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.

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.

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.

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 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) + 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 2 Calculate the percentage atom economy for the reaction below.

Example 2 Calculate 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

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 NH3 2 mol 34 g NaOCl 1 mol 74.5 g N2H4 1 mol 32 g NaCl 1 mol 58 g H2O 1 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.

Enthalpy of combustion The enthalpy of combustion is the heat energy given out when 1 mole of fuel burns completely in oxygen. The enthalpy of combustion of methane can be represented by the equation CH4(g) + O2 (g) CO2(g) + H2 O(l)

Calculations for you to try. 1. 0.25g of ethanol, C2H5OH, was burned and the heat given out raised the temperature of 500 cm3 of water from 20.1oC to 23.4oC. Calculate the enthalpy of combustion of ethanol 2. 0.01 moles of methane was burned and the energy given out raised the temperature of 200cm3 of water from 18oC to 28.6oC. Calculate the enthalpy of combustion of methane. 3. 0.1g of methanol, CH3OH, was burned and the heat given out used to raise the temperature of 100 cm3 of water at 21oC. Use the enthalpy of combustion of methanol in the data booklet to calculate the final temperature of the water. 4. 0.2g of methane, CH4, was burned and the heat given out used to raise the temperature of 250 cm3 of water Use the enthalpy of combustion of methane in the data booklet to calculate the temperature rise of the water.