Chapter 17

Gases of the Atmosphere

oxygenoxygen

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

The Nitrogen CycleAtmospheric

nitrogen

Atmosphericfixation

and deposition

Animalmanures

and biosolids

Industrial fixation(commercial fertilizers)

Crop harvest

Volatilization

Denitrification

Runoff anderosion

Leaching

Organicnitrogen

Ammonium(NH4)

Nitrate(NO3)

Plant residues

Biologicalfixation by

legume plants Plantuptake

Immobilization

Mineralization

Input to soilComponent Loss from soil

-

+

oxygen

Nitrogen• Nitrogen is lost from the

soil1. Absorption by the root of

green plants2. Loss in drainage water3. 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.

Nitrogen2. Nitrogen fixation –

1. Free living nitrogen fixing bacteria (in the presence of oxygen) convert atmospheric nitrogen into amino compounds – eventually changed to nitrates by nitrification

2. 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

Nitrogen3. Nitrogen fixation –

3. 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

4. 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

• 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

MnO2

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 high-energy 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