2 Learning outcomesAtom economy is derived from the principles of green chemistry.Atom economy is a measure of the proportion of reactantsthat become useful products.
3 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.
4 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.
5 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.
6 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.
7 Atom economyIn 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.
8 Atom economyA 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.
9 High atom economyAll reactant atoms included in the desired product.
10 Low atom economySome reactant atoms not included in the desired product.
11 Example 1 C(s) + 2H2O(g) → CO2(g) + 2H2(g) What is the percentage atom economy for the following reaction for making hydrogen by reacting coal with steam?C(s) H2O(g) → CO2(g) H2(g)12 g (2 + 16) g [12 + (2 × 16)] g 2(2 × 1) g12 g g g gTotal mass of reactants Mass of desired product= = 48 g = 4 g
12 Example 1 (contd) % atom economy = mass of desired product × 100 total mass of reactants= × 10048= 8.3%This reaction route has a very low atom economy and is an inefficient method of producing hydrogen.
13 Example 2Calculate the percentage atom economy for the reaction below.C6H C6H12Total mass of reactants Mass of desired product= [(6 × 12) + (12 × 1)] = [(6 × 12) + (12 × 1)]= 84 g = 84 g
14 Example 2 (contd) % atom economy = mass of desired product × 100 total mass of reactants= × 10084= 100%This reaction route has a very high atom economy as all reactant atoms are incorporated into the desired product.
15 Example 3Hydrazine (N2H4) is used for rocket fuel. Calculate the atom economy for hydrazine production.Total mass of reactants Mass of desired product= = g = 32 gNH32 mol34 gNaOCl1 mol74.5 gN2H41 mol32 gNaCl1 mol58 gH2O1 mol18 g
16 Example 3 (contd) % atom economy = mass of desired product × 100 total mass of reactants= × 100108.5= 30%This reaction route has an atom economy of 30%. The remaining 70% is waste product (NaCl and H2O).
17 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.