MISS CHOHANS NOTES ON GREEN CHEMISTRY UNIT 2 EDEXCEL

GREEN CHEMISTRY

Sustainability
The five principles of chemical sustainability:
e.g. reduce the need to use finite raw materials such as
(i) using industrial processes that reduce or crude oil
eliminate hazardous chemicals and which
less harmful or corrosive reactants
involve the use of fewer chemicals

(ii) designing processes with a high atom Using more readily available starting materials
economy that minimise the production of
e.g. Designing processes with high atom economy that
waste materials, often through discovering
minimise production of waste products.
catalysts for reactions with higher atom
economies e.g. Develop ways of making polymers/ fuels from
plant-based substances
(iii) using renewable resources such as plant-
based substances, e.g. less toxic, corrosive waste products

(iv) making more efficient use of energy and Recycle heat in a process to heat the incoming reactants
seeking alternative energy sources such as
solar energy, rather than consuming finite Ensuring that any waste products produced are non-

resources such as fossil fuels that will toxic, and can be recycled or biodegraded by being

eventually be exhausted, broken down into harmless substances in the

environment;
(v) reducing waste and preventing pollution of
the environment. Develop biodegradable polymers

The idea of “green chemistry” is to obtain all the valuable products, but in ways that do not damage the environment.
Green chemistry is about making the chemical industry more sustainable.

The Chemical industry needs to become greener to;

 Reduce waste.
 Develop more economically viable processes.
 Reduce their environment impact.
 Save limited resources.
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Five ways in which the Chemical industry can become Greener

 Change to renewable resources.
 Find alternatives to very hazardous chemicals.
 Discover catalysts for reactions with high atom economies,
e.g. the development of methods used to produce ethanoic acid based on catalysts

of cobalt, rhodium and iridium

 Make more efficient use of energy,

e.g. the use of microwave energy to heat reactions in the pharmaceutical industry

 Reduce waste and prevent pollution of the environment.

Changing to renewable sources

If renewable resources replace non-renewable materials raw materials will be conserved and the process becomes more
sustainable. Plants can be used as a source of certain organic compounds instead of oil. These can be extracted directly
from plants or derived from processing plant material, such as using fermentation to produce ethanol from plant starch
and sugars.

Use of alternatives to hazardous chemicals

This involves replacing dangerous chemicals used in a process with less harmful chemicals which can do the same job. It
includes changing processes so that less risky reagents are used or less destructive intermediates are formed. It also
includes using less damaging solvents for a process.

Use of more efficient catalysts

Introduction of more efficient catalyst enables lower temperatures and pressures to be used and so saves energy. More
efficient catalysts create fewer bye-products and so reduce waste.

Reduction of energy use

Changing to processes that use lower pressures and temperatures reduce the energy required to make a given product.
Use of exothermic reactions to provide heat reduces the amount of non-renewable energy used, for example introducing a
heat transfer system (using hot products to heat incoming reactants). Use of microwaves instead of hotplates allows more
efficient heating, as it heats the relevant molecules directly.

Reduction in the amount of waste

Recycling materials means that less waste is produced. Making biodegradable products means that they can be broken
down by natural processes and so reduce the materials that need to be treated after use.
MISS CHOHANS NOTES ON GREEN CHEMISTRY UNIT 2 EDEXCEL

Examples
Polylactic acid
An example of this is the production of the biodegradable plastic, polylactic acid, PLA. The lactic acid, 2-
hydroxypropanoic acid, for this process can be produced by using bacteria to carry out the conversion from
plant starch and sugars.

H H
H C H H C H
O O
n O C C + nH2O
H O C C
H O H
H
n

Since lactic acid is a substance produced by living organisms it means that the PLA is biodegradable.

Roundup
A new process for the production of a herbicide called “Roundup” was developed. The original process required the use of
methanol (toxic) and hydrogen cyanide (extremely toxic), but the new process, using a copper catalyst, does not use these
materials. The new process utilizes endothermic reactions, so it is safer and easier to control. The original process produced
14% waste, whereas the new process re-uses the catalyst and recycles any other unconverted chemicals, and so has no
waste. The new process uses fewer steps and so produces a higher yield.

Ethanoic acid
Ethanoic acid can be manufactured from methanol and carbon monoxide:
CH3OH + CO  CH3CO2H
A new process uses a combination catalyst consisting of iridium with ruthenium compounds. This catalyst produces purer
ethanoic acid and so reduces the energy required for the purification.

Summary
The table below summarises which of the new processes fulfils the five main ways of making the chemical industry more
sustainable.

PLA Roundup Ethanoic acid

Renewable resources Yes

Alternative to hazardous chemicals Yes
Development of new catalyst Yes Yes
More efficient energy use Yes
Reduction in waste Yes Yes
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Main Industrial Effects on the Environment

Global Climate Change

Products or waste products may enter the atmosphere and absorb Infra-red radiation, thus leading to increased global
climate change - anthropogenic climate change

e.g. Carbon dioxide, methane.

Acid Rain

Products or waste products may enter the atmosphere and dissolve in clouds to form acid rain, thus leading to change of
soil pH and river water pH etc.

e.g. Sulphur dioxide, nitrogen oxides, carbon dioxide.

Ozone Depletion

Products or waste products may enter the atmosphere and lead to ozone depletion, thus leading to increased levels of UV
radiation reaching the Earths surface.

e.g. CFC’s, products from combustion of plastics and Nitrogen oxides.

The ‘Greenhouse Effect’ and Global warming

A greenhouse gas absorbs Infra-Red radiation re-radiating from the earth

Infrared radiation is absorbed by C=O, O–H and C–H bonds in

H2O, CO2 and CH4. The polarity of the molecules changes
 Carbon dioxide (CO2), methane (CH4 ) and water
when their bonds vibrate
vapour (H2O) are all greenhouse gases. (They trap
the Earth’s radiated infra-red energy in the
atmosphere). The ‘Greenhouse Effect’ of a given gas is dependent

 Water is the main greenhouse gas (but is natural), both on its atmospheric concentration and its ability to absorb
followed by carbon dioxide and methane infrared radiation and also its residence time. (Time it stays in
atmosphere)

 Concentrations of Carbon dioxide in the atmosphere have risen

The Earth is thought to be getting warmer,
significantly in recent years due to increasing burning of fossil
and many scientists believe it is due to
fuels.
increasing amounts of greenhouse gases in
 Carbon dioxide is a particularly effective greenhouse gas and its
the atmosphere
increase is thought to be largely responsible for global warming.

Global warming could see rising sea levels, flooding, polar ice melting, changing air currents, changing weather patterns,
more extreme weather.

Human contributions to climate change are called anthropogenic

There has always been natural causes for climate change over hundreds of thousands of years, caused by changes in the
sun’s activity and volcanic activity
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Role of chemists in minimising climate change resulting from global warming

Chemists provide scientific evidence to Carbon Capture and Storage, CCS

governments to verify that global warming is taking Carbon dioxide could potentially be stored in a
place such as monitoring atmospheric changes number of different ways:

 removal of waste carbon dioxide as a

Chemists are investigating solutions to liquid injected deep in the oceans or on
environmental problems, such as developing carbon the seabed.
capture and storage, CCS.  storage in geological formations or under
the sea-bed by the reaction of carbon
Chemists can monitor progress against initiatives such dioxide with metal oxides to form stable
as the Kyoto protocol solid carbonates, which can be stored

Chemists can also develop

 alternative energy sources such as developing fuel cells or developing

solar power or fuels that do not produce CO2
 Develop more efficient engines for transport or lean burn engines
 Develop uses for carbon dioxide e.g. dry cleaning or making
decaffeinated coffee OR blowing agent in polymer or making fizzy
drinks

Carbon Neutrality
The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”

Ethanol as biofuel

A biofuel is a fuel produced from plants

Ethanol produced from fermentation is a biofuel.

It can be argued that ethanol produced from this method is classed as carbon– neutral as any carbon
dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis
when the plant grew.
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Removal of CO2 by photosynthesis

This does not take into account any
6CO2 + 6H2O  C6H12O6 + 6O2 energy needed to irrigate plants,
fractionally distil the ethanol from the
Production of CO2 by fermentation and combustion
reaction mixture or process the fuel. If
C6H12O6  2CH3CH2OH + 2CO2 Equations to show no the energy for this process comes from
net contribution to CO2 fossil fuels then the ethanol produced
2CH3CH2OH + 6O2  4CO2 + 6H2O
is not carbon neutral

Energy would be used to distil the ethanol water mixture after fermentation. Energy required to manufacture Fertilisers
and insecticides to grow plants for biofuels in good yield. Energy is required to transport fuel to the power plant

Apparent benefits may be offset by unexpected and detrimental side effects. Note the advantages and disadvantages of
using biofuels.

Advantages of using Biofuels Disadvantages of Biofuels

 Reduction of use of fossil fuels which are finite  Less food crops may be grown
resources biofuels are renewable  Land not used to grow food crops
 Use of biodiesel is more carbon-neutral  Rain forests have to be cut down to provide
 Allows fossil fuels to be used as a feedstock for land
organic compounds  May reduce biodiversity Shortage of fertile
 No risk of large scale pollution from soils
exploitation of fossil fuels new jobs created to
grow crops on new farmland

Hydrogen

A ‘hydrogen economy’ may contribute largely to future

energy needs but limitations include:
Hydrogen is readily available by the electrolysis of
(i) public and political acceptance of hydrogen as a water, but this is expensive.
fuel, with its risk of explosion.
To be a green fuel the electricity needed would need to
(ii) handling and maintenance of hydrogen systems, be produced from renewable resources
(iii) initial manufacture of hydrogen, requiring
energy
MISS CHOHANS NOTES ON GREEN CHEMISTRY UNIT 2 EDEXCEL

The Ozone Layer

The naturally occurring ozone (O3) layer in the upper

Ozone in the lower atmosphere is a pollutant
atmosphere is beneficial as it filters out much of the
and contributes towards the formation of
sun’s harmful UV radiation
smog

Ozone is continuously being formed and broken down

in the stratosphere by the action of ultraviolet radiation:

O + O 2  O3

Ozone formation

UV light causes an O2 molecule to split into free

radicals Ozone depletion

O2 + UV-light → O + O This is the reverse of the formation reaction. The

When the free radical hits another O2 molecule ozone energy is supplied by ultraviolet light

forms

O + O 2 → O3

There is a continuous cycle of formation and depletion of ozone

rate of ozone formation = rate of ozone removal

So there is a constant amount of ozone in the atmosphere

 The frequency of ultra-violet light absorbed equals the frequency of biologically damaging ultra-
violet radiation.
 These reactions therefore filter out harmful UV from reaching the Earth’s surface and allow life to
survive on earth.
 UV light can increase risk of skin cancer and increase crop mutation.

Destruction of Ozone
Layer
Chlorine radicals are formed in the upper
atmosphere when energy from ultra-violet

Radicals from CFCs, and NOx from thunderstorms radiation causes C–Cl bonds in

or aircraft, may catalyse the breakdown of ozone chlorofluorocarbons (CFCs) to break.

The C-F bond is much harder to break than the C-

Cl bond.

CF2Cl2 → CF2Cl + Cl
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The chlorine free radical atoms catalyse the The regenerated Cl radical
decomposition of ozone due to these reactions means that one Cl radical
because they are regenerated. (They provide an could destroy many
alternative route with a lower activation Cl + O3  ClO + O2
● ●
thousands of ozone
energy) ClO + O  O2 + Cl
● ● ●
molecules
Overall equation O3 + O  2O2●

Aircraft releasing NO is a problem because they release it closer to the ozone

layer

NO + O3  NO2 + O2
NO2 + O●  O2 + NO
Overall equation O3 + O●  2O2

Legislation to ban the use of CFCs was supported by HFCs (Hydro fluoro carbons) e.g.. CH2FCF3 are
chemists and that they have now developed now used for refrigerators and air-conditioners.
alternative chlorine-free compounds These are safer as they do not contain the C-Cl
bond

CFC’s still concern us because CFCs are still entering the atmosphere from disused items and are still used
for some purposes and by some countries. CFCs have a long lifetime in the atmosphere and it takes a long
time for CFCs to reach upper atmosphere
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