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Environmental Chemistry
Senior Chemistry
Cycles in nature
In the environment, there are many elements that can take on different forms
depending on the chemical reactions that are taking place.
Carbon and oxygen are
involved in a continuous
cycle on Earth with living
organisms.
Nitrogen compounds are varied and are
involved in a more complex cycle with
different organisms on Earth.
Oxygen (or not) in decomposition
When complex organic compounds containing C, N, P and S from living organisms breaks
down, there are two possible chemical pathways leading to two different sets of products
Aerobic – with oxygen
Anaerobic – without oxygen

CO2

CH4

SO42-

H2S

PO43-

PH3

NO3-

NH3
Notice that these products are highly
oxidised due to the presence of
oxygen. In fact, they are in their
highest possible oxidation states
Notice that these products do not
contain oxygen and exist in their
lowest possible oxidation states
Carbon and oxygen cycles
Where on the diagrams below can you see the complimentary processes of
photosynthesis and aerobic respiration? What are the chemical equations?
Diagram 1
Diagram 2
Carbon and oxygen cycles
Where on the diagrams below can you see the complimentary processes of
photosynthesis and aerobic respiration? What are the chemical equations?
Carbon cycle
Photosynthesis: 6CO2 + 6H20 + sunlight  C6H12O6 + O2
Carbon and oxygen cycles
Where on the diagrams below can you see the complimentary processes of
photosynthesis and aerobic respiration? What are the chemical equations?
Carbon cycle
Oxygen cycle
Aerobic respiration: C6H12O6 + O2  6CO2 + 6H20 + energy
Nitrogen cycle
Nitrogen makes up approximately 78% of our atmosphere as N2. This form
of nitrogen can be altered by natural and industrial processes.
Natural processes
1. Lightning strikes convert nitrogen gas into oxides
N2 + O2  2NO (nitric oxide)
NO + O2  2NO2 (nitrogen dioxide)
NO2 + H2O  soluble nitrates (nutrients for plants)
2. Nitrogen fixing bacteria found on root nodules of legumes
(e.g. beans, peas, lentils, clover)
N2  NH3/NH4+
3. Decomposition by decomposers (bacteria, fungi, worms)
Proteins  amino acids  NH3, NH4+, NO3-
Source:
http://media1.shmoop.com/images/biology/b
iobook_eco_12.png
Nitrogen cycle
Nitrogen makes up approximately 78% of our atmosphere as N2. This form
of nitrogen can be altered by natural and industrial processes.
Industrial processes
1. Fertiliser manufacturing (Haber process)
N2 + 3H2  2NH3 (Haber process to produce ammonia)
Ammonia is then converted to other soluble fertilisers. These convert
to nitrates that can be absorbed by plants:
•
•
•
Urea (NH2CONH2)
Ammonium nitrate (NH4NO3)
Ammonium sulfate ((NH4)2SO4)
2. Internal combustion (same as lightning)
N2 + O2  2NO (nitric oxide)
NO + O2  2NO2 (nitrogen dioxide)
NO2 + H2O  soluble nitrates (nutrients for plants)
Source:
http://media1.shmoop.com/images/biology/b
iobook_eco_12.png
Fertilisers

Plants need nutrients to grow and they must be soluble in
order for them to be absorbed by the roots of plants.

Specifically, the 3 major nutrients needed by plants are:

N (nitrogen, often in the form of ammonium nitrate or sulfate
and urea; nitrate is the form that plants can absorb, so if the
form of the fertiliser is not a nitrate, they must be converted
to nitrates by soil bacteria)

P (phosphorous, often in the form of soluble phosphates like
P2O5 ; plants absorb phosphorous in the form of dihydrogen
phosphate, which can be produced from phosphate in acidic
soils or is manufactured as ‘superphosphate’)
PO43- + H+  H2PO4-

K (potassium, often in the form of potash K2O)
Greenhouse effect
How does a green house become warmer inside than outside?
Greenhouse effect
How is the Earth like a greenhouse and what is the “Enhanced Greenhouse Effect”?
• Short UV wavelengths
penetrate the Earth’s
atmosphere.
• Longer IR wavelengths are reradiated from the Earth, but
do not escape as easily,
warming the planet (natural)
• With increased greenhouse
gases, humans are increasing
this effect – “Enhanced
Greenhouse Effect”
Source:
http://www.landlearnnsw.org.au/__data/assets/
image/0012/301332/greenhouse-effect.jpg
Greenhouse gases
Source: http://static.trunity.net/files/221601_221700/221660/screen-shot-2012-11-20-at-4.01.21-pm.png
Greenhouse
effect consequences
Question: What
evidence of these
effects do we see in
news reports today?
Acid rain
We have already seen that carbon dioxide combines with water making carbonic
acid. This means that rain is naturally acidic, as low as 5.6. Any value below this
is considered to be acid rain, caused by the release of SO2 and oxides of nitrogen
CO2 + H2O  H2CO3 (carbonic acid)
The question is, how acidic is the rain and how do we determine this?
pH
pH is a measure of the acidity of a solution. More
accurately stated it is a measure of the hydrogen ion
concentration on a logarithmic scale. pH stands for
‘potential of hydrogen’
Mathematically this is ,
pH = - log [H3O+]
or
pH = - log [H+]
The inverse of the above expression is
[H+] = 10-pH
So, in a neutral solution, the concentration of hydrogen
is 10-7 M:
pH = -log[H+] = -log(10-7)
pH = 7
pOH
pOH is a measure of the basicity of a solution. More
accurately stated it is a measure of the hydroxide ion
concentration on a logarithmic scale.
Mathematically this is ,
pOH = - log [OH-]
The inverse of the above expression is
[OH-] = 10-pOH
So, in a neutral solution, the concentration of hydrogen
is 10-7 M:
pOH = -log[OH-] = -log(10-7)
pH = 7
Also,
pH + pOH = 14
The logarithmic scale
The log scale is based on multiples of
ten where the log is the value of
exponent when the number is written
as an exponent.
To calculate pH, just use the
log button on your calculator
You try:
Number Representation and Logarithms
Exponent
Number
Log of the Number
Notation
1000
103
3
2
100
10
2
1
10
10
1
1
100
0
-1
0.1
10
-1
-2
0.01
10
-2
0.001
10-3
-3
-4
0.0001
10
-4
If an acid has an H+
concentration of 0.0001 M, find
the pH.
Solution:
First convert the number to exponential
notation, find the log, then solve the pH
equation.
H+ = 0.0001M = 10-4; log of 10-4 = -4;
pH = - log [ H+] = - log (10-4) = - (-4) = +4 =
pH
pH scale
The pH scale, (0 - 14), is the full set of pH numbers which indicate the concentration of
H+ and OH- ions in water.
pH Scale Principle:
H+ ion concentration and pH relate inversely.
OH- ion concentration and pH relate directly.
The following statements may be made about the
pH scale numbers.
Complete the last two
a. Increasing pH means the H+ ions are
decreasing.
b. Decreasing pH means H+ ions are increasing.
c. Increasing pH means OH- ions are
d. Decreasing pH means OH- ions are
pH exercises
Condition
Most H+ ions: pH = 4; or pH = 5.
Most OH- ions: pH = 10; or pH = 13.
Least H+ ions: pH = 12; or pH = 13.
Least OH- ions: pH = 8; or pH = 9.
If acid was added to a solution of pH 4, the pH would increase or
decrease?
If acid was removed from a solution of pH 3, the pH would inc. or dec.?
If base were added to a solution of pH 9, the pH would inc. or dec.?
If base were added to a solution of pH 2, the pH would inc. or dec.?
If acid were added to a solution of pH 13, the pH would inc. or dec.?
If base were removed from a solution of pH 12, the pH would inc. or dec.?
Answer
pH exercises (solutions)
Condition
Answer
Most H+ ions: pH = 4; or pH = 5.
pH = 4
Most OH- ions: pH = 10; or pH = 13.
pH = 13
+
Least H ions: pH = 12; or pH = 13.
pH = 13
Least OH ions: pH = 8; or pH = 9.
pH = 8
If acid was added to a solution of pH 4, the pH would increase or
Decrease
decrease?
If acid was removed from a solution of pH 3, the pH would inc. or dec.?
Increase
If base were added to a solution of pH 9, the pH would inc. or dec.?
Increase
If base were added to a solution of pH 2, the pH would inc. or dec.?
Increase
If acid were added to a solution of pH 13, the pH would inc. or dec.?
Decrease
If base were removed from a solution of pH 12, the pH would inc. or dec.? Decrease
Acid rain
Recall, in our previous discussion about non-metal oxides, oxides of sulfur and nitrogen form
strong acids (completely dissociate) that lead to acid rain.
SO2 + H2O  H2SO3 (sulfurous acid)
SO3 + H2O  H2SO4 (sulfuric acid)
2NO2 + H2O  HNO3 + HNO2 (nitric and nitrous acid)
These non-metal oxides are all gases
Their acidic products all contribute to the acidity of rain.
Note: strong acids completely dissociate to their ions, so that a monoprotic acid will have the
same hydronium ion concentration as the original concentration of the acid.
Acid rain effects
Many statues are made from calcium carbonate and acid rain
reacts with these statues to erode their features and in some
cases, create cracks in them.
CaCO3 + H2SO4  Ca2+ + SO42- + H2O + CO2
In the example reaction above, you can see that soluble
sulfate ions are produced, which can erode the surface of a
statue or penetrate the pores/cracks, which can lead to
serious damage when the calcium sulfate (gypsum)
crystallises.
Acid rain effects -statues
Many statues are made from calcium and or magnesium
carbonates and acid rain reacts with these statues to
erode their features and in some cases, create cracks in
them.
CaCO3 + H2SO4  Ca2+ + SO42- + H2O + CO2
In the example reaction above, you can see that soluble
sulfate ions are produced, which can erode the surface
of a statue or penetrate the pores/cracks, which can
lead to serious damage when the calcium sulfate
(gypsum) crystallises and expands the cracks.
The reactions between calcium carbonate and nitric
acid is similar, producing nitrate ions instead of sulfate.
Source: http://www.buzzle.com/img/articleImages/168126-8920-53.jpg
Acid rain - metals
Reactive metals also are affected by acid rain. Metals such as iron, zinc (found in
galvanised iron) and copper (see statue of liberty below) react are oxidised by
acidic precipitation.
Fe + 2H+  Fe2+ + H2
Cu + 2H+  Cu2+ + H2
The Statue of Liberty in New
York is made of copper and
shows what happens when
the pure metal is oxidised to
copper oxide.
Source:
http://userscontent2.emaze.com/images/09
0c6d2f-f7de-4853-b5a0b547a4e73f81/5d4ebc4e-0866-4c57-96cfc4dd44795736.jpg
Acid rain – metal mobilisation
Metals such as aluminium are mostly in non-toxic forms such as aluminium hydroxide
in soils.
When the pH falls below 5, aluminium ions are dissolved in the water (mobilisation)
and become toxic to plants, stunting root growth and reducing the uptake of
important nutrients like calcium.
Aluminium ions reduce populations of soil microbes that act as decomposers, which
release important nutrients into the soil. They can also clog the gills of fish causing
them to die.
Other toxic metals that are mobilised by lower soil pH include lead, mercury, zinc,
copper, cadmium, chromium, manganese, and vanadium.
Source:
http://acidraintoxicity.weebly.com/
uploads/4/2/9/1/42911791/9032843
_orig.jpg
Photochemical smog
Oxides of Nitrogen (NOx)
Nitrogen and oxygen in the air can react at high temperatures
that are present inside car engines:
N2(g) + O2(g)  2NO (g)
2NO(g) + O2(g)  2NO2(g)
Photochemical smog
Nitrogen dioxide combines with unburnt hydrocarbons (UHCs)
and other chemicals to produce the brown haze known as
photochemical smog. This causes respiratory problems in many
people.
In addition, NO2 can react with UV light to produce toxic ozone O3
in the troposphere.
NO2(g) + UV  2NO(g) + O(g)
O(g) + O2(g)  O3(g)
www.birmingham.gov.uk
Photochemical smog
Chemical causing photochemical smog can be designated as “primary” or
“secondary” pollutants. Primary are emitted into the atmosphere directly
whereas secondary are produced from reactions involving primary pollutants.
Primary pollutants
Secondary pollutants

NO (major pollutant)

O3 (from NO2)

Hydrocarbons (UHCs) (major
pollutant)

NO2 (from NO)

HNO3 (from NO2)

H2SO4 (from SO2)

PAN (peroxyacyl nitrates) (from
UHCs)

CO

SO2

CO2
Catalytic converters
Catalytic Converter: Rhodium, palladium
and platinum catalysts are coated on
a ceramic honeycomb block to
remove unburnt hydrocarbons, nitric
oxide and carbon monoxide from
motor vehicles.
Chemistry
www.chem.brown.edu
Catalytic converters - efficiency
The efficiency of the catalytic converter
depends upon the conditions of the exhaust.
Air:fuel ratio is critical to the efficient
removal of pollutants.
As seen in the graph (right), low oxygen
results in incomplete combustion of HC and
CO whereas to much oxygen results in a
greater amount of NOx.
Temperature is also a key factor in the efficiency
of the catalytic converter. You can clearly see
from the graph (left) that the removal of
pollutants is greatest when above 4000C. This
means at start up, an engine is producing more
pollutants than when it is hot.
Source: https://gsf165.wordpress.com/2013/06/26/catalytic-converter/
Water treatment - turbidity

Turbidity is a measure of the
suspended particles in a sample of
water. This is often caused by erosion
or run-off

The more turbid a body of water, the
more toxic contaminants there can be,
including bacteria, protozoa, etc.
Water treatment - turbidity

Alum or aluminium sulfate is a common chemical that is used to reduce the
turbidity of water. It forms insoluble aluminium hydroxide when added to
water.

Water is turbid due to fine particles that are too small to settle. Alum is used
as a flocculent to aggregate the particles so that they are heavy enough to
settle to the bottom of the container. See the video on the next slide.
Two theories regarding how the flocculation occurs:
1. Neutralisation – because the silt particles are negatively
charged on their surfaces, they repel each other. The
charged aluminium particles adsorb onto the silt particles
and make them less repulsive, resulting in aggregation.
2. Sweep-floc – this theory suggests that the flocculent
precipitate (Al(OH)3), collides with and drags the colloid
particles down with them.
Water treatment - flocculation

Movie too large to download
Water treatment - chlorination
At water treatment facilities, Cl2 gas is added to the water supply towards the
end of the process in order to kill any remaining bacteria in the water.
In water, chlorine gas combines with water to form hypochlorous acid (HOCl).
Hypochlorous acid is a weak acid and dissociates further to hypochlorite (OCl-).
Although chlorine
gas and
hypochlorite will
kill bacteria, this
is the most
effective sanitiser
Water treatment - chlorination
The effectiveness of chlorine in water
purification is strongly related to the pH of the
solution. This is due to the equilibria that are
established.
For you to do: use the equilibria above and
the graph to the right to explain the effect of
pH on the equilibria between chlorine, water,
hydrochloric and hypochlorous acids.