o level chemistry for Composition of air

Composition of air

• The atmosphere is a layer of air containing mixture of several gases. This mixture composition varies according to time and place. 
• The composition of water vapour varies from 0-5%, depending on the humidity of air.

Fractional distillation to separate components of air

• Air is the main source of oxygen, nitrogen, and noble gases.
• These are separated by first liquefying the air and then separating the components of liquid air by fractional distillation.

1. air is filtered to remove dust
2. any carbon dioxide or water vapour in the air must be removed, otherwise when air is cooled, they would solidify and block up the pipes.
3. Carbon dioxide gas is removed by bubbling the air through an alkali like sodium hydroxide.
1. 2NaOH + CO₂ --> Na₂CO₃ + H₂O
4. Water is removed in a drying tower with a drying agent such as silica gel.
5. The air is then compressed to about 200 atmospheric pressure. On compression, a gas becomes hotter. The compressed air is then suddenly allowed to expand through a fine nozzle and it becomes colder. This cold air is returned to the compressor, and on its way, it helps to cool more incoming air. This process of compression and expansion is continued until the temperature drops to -200^oC (73K). At this temperature, all the gases except neon and helium would have been liquefied.
6. The liquid air is pale blue because of the presence of liquid oxygen. It is passed into a fractionating tower, where it is slowly warmed up. This first gas to boil off is nitrogen (-196^oC).
7. When all the nitrogen has been given off, the temperature rises to -186^oC and argon gas is boiled off. 
8. Oxygen is next, at -183^oC. The gases are boiled off separately and are collected and stored under pressure in cylinders.

Percentage Composition of Oxygen in Air

• A known volume of air is passed through tube with burning copper powder and oxygen in air will react with hot copper powder to produce black copper oxide:
• 2Cu(s) + O₂(g) --> 2CuO(s)
• If oxygen is depleted, the readings on both syringes will be steady and the reaction has completed. Hence, to find the volume of oxygen in air collected in syringe:
• Volume of O₂ = Initial volume of air – Final volume of air
• For instance, the initial volume of air in one syringe is 80cm³ and the final volume is 64cm3. Hence, the percentage volume of O₂ in air is:

Oxygen

A. Respiration

Oxygen is essential for all plant and animal life on earth. The process by which living organisms produce energy from their food is called respiration. Oxygen is essential for this process:

sugar + oxygen ---> carbon dioxide + water + energy

• When we breathe in, we take air into our lungs.
• oxygen from the air dissolves in the blood in our lungs
• the dissolved oxygen is then taken to the cells in all parts of our body
• oxygen reacts with sugars in the cells to produce energy

B. Combustion/Burning

3 things are required for combustion

1. fuel
2. heat
3. oxygen

• MOST substances react with O₂ to in exothermic reaction, which is called combustion. If flames are produced during combustion, it’s called burning.
• ALL carbon compounds burn in O₂ to produce CO₂ while ALL hydrogen containing compounds burn in O₂ to produce H₂O.
• When adequate supply of oxygen is available during burning, it will create a complete combustion. If otherwise, the combustion is incomplete.
• E.g. CH₄(g) + 2O₂(g) --> CO₂(g) + 2H₂O(g), makes up a complete combustion

Test for combustion

• When air hole is closed, air cannot enter supplying oxygen, and hence soot (unburnt carbon) and CO is produced from incomplete hydrocarbon gas combustion. 
• As a result, flame is yellow due to glowing specks of hot soot in heat and the flame is not hot. 
• When air hole is opened, air supplies plenty of oxygen, allowing complete combustion.

Uses of Oxugen

• As rocket fuel
• In steel making, to burn off impurities
• In oxy-acetyline cutting and welding
• In oxygen tanks for deep sea divers and mountain climbers to provide oxygen
• For respiration for most animals
• Used as oxygen tents in hospital to aid patients with respiratory problems

Oxides: oxygen combined with elements

Oxygen is a reactive gas and will combine directly with most metals and non-metals to form oxides

Reaction with metals

• Most metals, except silver and gold, combine directly with oxygen to form metal oxides.
• Most metal oxides are basic oxides 
• those that dissolve in water form alkalis
• eg sodium + oxygen --> sodium oxide

Reaction with non-metals

• non-metals like carbon, sulphur and phosphorus burn in oxygen to form acidic oxides
• eg carbon + oxygen --> carbon dioxide

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Many elements burn in oxygen with colored flames to produce the corresponding oxide.

Across the periodic table, the properties of the oxides of these elements change from basic to acidic in nature (metals to non-metals)

Basic oxides are formed by metals in Groups I, II, and III.

They are generally ionic oxides and are white solids.

Metal oxides

Element	Reaction	Product	Equation	pH in solution
potassium	burns with a lilac-colored flame and forms a white smoke	potassium oxide	4K + O2 --> 2K2O	12
sodium	burns with a yellow-colored flame forming a white smoke	sodium oxide	4Na + O2 --> 2Na2O	11
magnesium	burns with a brilliant white-colored flame forming a white smoke	magnesium oxide	2Mg + O2 --> 2MgO	8
iron	glows red hot and burns with white sparks	iron(II) and iron(III) oxide	3Fe + 2O2 --> Fe3O4	insoluble
copper	just glows red hot, and when cooled is covered with a black coating of the oxide	copper(II) oxide	2Cu + O2 --> 2CuO	insoluble

Non-metal oxides

Element	Reaction	Product	Equation	pH in solution
phosphorus	- white phosphorus: catches fire & white smoke - red phosphorus: slower reaction	phosphorus(V) oxide	P4 + 5O2 --> P4O10	3
sulphur	burns easily with a blue flame, producing a pungent gas	sulphur dioxide	S + O2 --> 2O2	3
carbon	glows red, but the reaction is slow. a colorless gas is produced	carbon dioxide	C + O2 --> CO2	5

The non-metals in Groups IV, V, VI and VII form covalent oxides.

Such oxides are mainly gases or liquids but giant covalent oxides like silicon dioxide are solids

Aluminium oxide is both basic and covalent and is therefore an amphoteric oxide.

Air Pollution: Harmful substances in the air

Main Pollutant gases:

1) carbon monoxide, CO

Comes from: 

- When fuels like petrol and diesel are burnt in an internal combustion engine, the amount of oxygen present is limited, so carbon monoxide gas is formed, instead of carbon dioxide gas.

- Unburnt hydrocarbons

- forest fires

Hazards: 

- Combines with haemoglobin when inhaled, which produces carboxyhaemoglobin that reduces efficiency of haemoglobin to transport oxygen. 

- Cells then die.

Prevention:

- Install catalytic converters in cars

- Reduce number of cars on road

- Create efficient engines in cars to ensure complete hydrocarbon combustion

2) sulphur dioxide, SO₂

Comes from: 

- Combustion of fossil fuels containing sulphur impurities

- volcanic eruptions

Hazards:

- Lung irritant

- eye irritant

- acid rain

Prevention:

- Prevent using fuels containing sulphur impurities, e.g. coal

- Reduce the sulphur impurities inside fossil fuels

- Spray exhaust gases from factories with water/hydrated CaO/alkalis to absorb sulphur dioxide before it’s released into the atmosphere

- Add CaO to soil and rivers to neutralize acid rain

3) oxides of nitrogen, NO

Comes from:

- Lightning activity

- forest fires

- internal combustion engines (as nitrogen oxides are formed by oxygen and nitrogen under high temperature) --> Poisonous oxides of nitrogen are also formed from the electrical spark which passes through the air/petrol mixture.

- power stations

Hazards:

- Eutrophication

- lung damage

- acid rain

Prevention:

- Install catalytic converters in cars

- Design car engines which run at lower temperatures

Notes: Reactions of oxides of sulphur and nitrogen

The oxides of sulphur and nitrogen are acidic gases and are water-soluble. They dissolve to form acid rain.The main source of these pollutant gases is from the burning of fuels, especially those in petrol and diesel engines.

All oils and fuels contain sulphur, and when they are burnt, sulphur dioxide gas is formed. In power stations, large quantities of this gas are produced, which dissolve in water in the atmosphere to form sulphurous acid (sulphuric acid)

SO₂ (g) + H₂O (l) --> H₂SO₃ (aq)

4) Methane

Comes from:

- Decomposition of vegetable matter

- rice field

- cattle ranching

- natural gas

- mines

Hazards:

- highly flammable

- greenhouse gas

Prevention:

- Cattle and other ruminant animals should be given improved diet

- Animal manure and rotting vegetation can be used as biomass fuel

5) Unburnt hydrocarbons

Comes from: 

- Internal combustion engines

- Because of the limited supply of air inside the engines some of it remains unburnt and escapes asgaseous hydrocarbons. 

Hazards:

- Carcinogenic

- forms photochemical smog

- can act as greenhouse gases contributing to global warming.

Prevention:

- Install catalytic converters in cars

- Reduce number of cars on road

- Create efficient engines in cars to ensure complete hydrocarbon combustion

6) Ozone

Comes from:

- It is formed when an electrical spark passes through air. This is because it reacts with the UV radiation in sunlight to produce a 'photochemical smog'.

- It is an allotrope (two/three different forms of a pure element) of oxygen having structural formula O3 having characteristic odour. 

- High up in the atmosphere ozone is beneficial as it helps to filter out high levels of UV radiation

Hazards

- It reacts with unburnt hydrocarbons to form photochemical smog that causes headache, eye, nose and throat irritation.

- It corrodes and kills plants and trees

Prevention

- Don’t use CFCs/replace it with HCFCs which destroys faster.

Notes: Catalytic converters

- One way to reduce pollution from cars is to fit catalytic converters to our exhausts. 

- Inside the converter is a special metal-like platinum which acts as a catalyst. 

- It converts the poisonous exhaust gases of CO and oxides of nitrogen into harmless gases like carbon dioxide and nitrogen. 

- It does this by transferring oxygen atoms from the oxides of nitrogen to the CO.

2CO (g) + 2NO (g) --> 2CO₂  (g) + N₂ (g)

7) Dust and Smoke

- The larger, heavier dust particles will settle quickly but the smaller particles may remain suspended in the air for a long time. 

Comes from:

- building work

- mining activities

- forest fires

- incomplete combustion of fuels.

Hazards:

- irritate lungs, causing bronchitis and other lung-related diseases.

8) Lead compounds

Comes from:

- Combustion of leaded petrol in car engines

- lead compounds are added to petrol to make it heavier so that it does not ignite too soon. 

Hazards:

- when breathed in can build up inside the body and are toxic and poisonous

- Causes lead poisoning which leads to brain damage.

Natural compounds of carbon

Carbon Cycle

- Carbon dioxide is produced mainly by respiration. Here, sugars such as glucose are converted into carbon dioxide and water, giving out energy (exothermic)

Respiration of glucose equation:

- Carbon dioxide is also produced by combustion of fuels, in factories, and in the home

- The carbon dioxide is then absorbed by plants, by photosynthesis. Energy is absorbed (endothermic) from the sun, and used to build up simple sugars.

Photosynthesis equation:

- Animals eat plants, and in turn, they themselves get eaten by other animals. So the carbon originally in the atmosphere ends up in every living plant and animal. Upon death, the carbon is released by bacteries and fungi, to return to the atmosphere as carbon dioxide.

The cycle is then repeated.

Revision Questions and Answers

A

1. Which atmospheric pollutants, emitted by internal combustion engines, are reacted together to convert them to less harmful products?

a. carbon monoxide and nitrogen dioxide

b. carbon monoxide and unburned hydrocarbons

c. nitrogen dioxide and sulfur dioxide

sulfur dioxide and unburned hydrocarbons

2. The global atmospheric concentration of carbon dioxide has increased in the last 200 years. What could be causing this increase?

1. emissions from motor vehicles
2. photosynthesis
3. power stations using coal and oil

a. 1 and 2 only

b. 1 and 3 only

c. 2 and 3 only

d. 1, 2, and 3

3. Which statements about the pollutant carbon monoxide are correct?

1. it is a colourless, odourless gas

2. it is formed by incomplete combustion of natural gas

3. it reacts with haemoglobin in the blood

a. 1 and 2 only

b. 1 and 3 only

c. 2 and 3 only

d. 1, 2, and 3

4. Which gas is not produced when hydrocarbons are burnt in the internal combustion engine a. carbon dioxide b. carbon monoxide c. hydrogen d. nitrogen oxides

5. Photochemical smog is seen in many industralised cities. Which of the following is not responsible for its formation? a. nitrogen dioxide b. sulphur dioxide c. pentane d. ozone

MCQ Answers

1. a

2. b

3. d

4. c

5. b

Structured Question Worked Solutions

1. The exhaust gases of a motor car contain several pollutants, among them carbon monoxide and oxides of nitrogen. The flowchart below shows the arrangement of a two-stage catalytic converter in a motor car engine and exhaust system.

In the first converter, carbon monoxide reacts with oxygen.

In the second converter, nitrogen oxides decompose into nitrogen and oxygen.

ai. Explain how carbon monoxide is produced in a motor car engine.

aii. Give the name of the gas that is produced in the first converter. Write an equation to show its production.

aiii. Why is carbon monoxide a pollutant?

b. Two oxides of nitrogen are nitrogen dioxide, NO₂, and nitrogen monoxide, NO.

i. Write an equation to show the decomposition of one of these oxides.

ii. Why are oxides of nitrogen pollutants?

ci. What is a catalyst?

cii. Suggest why the catalysts in the first and second converters are different.

Solution

1ai. As a result of incomplete combustion of petrol

1aii. carbon dioxide

2CO (g) + O₂ (g) --> 2CO₂ (g)

1aiii. Carbon monoxide, when breathed in, prevents blood from transporting oxygen. This is because CO can become strongly bonded to haemoglobin (present in red blood cells) and thus prevents the haemoglobin from transporting oxygen. The victim may die after some time due to lack of oxygen.

1bi. 2NO₂ (g) --> N₂ (g) + 2O₂ (g)

1bii. They are acidic and cause acid rains, thereby corroding buildings and killing plants.

1ci. A catalyst is a substance that changes the rate of a reaction and itself remains chemically unchanged at the end of the reaction.

1cii. Because different catalysts catalyse different reactions. The catalyst that catalyses the production of CO₂ from CO may not catalyse the decomposition of oxides of nitrogen and vice versa.

2a. Pure oxygen is obtained from air and is used as an aid to breathing in hospital oxygen masks.

i. describe in outline how pure oxygen is obtained from air

ii. give a commercial use for oxygen other than as an aid to breathing

b. One problem with oxygen is that it is the essential element in causing corrosion of metals. One method of limiting corrosion is known as 'sacrificial protection'. Explain what is meant by sacrificial protection.

Solution

2ai. By fractional distillation of liquid air. Fractional distillation is a method used to separate liquids based on the difference in their boiling points. 

2aii. acetylene in welding

2b. A metal more reactive than the metal to be protected is connected to the metal to be protected. This more reactive metal corrodes in preference to the protected metal. This is called sacrificial protection.

3. Complete the following table about atmospheric pollutants.

atmospheric pollutant	source of pollutant	effect of pollutant
carbon dioxide	complete combustion of fossil fuels
carbon monoxide
methane
sulphur dioxide

Solution

atmospheric pollutant	source of pollutant	effect of pollutant
carbon dioxide	complete combustion of fossil fuels	greenhouse effect
carbon monoxide	incomplete combustion of fossil fuels	poisonous to human because it prevents haemoglobin in red blood cells from transporting oxygen
methane	decomposition of sewage and vegetation	a greenhouse gas which traps radiation by reflecting it back to earth. This causes a rise in the earth's temperature resulting in global warming
sulphur dioxide	combustion of fossil fuels containing sulphur and sulphur compounds in power stations and motor vehicles	dissolves in rain to form acid rain. Also lowers pH of water in rivers and soil causing harm to both animals and plants

4. Ozone occurs in the upper atmosphere.
a. why is ozone in the upper atmosphere important?
b. state one type of compound that is responsible for ozone depletion

Solution

4a. The ozone layer acts as a barrier that absorbs harmful UV rays from the sun, preventing it from reaching earth.

4b. chlorofluorocarbons

5. Coal-burning power stations produce sulphur dioxide and oxides of nitrogen. These two gases cause acid rain.

a. Nitric oxide, NO, is made in a power station when nitrogen and oxygen react together. Write the equation for this reaction.

b. Many coal-burning power stations are now fitted with a flue gas desulphurisation plant which removes sulphur dioxide and nitrogen dioxide from the gaseous emissions. In a flue gas desulphurisation plant, powdered calcium carbonate reacts with sulphur dioxide as shown.

SO₂ (g) + CaCO₃ (s) --> CaSO₃ (s) + CO₂ (g)

i. suggest why the calcium carbonate is powdered

ii. calculate the mass of calcium carbonate needed to react with 8000 kg of sulphur dioxide

iii. nitrogen dioxide also reacts with calcium carbonate. suggest the name of the solid product of this reaction.

c. In the air sulphur dioxide reacts with nitrogen dioxide forming sulphur trioxide. The reactions that take place are shown in the equation

SO₂ + NO₂ --> SO₃ + NO
2NO + O₂ --> 2NO₂

Suggest the role of nitrogen dioxide in these reactions. Explain your answer.

d. Sulphur dioxide is used in the Contact Process to make sulphuric acid. Describe the conditions and name the catalyst in the Contact Process.

Solution

5a. N₂ + O₂ --> 2NO

5bi. To increase the surface area in order to absorb more SO₂ gas

5bii. 

Mr of SO₂ = 64

no. of moles of SO₂ = (8000 X 10³) / 64 = 1.25 X 10⁵

no. of moles of CaCO₃ reacted = 1.25 x 10⁵

mass of CaCO₃ reacted = 1.25 x 10⁵ x 100 = 1.25 x 10⁷ = 12500 kg 

5biii. calcium nitrate

5c. As an oxidising agent because it oxidises SO₂ to SO₃ in reaction 1

As a catalyst because NO₂ is reformed at the end of reaction 2 and therefore, the original role of NO₂ in reaction 1 has not been used up at the end of the reaction.

5d. 450^oC, 2 atmospheric pressure

catalyst: Vanadium(V) oxide

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