­ Microwaves

Translational energy: is even less, it is so small that…

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Xavier




The following can occur with high frequency EM radiation, in increasing energy:

Electrons may be excited to a higher energy level (electrons excited, return to ground
state, light is emitted). E.g. with chlorine, green light is emitted.
Electrons in a bond may photodissociate into radicals
Electrons are able to…

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Xavier





The sun radiates a wide spectrum of energy.

Some corresponds to the energy required to break chemical bonds ­ even in DNA.
This can cause damage in genes and cause skin cancer.
Years of exposure can make people look wrinkly and leathery: radiation damages proteins of
connective tissue beneath…

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Xavier





All reactions involve the breaking and remaking of chemical bonds.

Bond breaking = bond fission

Two types = Heterolytic fission and Homolytic fission

Heterolytic fission

Both shared electrons go to just 1 atom
Two ions are formed
It is common when a bond is already polar

E.g. bromomethane







Homolytic…

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Xavier











Even O2 has two unpaired electrons, and so is a biradical: however it is relatively stable.

Radicals are reactive. Filled outer electron shells are more stable than unfilled ones.

When a Cl radical collides with a H2 molecule, it will steal the electron from the H2 bond and form…

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Xavier




CH3Cl + Cl2 CH2Cl2 + HCl

For every Cl2 molecule used, 1 H atom is replaced on the halogenoalkane, and one HCl (H-Hal) is
formed.

In summary,

Radical chain reactions usually occur in the gas phase or in a non-polar solvent
They are often initiated by heating or by…

Page 10

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Xavier





Ozone is being made and destroyed all the time.
If left to themselves, the rate of proucing ozone = the rate of destroying ozone.
I.e. they are in a "steady state." But not with human inteference!

Reaction (2) should equal reactions (4) and (5). However, when taking measurements we…

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