atom economy. learning outcomes atom economy is derived from the principles of green chemistry. atom...
TRANSCRIPT
Atom Economy
Learning outcomes
Atom economy is derived from the principles of green
chemistry.
Atom economy is a measure of the proportion of reactants
that become useful products.
% atom economy =Mass of desired product(s)
Total mass of reactantsx 100
What is green chemistry?
The sustainable design of chemical products and
chemical processes.
It minimises the use and generation of chemical
substances that are hazardous to human health or the
environment.
Green chemistry principles
Better to prevent waste than to treat it or clean it up.
Chemical processes should aim to incorporate all reactants
in the final product.
Chemical processes should aim to use and generate
substances with minimal toxicity to human health and the
environment.
The green chemical industry
Modern chemists design reactions with the highest possible
atom economy in order to minimise environmental impact.
Chemists achieve this by reducing raw material and energy
consumption.
Percentage yield
Historical method for evaluating reaction efficiency.
Measures the proportion of the desired product
obtained compared to the theoretical maximum.
Gives no indication of the quantity of waste produced.
% yield =Actual yield
Theoretical yieldx 100
Atom economy In an ideal reaction, all reactant atoms end up within
the useful product molecule. No waste is produced!
Inefficient, wasteful reactions have low atom economy.
Efficient processes have high atom economy and are
important for sustainable development. They conserve
natural resources and create less waste.
Atom economy
A measure of the proportion of reactant included in the final useful product.
A reaction may have a high percentage yield but a low percentage atom economy, or vice versa.
% atom economy =Mass of desired product(s)
Total mass of reactantsx 100
High atom economy
All reactant atoms included in the desired product.
Low atom economy
Some reactant atoms not included in the desired product.
Example 1
What is the percentage atom economy for the following reaction
for making hydrogen by reacting coal with steam?
C(s) + 2H2O(g) → CO2(g) + 2H2(g)
12 g 2(2 + 16) g [12 + (2 × 16)] g 2(2 × 1) g
12 g 36 g 44 g 4 g
Total mass of reactants Mass of desired product
= 12 + 36 = 48 g = 4 g
Example 1 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 4 × 100
48
= 8.3%
This reaction route has a very low atom economy and is
an inefficient method of producing hydrogen.
Example 2Calculate the percentage atom economy for the reaction below.
C6H12 C6H12
Total mass of reactants Mass of desired product
= [(6 × 12) + (12 × 1)] = [(6 × 12) + (12 × 1)]
= 84 g = 84 g
CCH3
CH3
CH3
CH CH2
CH3
C C
CH3
CH3
CH3
acid
Example 2 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 84 × 100
84
= 100%
This reaction route has a very high atom economy as all
reactant atoms are incorporated into the desired product.
Example 3
Hydrazine (N2H4) is used for rocket fuel. Calculate the atom
economy for hydrazine production.
Total mass of reactants Mass of desired product
= 34 + 74.5 = 108.5 g = 32 g
NaCl H 2O++ +N2H42NH3 NaOCl
NH3
2 mol
34 g
NaOCl1 mol
74.5 g
N2H4
1 mol
32 g
NaCl1 mol
58 g
H2O1 mol
18 g
Example 3 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 32 × 100
108.5
= 30%
This reaction route has an atom economy of 30%. The
remaining 70% is waste product (NaCl and H2O).
Catalysts
Have a crucial role in improving atom economy.
Allow the development of new reactions requiring fewer
starting materials and producing fewer waste products.
Can be recovered and re-used.
Allow reactions to run at lower temperatures, cutting energy
requirements.