Chapter 17
oxygen
Gases of the Atmosphere
The industrial importance of Air
• Critical as a raw material to the
chemical industry
• Individual gases from air have many
uses
Separation of Air
• Gases in the air may be separated by
fractional distillation
• Liquid air is separated by fractional
distillation
Nitrogen
• Nitrogen comprises 78% by volume of the
atmosphere
• It exists as stable N2 molecules and is
chemically comparatively inert
Nitrogen
• Physical and Chemical Properties
– Colourless, odourless, tasteless, non-toxic, diatomic
gas
– Dissolves only slightly in water
– Melting point is -210°C
– Boiling point is -196°C
– Major chemical property is that it is relatively
unreactive
– Nitrogen does not support combustion, it will not burn
and a match placed in a sample of N2 will stop
burning
– Lack of reactivity is due to the strong triple covalent
bonding within a nitrogen molecule. It requires a
great deal of energy to break this bond.
Nitrogen
• Industrial Uses of Nitrogen
– Can be used to prevent fires
– Prevents oxidation so can be used to stop
light bulb filaments from burning away
– Foods such as peas and bacon are packed in
nitrogen to stop them spoiling by oxygen in
the air
– At -196°C N2 is a cold unreactive liquid –
quick freezing foods – preserves flavour and
texture
– Liquid N2 is inexpensive
Nitrogen
• Laboratory Preparation of Nitrogen
– N2 can be prepared by warming ammonium
nitrite, which decomposes to form steam and
nitrogen
• NH4NO2 (s)  N2 (g) + 2H2O (g)
– OR a solution of ammonium chloride can be
added slowly to a warm solution of sodium
nitrite
• NH4+ (aq) + NO2- (aq)  N2 (g) + 2H2O (l)
Nitrogen
• Industrially prepared by
– Fractional distillation of liquid air
– Passing air over red-hot coke. The carbon
combines with oxygen to form carbon dioxide,
while nitrogen remains unchanged.
Nitrogen
• Nitrogen compounds are essential for living
things
• Nitrogen is important in the production of
proteins
• Animals rely on plants as a source of several
amino acids
• Plants use up all the soluble nitrates and
ammonium salts in the soil, the supply must
be replaced to meet demand
• This occurs through the nitrogen cycle
Component
Input to soil
Loss from soil
The Nitrogen Cycle
Atmospheric
nitrogen
Atmospheric
fixation
and deposition
Industrial fixation
(commercial fertilizers)
Crop
harvest
Animal
manures
and biosolids
Volatilization
Plant
residues
oxygen
Runoff and
erosion
Biological
fixation by
legume plants
Plant
uptake
Denitrification
Organic
nitrogen
Ammonium
(NH+4)
Nitrate
(NO3)
Leaching
Nitrogen
• Nitrogen is lost from the
soil
1. Absorption by the root
of green plants
2. Loss in drainage water
3. Denitrification – certain
bacteria in the soil can
convert nitrates into N2
which escapes into the
air. These bacteria are
active in the absence of
oxygen.
Nitrogen
• Nitrogen is returned to the soil
1. Nitrification – the remains of animals and
plants in the soil decay. Ammonium
compounds are converted to nitrates.
Nitrogen
2. Nitrogen fixation –
1.
2.
Free living nitrogen fixing
bacteria (in the presence of
oxygen) convert atmospheric
nitrogen into amino
compounds – eventually
changed to nitrates by
nitrification
Nitrogen-fixing bacteria are
found in the root nodules of
some plants fix nitrogen in the
soil to amino compounds.
When the plants or bacteria
die the nitrogenous
compounds are returned to
the soil and changed into
nitrates
3.
Nitrogen fixation –
3.
4.
Nitrogen
Lightning – oxygen and
nitrogen in the air react in the
presence of lightning to form
nitrogen monoxide. Lightning
flashes have sufficient energy
to split the triple bond.
Nitrogen monoxide reacts
rapidly with oxygen to form
nitrogen dioxide
Fertilisers added to the soil
and initially created by the
Haber process
•
N2 (g) + H2 (g)  2NH3 (g)
Nitrogen
• Algal blooms
– Eutrophication results from over-use of
nutrients, which encourages algal growth in
waterways
Review check
Review
• Complete revision question 1
Oxygen
• Constitutes 21% of air by volume
Oxygen
• Physical and chemical
properties
– Colourless, odourless,
diatomic gas
– Boils at -183°C
– Freezes at -219°C
– Reacts directly with most
other elements
– Forms compounds with all
elements except helium,
neon, argon and krypton
– Only slightly soluble in water
– Soluble enough to sustain
the life of aquatic plants and
animals
Oxygen
• Industrial and other uses of oxygen
– Almost all combustion processes
require oxygen
– In the steal industry, oxygen is used to
burn off the carbon which is present
as an impurity in cast iron and which
makes iron brittle.
– In hospitals, oxygen is used to revive
accident victims and assist patients
with breathing difficulties
– In the chemical industry oxygen is
used in the production of a number of
chemicals
Oxygen
• Laboratory preparation of oxygen
– Using MnO2 as a catalyst H2O2 decomposes
to produce H2O and O2
MnO2
• H2O2 (aq)
H2O (l) + O2 (g)
– Water can be dropped into sodium peroxide
– A mixture of potassium chlorate and
manganese (IV) oxide can be gently heated.
The heated manganese (IV) oxide is a
catalyst. Oxygen will be released
Ozone
• Ozone is an allotrope of oxygen with the
chemical formula O3.
• Has a different structure to the oxygen we
breathe, but is still composed of oxygen
atoms
• Light blue gas with a distinctive odour
• Can be helpful or harmful depending on
where it is found in the atmosphere
Ozone
• Lightning
– Each time a spark of
lightning jumps, ozone
is produced.
– Sufficient energy is also
provided for nitrogen to
react with oxygen in the
air to form oxide of
nitrogen.
– These react with
rainwater to produce
nitric acid, which
increases the nitrogen
content of soil
Ozone
Too much in the
troposphere harmful
Stratosphere
ozone protects
Ozone
• Tropospheric ozone is harmful to life
– The concentration of ozone is increased as a
consequence of reactions between gases such as
nitrogen oxides and some hydrocarbons in the
presence of sunlight
– Ozone is a poisonous gas and pollutant
– It can cause rubber to deteriorate breaking the C
double bond
– It is a powerful oxidant and is irritating to the eyes and
mucous membranes
– Is highly toxic to plants, and can cause fatigue, lack of
coordination and lung disturbances in people
Ozone
• Photochemical smog
– Occurs when the sun acts on emissions of
nitrogen and hydrocarbons
– Hydrocarbons and Nox (NO(g) and NO2 (g))
react in the presence of sunlight and NO2 and
ozone
Ozone
• The ozone in the stratospheric is vital to
the survival of life on Earth
• Ozone absorbs about 97% of the highenergy ultraviolet radiation from the sun
• This radiation causes sunburn, wrinkles,
cataracts, immune system damage and
skin cancer
Carbon Dioxide
• Is a colourless, odourless gas that is
present only in small amounts and yet is
an extremely important gas in our
atmosphere.
• In small quantities it is essential
• In large quantities it can be regarded as a
pollutant.
• It is needed in photosynthesis
Carbon Dioxide
• Photosynthesis and respiration can be
considered as opposite processes and
contribute to a balance between important
gases oxygen and carbon dioxide in the
atmosphere
• animation
Carbon Dioxide
• Preparation of carbon dioxide
– Carbon dioxide gas is generated
in Kipp’s apparatus.
– Hydrochloric acid is allowed to fall
onto marble chips
– The resultant gas is bubbled
through water to dissolve acid
spray and dried by concentrated
sulfuric acid
– The gas is collected in the flask
Carbon Dioxide
• Industrial production and uses of carbon
dioxide
– Limewater is used to test for carbon dioxide.
It turns milky in the presence of the gas.
– Quicklime (calcium oxide) is produced in the
heating of limestone
– Carbon dioxide is given off in the fermentation
of various sugars
• C6H6O6 (aq)  2C5H5OH(aq) + 2CO2 (g)
ethanol
Carbon Dioxide
• Carbon dioxide is used in fire
extinguishers because it does not support
combustion
• Also as a refrigerant (dry ice), in soft
drinks and as a solvent for some organic
compounds
Carbon Monoxide
• Is produced when carbon or hydrocarbons
are burnt in the presence of insufficient
oxygen
• It is a pollutant gas, even in small
quantities, and is colourless and odourless
• It bonds to the haemoglobin in blood more
easily than oxygen and prevents oxygen
from being carried to the body tissues
The Carbon – Oxygen Cycle
The Gases in Air
• The Noble Gases
– Are unreactive as they have a full outer shell
of electrons
– All of group 18 of the periodic table are gases
at room temperature.
– 8 electrons in the outer shell (Helium – 2)
Review
• Complete the multiple choice revision
questions pages 420, 421