Chapter 19
Green Chemistry
What is Green Chemistry


The design of chemical processes and
products that reduce or eliminate the use
and generation of hazardous substances in
their manufacture and application
Green chemistry aims to avoid problems
before they happen and to consider the
impacts of the manufacture use, final
disposal and break-down of chemical
products
12 Principles of Green Chemistry
•1
• Preventing wastes
– It is better to prevent waste than to clean it
up after it forms
12 Principles of Green Chemistry
•2
• Atom economy
– Waste at little material as possible
12 Principles of Green Chemistry
•3
• Less hazardous synthesis
– Chemical manufacturing processes should
be designed to use reactants and generate
products that have little or no toxicity
12 Principles of Green Chemistry
• 4
• Designing safer
chemicals
– Chemical products
that do their jobs
well but are nontoxic will create
fewer medical and
environmental
problems
12 Principles of Green Chemistry
•5
• Using safer solvents
– The use of additional chemicals such as
solvents should be avoided, and if they are
used they should be non-toxic substances
12 Principles of Green Chemistry
•6
• Energy efficiency
– Reducing energy requirements by
conducting processes at room temperature
and pressure reduces costs and lowers the
impact on the environment.
12 Principles of Green Chemistry
• 7
• Using renewable
feedstock
– Starting materials for a
chemical product should,
where possible, be
renewable resources
– Eg nylon from glucose
instead of benzene;
biodiesel from crops
12 Principles of Green Chemistry
•8
• Reducing use of derivatives
– Derivatives are chemical used for a
temporary effect in the chemical process
– Derivatives cause a temporary change in
properties and are then removed as waste
12 Principles of Green Chemistry
•9
• Using catalysts
– Catalysts speed up reactions and allow
reactions to occur at a lower energy
12 Principles of Green Chemistry
• 10
• Design for
degradation
– The proper disposal
of chemical products
at the end of their
use should be part of
the design process.
– The products should
be easily able to
bread down into
non-toxic materials
12 Principles of Green Chemistry
• 11
• Using real-time
pollution prevention
– This includes
monitoring and
controlling
hazardous
substances while the
chemical process is
occurring and
12 Principles of Green Chemistry
• 12
• Preventing accidents
– The choice of substances and
their states used in a chemical
process should remove the
potential for chemical
accidents, including
accidental releases,
explosions and fires
– A toxic substance in solid
form becomes much more
dangerous if changed into its
liquid or gas form
Green Chemistry Strategies
• Green chemistry strategies seek
innovative methods to prevent the use or
generation of hazardous substances
initially.
• Risk = exposure x hazard
• Green chemistry involves inventing new
and innovative methods of reducing
chemical hazards while synthesising
superior products in a more efficient and
economical way.
Green Chemistry Strategies
• Green chemists will first analyse and
assess the potential hazards
associated with an existing process.
• They identify materials involved in
every step of the chemical
transformations – reactants, reagents,
solvents, products and by products –
as well as looking at the reaction
conditions and energy input
Green Chemistry Strategies
• Identifying hazards and inefficiencies
will lead to modification of the process
• The efficiency of the new process is
tested to determine if it is more
effective and less hazardous.
• This process of evaluation and testing
will be repeated until a greener
alternative procedure is established
Atom Economy
The atom economy of a chemical reaction is
a measure of the amount of starting materials
that become useful products.
Inefficient, wasteful processes have low atom
economies.
Efficient processes have high atom
economies, and are important for sustainable
development, as they use fewer natural
resources and create less waste.
• The atom economy of a reaction can be
calculated:
Atom Economy
• Note that, because the total mass of
products equals the total mass of
reactants, you can put that into the
bottom of the fraction in the calculation like
this:
Atom Economy
• For example, what is the atom economy for making hydrogen by
reacting coal with steam?
• Write the balanced equation:
– C(s) + 2H2O(g)
→
– C(s) + 2H2O(g)
– 12
2 × 18
→
CO2(g) + 2H2(g)
• Write out the Ar and Mr values underneath:
CO2(g) + 2H2(g)
44
2×2
• Remember that the Ar or Mr in grams is one mole, so:
– total mass of products = 44 + 4 = 48g (note that this is the same as the
reactants: 12 + 36 = 48g)
– mass of desired product (H2) = 4g
– % atom economy = 4⁄48 × 100 = 8.3%
• This process has a low atom economy and is therefore an inefficient
way to make hydrogen. It also uses a non-renewable resource: coal.
Complexities and challenges for
green chemistry
• Issues for green chemistry processes include
starting materials, hazards of the process,
number of steps, whether the process can be
scaled up, energy use, waste produced,
products produced, how the operation is to be
run, and cost
Review
• Complete the multiple choice questions
1, 2 page 470
• Complete the review questions 2, 7,
page 470
• Complete the worksheet – percentage
yield and atom economy