The chemistry of atmospheric and water pollution.

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Human activity has caused changes in the composition and the structure of the atmosphere. Chemists monitor these changes so that further damage can be limited.

Point 4.1 – Describe the composition and layered structure of the atmosphere.

The atmosphere contains 4 distinct layers. They are: troposphere, stratosphere, ionosphere (which is made up of two layers called mesosphere and thermosphere).

Troposphere: The troposphere is the layer which is closest to the earth’s surface. It is approximately 0-15km above earth’s surface. The thermosphere contains 3 main gases. Nitrogen is the most abundant gas at 78% (by volume). The nitrogen is part of the nitrogen cycle. The second most abundant gas is oxygen (21%) which is necessary for the process of respiration in living things. Argon is next at 0.9% and other gases such as carbon dioxide, Ne, He and variable amounts of water vapour are present. As altitude increases, the temperature decreases (because of low pressure). Here is where all the weather takes place.

Stratosphere: The stratosphere lies above the troposphere. It is approximately 15-50km above the earth’s surface. Important gases in this layer include nitrogen, oxygen and especially ozone. In the upper stratosphere there is the ozone layer. Temperature rises as altitude rises in this layer because particles (e.g. ozone) in the stratosphere absorb harmful U.V. radiation from the sun. The temperature range is -50ºC to 30ºC.

Ionosphere: This layer is above the stratosphere and is comprised of two layers – mesosphere and thermosphere. It is approximately 50-150km above the earth’s surface. The mesosphere lies below the thermosphere. The ionosphere mainly consists of mainly ions and atomic particles such as O2+ and NO+. Oxygen atoms (radicals) exist in the ionosphere and free electrons that would not be stable at lower altitudes. The reason for the ions in the ionosphere is that the U.V. radiation from the sun breaks the bonds of molecules to form atoms and free radicals. Temperature in the ionosphere is about -50ºC.

Point 4.2 – Identify the main pollutants found in the lower atmosphere and their sources.

Here is a table showing the main pollutants found in lower atmosphere (troposphere) and their sources:

Note: Ozone is also formed in the lower atmosphere via photochemical smog. The nitrogen oxides in the photochemical smog absorb the U.V. radiation from sun and this produces an oxygen atom which then reacts with an oxygen molecule to produce ozone.


NO2 (g) + U.V.  NO (g) + O. (g)

O. (g) + O2 (g)  O3 (g)

Point 4.3 – Describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant.

In the troposphere as outlined above, ozone can be produced from the breakdown of nitrogen oxides by sunlight (see 2 equations above). Ozone is a toxic, pungent gas which can be harmful for humans at concentrations greater than 0.12ppm (asthmatics). It causes irritation of the eyes and airways, coughing and increased incidence of respiratory complications such as asthma and bronchitis. Because ozone is a strong oxidising agent, excessive concentrations can disrupt biochemical reactions inside the body. Thus it is a dangerous pollutant in the lower atmosphere.

However in the stratosphere, ozone has a very important role. It acts as a shield, preventing harmful U.V. rays from reaching earth’s surface. The ozone molecule absorbs the UV radiation from the sun and it decomposes into an oxygen molecule and an oxygen atom (radical):

O3 (g) + UV  O2 (g) + O. (g)  

*** Make sure to put a dot next to oxygen atom to show that is a radical.

It is important that a large percentage of the UV rays reaching the earth be absorbed by the atmospheric ozone in order to maintain life on earth. This is because UV is damaging to humans causing skin cancer and it is able to break the covalent bonds of biologically important molecules such as proteins and DNA. This can lead to gene mutations and the formation of cancer. Thus ozone is a very important shield which blocks harmful UV rays in order to maintain life on earth.

Point 4.4 – Describe the formation of a coordinate covalent bond.

A coordinate covalent bond is a covalent bond that has formed when one atom provides both electrons to form the shared bonding pair. A lone pair from one atom forms the bond. Once the coordinate covalent bond is formed, the properties are the same as a covalent bond and they cannot be distinguished.

Point 4.5 – Demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures.

The ammonium ion is formed from the reaction between a ammonia molecule and a proton (hydrogen ion):  

*** Note: the 2 red dots show a coordinate covalent bond (all electrons are donated by nitrogen when bonding with the hydrogen)

The Lewis dot electron diagram for all of the species is shown above. We can see that the nitrogen atom in the ammonia molecule contains a lone pair of electrons. So when ammonia reacts with the hydrogen ion, the lone pair is donated to the hydrogen ion thus forming the coordinate covalent bond.


The hydronium ion is formed from the reaction between a water molecule and a hydrogen ion (proton):

*** Note: the 2 red dots show a coordinate covalent bond (all electrons are donated by oxygen when bonding with the hydrogen)


From the Lewis dot electrons diagrams above, we can see that in the water molecule, oxygen has 2 pairs of lone electrons. One of these non-bonding pairs of electrons reacts with the hydrogen ion to form a coordinate covalent bond with the oxygen in the hydronium ion still left with a lone pair of electrons.

The ozone molecule is formed from the reaction between an oxygen molecule and an oxygen atom (radical):


        *** Note: the 2 red dots show a coordinate covalent bond (all electrons are donated by the oxygen molecule when bonding with the oxygen atom)

From the Lewis dot electron diagram above, we can see that when the oxygen reacts with an oxygen radical, an ozone molecule forms. There is still the double covalent bond (from the oxygen molecule) and there is a coordinate covalent bond between the oxygen molecule and oxygen atom. The unpaired electrons in the oxygen atom pair up so that there is space for the coordinate covalent bond to form.

Point 4.6 – Compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding.


Both ozone and oxygen contain oxygen atoms in their structure.

Both exist as gases in the atmosphere.

Both have dispersion forces between their molecules.


Ozone has a higher boiling point and melting point than oxygen. This is because it has a larger molecular mass which means there are stronger dispersion forces between ozone molecules than oxygen molecules. Thus more energy is needed to overcome the dispersion forces between ozone than in oxygen resulting in higher boiling and melting points. The fact that oxygen is non-polar and ozone is polar (small positive charge on central O atom) means that there are stronger intermolecular forces between ozone molecules; dispersion + dipole-dipole forces.

Ozone has a higher density than oxygen because it has a higher molecular mass. Thus ozone takes up more mass in a given volume than oxygen resulting in a higher density.

Ozone is more reactive than oxygen as it is a more powerful oxidant. This is because oxygen contains a stable double covalent bond while the ozone molecule has a double covalent bond and a single coordinate covalent bond. The single coordinate covalent bond is easier to break than the double covalent bond thus ozone is more reactive. When oxygen oxidises a metal, a metal oxide is produced:

2Mg (s) + O2 (g)  2MgO (s)

                     When ozone oxidises a metal, a metal oxide and oxygen is produced:

Mg (s) + O3 (g)  MgO (s) + O2 (g)

Point 4.7 – Compare the properties of the gaseous forms of oxygen and the oxygen free radical.

A free radical is a reactive particle that contains one or more unpaired electrons in its outer shell.  The oxygen atom (free radical) is an atom which has six valence (outer shell) electrons and has 2 unpaired electrons.


Because of the unpaired electrons in the oxygen free radical, it is very reactive and reacts immediately with other substances in order to obtain a full outer shell of electrons.  The similarity between the oxygen molecule, oxygen free radical and ozone is that they are all forms of the element oxygen. However the difference between the oxygen free radical, ozone and oxygen is that ozone and oxygen molecules have full outer shell of electrons which make them relatively stable compared to oxygen free radical. This is because it needs 2 more electrons to fill outer shell thus it is much more reactive than oxygen molecule and ozone molecule. Because it reacts so quickly, it has a short life span. The order of reactivity is: oxygen molecule < ozone < oxygen free radical (oxygen atom).

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Point 4.8 – Identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere.

Haloalkanes are carbon compounds that contain 1 or more halogen atoms in place of H atoms in the hydrocarbon. Chlorofluorocarbons (CFCs) are a class of haloalkanes in that they contain chlorine and fluorine atoms which substitute all of the hydrogen atoms in the compound. Example of a CFC include: CCl3F (trichlorofluromethane).

CFCs are synthetic in that they are produced from human activity. CFCs were developed to replace ammonia as a refrigerant in the 1930s. At the time, their properties were found to be ‘safer’ ...

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