Air and Water

Air

If a sample of clean air was taken from any location in the world and analysed, it would be expected that the composition (by volume) to be similar to below:

Fractional Distillation of Air

As we know from above air is a major source of oxygen, nitrogen and the noble gases. These gases are obtained by the fractional distillation of liquid air. It is slightly complicated and involves different steps but nevertheless must be learnt:

1. The air is obtained through intake pipes and passed through very fine filters to remove any solids.

2. This air is then cooled to -80°C (using heat exchnagers). At this temperature water vapour and carbon dioxide are solids and are removed as they can cause severe blockages.

3. The air minus water and carbon dioxide is cooled down further to below 200°C at which most of the air liquefies.

4. The liquid air is pumped into a fractional column where it is gradually allowed to warm. They turn into gases, which rise and condense in different fractions according to their boiling points.

5. The gases are collected separately before being stored in tanks and cylinders ready for use.

A quick outline of uses for the various gases produced:

Oxygen –

medical uses (incubators, etc.)

conversion of pig iron into steel

Nitrogen –

the most important use of nitrogen is in the Haber Process (studied later) to produce ammonia

provides inert atmosphere, esp. in packaging so contents don't react and go ‘off’ like in food

Noble Gases –

argon is used to fill light bulbs

neon is used in advertising signs

helium is used to fill airships and air balloons 

Pollutants

Notice at the beginning of the chapter the composition of air was based on clean air, i.e. not polluted.

In chemistry we are mainly concerned with 4 pollutants:

i) Carbon Monoxide

Burning of carbon containing substances in plentiful oxygen reacts in complete combustion and the formation of carbon dioxide:

C + O₂ = CO₂

However, the incomplete combustion of carbon (due to small presence of oxygen) results in the formation carbon monoxide.

2C + O₂ = 2CO

The major sources of carbon monoxide include – internal combustion engines in vehicles and factories.

Carbon monoxide is the most toxic of the pollutants. It binds onto haemoglobin in blood displacing blood. The result is cells not getting enough oxygen leading to fatigue and in large enough quantities, death.

ii) Sulfur Dioxide

The main source of sulfur dioxide is from heavy industry and power plants that burn hydrocarbons with sulfur impurities in them. To a lesser extent they also are released from volcanoes.

S + O₂ = SO₂

This sulfur dioxide dissolves in water in clouds to form sulfurous acid, which then is further oxidised to sulfuric acid.

SO₂ + H₂O = H₂SO₃

2H₂SO₃ + O₂ = 2H₂SO₄

These two acids are largely responsible for acid rain. Acid rain has adverse effects on both the environment and people. Acid rain lowers pH levels of waterways killing aquatic life, leeches away important nutrients in soils and kills vegetation. Humans are not directly affected as the concentration of sulfuric acid in rain is so low, but building, historic landmarks and statues. If they are made out of limestone their features become unidentifiable due to the following reaction:

CaCO₃ + H₂SO₄ = CaCO₄ + H₂O + CO₂ asthmatics

As a gas alone sulfur dioxide can cause bronchospasms in asthmatics.

iii) Lead Compounds

Lead compounds were once commonly used as additives in petrol. They are released during the combustion of leaded fuels. They are considered very bad pollutants as they have been linked to have adverse effects on humans such as causing mental disability in children and cancer.

iii) Nitrogen Oxides (NO_x)

The main source of nitrogen oxide is from internal combustion engines. It is NOT the fuel in the engine that creates the oxides, instead the high temperatures generated by the engine that allow the normally unreactive nitrogen and oxygen to react in the atmosphere.

The intense heat of an engine causes nitrogen and oxygen to react to produce nitrogen monoxide:

N₂ + O₂ = 2NO

The nitrogen monoxide then oxidises in air to form nitrogen dioxide.

2NO + O₂ + 2NO₂

The concerns about nitrogen dioxide arise from its similarity to sulfur dioxide in the way that it contributes to acidic rain. As a gas alone, nitrogen monoxide and nitrogen dioxide can cause bronchospasm in asthmatics. The nitrogen oxides also contribute to photochemical smog.

Catalytic Converters

The introduction of strict pollution laws meant that all cars in certain areas had to be fitted with catalytic converters. It is a device in the exhaust system of a vehicle that converts toxic pollutants to less toxic pollutants through catalysed reactions.

Car exhaust fumes cause the toxic pollutant gases carbon monoxide and nitrogen monoxide. We have studied the sources of these. The following reactions naturally occur to convert toxic pollutants into less toxic ones but at extremely slow rates:

2CO + O₂ = 2CO₂

2NO + 2CO = N₂ + 2CO₂

The catalyst in the converter in the exhaust system speeds up these reactions. Instead of the reactions taking place in the atmosphere at a slow rate the catalytic converter ensures these reactions take place before gases leave the exhaust.

We’ve discussed before that the removal of nitrogen monoxides are essential as they cause respiratory disease as well as photochemical smog. Carbon monoxides are very toxic therefore are preferred to be turned into carbon dioxide.

Earlier we learnt about lead compounds. Another reason why lead compounds aren’t used in cars, apart from their adverse affects, is that they ‘poison’ the catalyst in catalytic converters. This is due to the lead compounds coating the surface of the catalyst preventing it doing its job. 

Water

We now move onto our second major resource studied in this chapter – water. 

Tests for Water

We have two methods to test for water:

1. The presence of water turns anhydrous copper sulfate from white to blue hydrated copper sulfate:

CuSO₄ + 5H₂O = CuSO₄.5H₂O

2. The presence of water turns cobalt chloride paper from blue to pink.

CoCl₂ + 6H₂O = CoCl₂.6H₂O

The substances above need to be kept dry before the test. We do so using a desiccator – basically a sealed container with a bed of silica gel to absorb water in preference to other substances.

It should be noted that the boiling point and non-conductivity of pure water aren’t really considered tests especially if the water produced is so minute in volume.

Uses of Water

Water is by far the most important liquid on earth. It is a universal solvent, dissolving ionic and other polar compounds.

Industry uses huge amounts of water mainly for cooling. Other uses are outlined:

• electricity generation
• solvent use in refining ores, etc.
• water blasting and water jet cutting

This water does not have to be distilled or chlorinated at all. However, water used domestically such as for washing, cleaning and cooking blah blah needs to be reasonably clean.

Purification of Water Supply

Most drinking and cooking water is obtained from lakes and rivers. Pollution levels here are low, but pollutants still exist. To make the water suitable for domestic use, the water undergoes treatment including filtration and chlorination:

1. Water is pumped into screens, which remove solid floating debris.

2. Aluminium sulfate is added to coagulate (stick together) small pieces of clay so that they are easily removed.

3. The water is then filtered through coarse sand to remove larger, insoluble debris.

4. The water encounters more flocculants (chemicals that make particles move down to bottom of tank) and is filtered again through fine sand.

5. Chlorine gas is bubbled through the water to kill bacteria. This makes the water slightly acidic, so to reverse this appropriate amounts of sodium hydroxide (an alkali) is added.

In some countries where tooth decay is a particular problem fluoride is added to the water to combat tooth decay.

Corrosion

Corrosion is the general term given to the processes, which take place where metals and alloys are chemically attacked, by oxygen, water and/or any other substance found in their immediate environment.

When looking at the relationship between the reactivity series and corrosion it is noticed that the higher the metal in the reactivity series, the more faster it will corrode.

Rusting is the most well known example of corrosion. It only applies to iron and metallic substances containing iron, e.g. steel. Rust is a orange-red powdery substance mainly consisting of hydrated iron(III) oxide (Fe₂O₃.XH₂O). By looking at the formula it makes sense that rust can only occur when oxygen and water are present. It should be noted that rusting is encouraged by the presence of salt (hence why things rust quickly in coastal environments)

Rust Prevention

Rust is a very annoying substance and not only causes lots of damage to iron and steel structures but ruins them cosmetically.

Rust prevention methods do one or both of the following:

• isolating iron from environment by protective cover
• preventing oxidation of iron/steel

The exclusion of oxygen and water reaching metal surfaces is probably the most basic and can be achieved by:

• painting
• coating with plastic
• oiling/greasing (why sodium and potassium are kept in oil)
• plating with non-reactive metal
• protective oxide layer (relate to aluminium oxide)

The preventing of oxidation is slightly more complicated. There are two main methods:

Sacrificial Protection

This method is based on the idea that more reactive metals corrode in preference to the less reactive iron. The metals used for sacrificial protection are normally zinc and magnesium. Since the zinc and magnesium are more reactive it forms ions in preference to iron – protecting it from rusting.

As long as a more reactive metal is in contact with the iron, the structure will be protected. Not until all the reactive metal has reacted with the iron start rusting.

Galvanising

Galvanisation is very similar to sacrificial protection. Objects that are galvanised are usually dipped into a tub full of molten metal (usually zinc). The thin layer of zinc that results slowly corrodes away and loses electrons to the iron, thereby protecting it. If zinc in a particular area is scratched away the whole structure is still safe due to the above ideas of sacrificial protection. 

Ammonia

Ammonia is a colourless gas with the formula NH₃. It also has a very pungent smell. Most of the ammonia produced in the world is made using the Haber Process.

The Haber Process

The origins actually had a deeper sinister background, being developed for explosive manufacturing, but has since been used to make ammonia for agricultural reasons. It involved reacting nitrogen and hydrogen.

The sources of the raw materials need to be known:

• hydrogen – cracking of hydrogen or reaction of methane gas with steam to produce carbon monoxide and hydrogen
• nitrogen – fractional distillation of air

1. Nitrogen and hydrogen are pressurised to 200 atmospheres and passed over a catalyst of iron(III) oxide at a temperature of approximately 450°C.

2. Within the reaction chamber the following reaction occurs:

N₂ + 3H₂ = 2NH₃

Note that this reaction is both exothermic and reversible.

3. The reaction mixture is extracted and placed in a condensing chamber. The ammonia is separated as the condenser is allowed to cool. Since ammonia has the highest boiling point it liquefies and is pumped out while the hydrogen and nitrogen remains gaseous and resumed into the reaction chamber.

Uses of Ammonia

Worldwide the production of ammonia exceeds 100 million metric tonnes a year. Why so popular? It is used in:

• making explosives
• used to make fertilisers by reacting with acids to form ammonium salts
• household cleaning chemicals
• manufacture of nitric acid

i) Artificial Fertilisers

Over 80% of ammonia produced ends up in fertilisers. Most fertilisers contain the three most important nutrients to plants – nitrogen, phosphorous and potassium, hence the name NPK fertilisers.  Crops remove these nutrients while they grow so farmers add artificial fertilisers to replace these nutrients.

Ammonium Compounds

Ammonia reacts to form ammonium compounds.

ammonia – NH₃

ammonium – NH₄^- (polyatomic ion)

As we know the primary use for ammonium salts is as a component of NPK fertilisers to supply nitrogen to plants for the production of plant proteins.

Ammonium salts are ideal for fertilisers as they are all water soluble. They are prepared by reacting ammonia with the appropriate salt, for example:

NH₃ + H₂SO₄ = (NH₄)₂SO₄ (ammonium sulfate)

NH₃ + HNO₃ = NH₄NO₃ (ammonium nitrate = most common)

Decomposition of Ammonium Salts

Ammonium salts decompose on heating, however a different number of products are formed. In general all ammonium salts decompose to form ammonia.

NH₄Cl = NH₃ + HCl

(NH₄)₂SO₄ = NH₃ + NH₄HSO₄

(NH₄)₂CO₃ = NH₃ + NH₄HCO₃

and the one exception is ammonium nitrate: NH₄NO₃ = N₂O (nitrous oxide) + 2H₂O

Ammonia from salts is also displaced by reaction with alkalis (see chapter 8).

Carbon Dioxide

We are all familiar with carbon dioxide. Some of the sources include:

• reaction between an acid and a carbonate
• respiration (C₆H₁₂O₆ + 6O₂ = 6H₂O + 6CO₂)
• complete combustion of carbon containing substances

Carbon dioxide also has some important uses:

• as a refrigerant
• fire extinguishers
• carbonated drinks
• special effects (smoke machines)

The Carbon Cycle

The carbon cycle is a series of events that cycle carbon around the world. The cycle is as follows:

Plants absorb carbon dioxide from the air and use it to photosynthesise to produce glucose and oxygen

the carbon is now stored in the form of glucose in the plant. From here two things can happen

1. If the plant is eaten by an animal the transfer of glucose occurs. This glucose is used in respiration to produce water and carbon dioxide which returns back into the atmosphere

2. If the plant dies it anaerobically decays to form hydrocarbons which contain the original carbon

Hydrocarbons are extracted and these are burnt in power plants for energy. The reaction produces water and carbon dioxide which returns back into the atmosphere

Methane

Methane is a colourless and odourless gas. The main sources of methane are:

• decomposition of vegetation
• waste gas from digestion in animals

The Greenhouse Effect and Climate Change

Carbon dioxide and methane are similar in the fact they are both the main greenhouse gases. The sun sends heat energy to the earth through infra-red radiation. Naturally some of this radiation is absorbed by earth and some is reflected into space through the atmosphere. Greenhouses come into play as they trap this reflected heat energy within the atmosphere, hence causing the Greenhouse effect.

Greenhouse gases have always been present naturally and are responsible for why the world is suitable to live in. However in the recent years these have increased. More fuel combustion has taken place around the world resulting in more carbon dioxide being released. An increase in population has also led to the demand of cattle and sheep – huge producers of methane gases. This increase in greenhouse gases has led to an increase in trapping heat. Ultimately this leads to the average temperature of the earth rising. This is called global warming.