Air, Oxygen, Atmospheric Pollutants

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

Non-metal oxides

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 as gaseous 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

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.

Solution

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