Green Chemistry



Green Chemistry

Air pollution caused by the combustion of fossil fuels:

Sustainability is the development of processes which prevent the depletion of natural resources.

There are 5 principles of chemical sustainability:

1. Use industrial processes which will reduce or eliminate hazardous chemicals

Phasing out tetraethyl lead as a fuel additive, now using cycloalkanes

2. Minimise the production of waste materials

Cracking of distillation residues to gain chemical feedstock

3. Use renewable materials such as plant based substances

Fossil fuels being supplemented by biodiesel and bioalcohol

4. Seek alternative energy resources

Using solar, wind, tidal energy instead of consuming finite resources

5. Ensure waste products are non toxic

Catalytic converters remove NOx, CO and Unburnt hydrocarbons from exhaust gases from an internal combustion engine.

The 1992 Rio Declaration on Environment and Development defines the rights of people to be involved in the development of their economies and the responsibilities of human beings to safeguard the environment.

The Greenhouse Effect

Infrared radiation is absorbed by 3 covalent bonds:

C=O

O-H

C-H

The greenhouse effect of a given gas is dependent on two things:

1. Its atmospheric concentration

2. Its ability to absorb infrared radiation

It is important to control global warming as there can be damaging consequences:

▪ Rising Sea levels

▪ Melting polar ice caps

▪ Unusual weather patterns

▪ Increase in desertification

▪ Destruction of natural habitats

Scientists are trying to minimise climate change caused by the release of greenhouse gases:

The Ozone Layer

The ozone layer is in a region of the Earth’s upper atmosphere called the stratosphere. Ozone is constantly being formed and broken down by the action of UV radiation:

Oxygen molecules are broken down under high energy UV radiation forming two oxygen atoms:

An oxygen atom then reacts with an oxygen molecule forming a molecule of ozone;

Ozone is broken down when an ozone molecule reacts with an oxygen atom forming two molecules of oxygen:

A natural state of balance is reached, where ozone is broken down at the same rate at which it is produced;

Chloroflourocarbons

CFCs are compounds which contain chlorine, fluorine, carbon and hydrogen only.

They were widely used because of their chemical properties:

✓ Low reactivity

✓ Volatile

✓ Non Toxic

Fridges Air Conditioning Aerosols

CFCs cause depletion of the ozone layer by free radical substitution:

Initiation - Under UV radiation, the C-Cl bond of the CFC breaks, producing a chlorine radical.

CFCl3 Cl + CFCl2

Propagation (1) – Chlorine free radical reacts with an ozone molecule producing a free radical and oxygen molecule.

Cl + O3 ClO + O2

Propagation (2) - Free radical reacts with an oxygen atom producing a chlorine free radical and oxygen molecule.

ClO + O Cl + O2

Overall - ozone is depleted to oxygen molecules.

O3 + O 2 O2

Nitrogen Oxides, NO are a second type of free radical which deplete the ozone layer. These come from lightning or aircraft engines. The reaction proceeds in two propagation steps:

Propagation (1) – Nitrogen oxide free radical reacts with an ozone molecule producing a free radical and oxygen molecule.

NO + O3 NO2 + O2

Propagation (2) - Free radical reacts with an oxygen atom producing a nitrogen oxide free radical and oxygen molecule.

NO2 + O NO + O2

Overall - ozone is depleted to oxygen molecules.

O3 + O 2 O2

Alternatives to CFCs have been developed:

• Hydrocarbons

o Adv: do not deplete the ozone layer

o Disadv: Contribute to global warming

• Hydrochlorofluorocarbons

o Adv: Chemically unreactive, useful in aerosols, refrigerants, air conditioning, foam blowing

o Disadv: Found to deplete ozone and contribute to global warming

• Carbon Dioxide

o Adv: used as a blowing agent in expanded polymers. Liquid super-cooled CO2 can be used as a safer solvent than chlorinated hydrocarbons.

o Disadv: Contributes to global warming

Sustainability means meeting the demands of the population whilst preventing the depletion of natural resources, so that future generations are capable of meeting their own needs.

The Montreal Protocol is an agreement between 197 countries to stop the production of CFCs and replace them with HFCs and HCFCs. International cooperation was reached to promote the reduction of pollution levels. Pyrocol FEF should be used in fire extinguisher rather than CFCs because it does not produce toxic or ozone depleting waste products like other halogenated fire fighting materials.

Air pollution and the Catalytic Converter

Emissions from cars are some of the biggest air pollutants. The three main polluting gases are:

a) Carbon Monoxide

b) Oxides of Nitrogen

c) Unburnt Hydrocarbons (Volatile Organic Compounds)

Catalytic Converters

A catalytic converter is used to decrease the harmful emissions form an internal combustion engine. A three way catalytic converter uses three metals as heterogeneous catalysts, as they are in a different physical state to the reactants. These metals are platinum, rhodium and palladium. The catalyst provides a surface on which the reaction takes place.

• The CO and NO gas molecules diffuse over the catalytic surface. Some molecules are held on to the metal surface by adsorption.

• Temporary bonds are formed between the catalytic surface and the gas molecules. These bonds hold the gases in the correct orientation so that they can react together.

• After the reaction, the CO2 and N2 products are desorbed from the catalytic surface and diffuse away.

Catalysts and Sustainability

← Catalysts lower temperature requirements because they offer an alternative route with lower activation energy. Less energy is required, so less fossil fuels are burned, less CO2 produced.

← Selective catalysts mean fewer waste products are produced, because only the desired product is formed. This increases atom economy and promotes addition reactions.

← Enzymes are proteins with catalytic properties. They generate specific products and operate close to room temperature and pressure.

Problems with Polymers

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Carbon Monoxide is a poisonous gas, formed by the incomplete combustion of hydrocarbons. It binds to the haemoglobin in red blood cells, inhibiting the ability to carry oxygen.

Oxides of Nitrogen are formed under high temperatures in the car engine, when atmospheric nitrogen is oxidised to form nitrogen oxides. They cause the formation of low level ozone and reacts with water to form nitric acid, which falls as acid rain.

Unburnt hydrocarbons are released in car exhausts. They react with nitrogen oxides to form low level ozone, which causes breathing difficulties.

Infrared Radiation

This carbon dioxide molecule absorbs Infrared radiation, which causes the molecule to vibrate. The vibrating molecule will eventually emit some of this energy as radiation. This re-emitted radiation is absorbed by other gas molecules and at the Earth’s surface, increasing the temperature.

← Educating Society with information on climate change, through the media (books, journals, documentaries)

← Monitoring progress against agreements such as the Kyoto Protocol, which was an international agreement setting targets to reduce greenhouse gas emissions.

← Providing Scientific Evidence that global warming is taking place. Scientists use ice cores from Antarctica to estimate CO2 levels thousands of years ago – a proxy record.

Providing Information to change opinions

← Alternative energy resources such as solar power, wind, and development of hydrogen fuel cells.

← Carbon Capture and Storage – turned into a liquid and injected deep into the oceans or stored in geological formations. Waste CO2 can be reacted with metal oxides to make stable carbonates, such as Calcium Carbonate.

← More efficient engines for transport such as lean burn engines, hybrid engines or electric cars.

Reducing Global Warming

Developing New Technologies

O2 2O

O2 + O O3

O3 + O 2O2

O2 + O O3

Oxides of Nitrogen are formed under high temperatures in the car engine, when atmospheric nitrogen is oxidised to form nitrogen oxides. They cause the formation of low level ozone and reacts with water to form nitric acid, which falls as acid rain.

Carbon Monoxide is a poisonous gas, formed by the incomplete combustion of hydrocarbons. It binds to the haemoglobin in red blood cells, inhibiting the ability to carry oxygen.

Unburnt hydrocarbons are released in car exhausts. They react with nitrogen oxides to form low level ozone, which causes breathing difficulties.

2CO + O2

C7H16 + 11O2

2NO + 2CO

2CO2

7CO2 + 8H2O

N + 2CO2

Burning polymers under controlled conditions produces heat energy. Plastics are turned into briquettes which are then burned to produce electricity. Polymers can also be cracked to give new alkenes, like synthesis gas. This can be used as a chemical feedstock for conversion into useful products or as a fuel in oil refineries.

Polymers cannot be incinerated because toxic fumes are released – PVC is particularly hazardous because Hydrogen chloride and PCBs (polychlorinatedbiphenyls) are formed.

Polymers have identification codes (numbers 1-7) and are separated into these groups using optical scanning techniques, to make sorting quicker and less labour intensive. They are then processed, chopped, washed and the impurities are then removed, before being melted and remoulded.

Polymers must be recycled as they are not biodegradable. However, recycling is made very hard because plastics are difficult to separate.

Chemists are developing new polymers which are biodegradable and compostable. They can be derived from raw materials such as starch, maize, cellulose and lactic acid.

What can chemists do to resolve it?

What is the issue?

Polymers contain non polar bonds, such as C-H bonds, or strong bonds, like C-F bonds. This makes them unreactive, and so they are non-biodegradable. They cannot be broken down by organisms in the natural environment. They exist in the environment for a very long time, and so contribute to overflowing landfill sites.

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