Atom Economy
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. Mass of desired product(s) % atom economy = x 100 Total mass of reactants 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 Actual yield % yield = x 100 Theoretical 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. 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 Mass of desired product(s) % atom economy = x 100 Total mass of reactants 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. 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 12 g 36 g 44 g 2(2 × 1) g 4g Total mass of reactants Mass of desired product = 12 + 36 = 48 g =4g 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 2 Calculate the percentage atom economy for the reaction below. CH3 H3C C acid H3C CH CH2 CH3 C6H12 Total mass of reactants = [(6 × 12) + (12 × 1)] = 84 g CH3 C H3C C CH3 C6H12 Mass of desired product = [(6 × 12) + (12 × 1)] = 84 g 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. 2NH3 + NaOCl N2H4 NH3 2 mol NaOCl 1 mol N2H4 1 mol NaCl 1 mol H2O 1 mol 34 g 74.5 g 32 g 58 g 18 g + NaCl Total mass of reactants Mass of desired product = 34 + 74.5 = 108.5 g = 32 g + H2O 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.
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