Yield and Atom Economy of Chemical Reactions

1. Explain why reactions with a high percentage yield are preferred in industrial manufacturing processes.

Answer: The higher the percentage yield of a reaction, the more efficient the process is/the less waste is produced/the greater the products for the manufacture.

2. Write the equation used to determine the percentage atom economy for a desired product in a reaction.

Answer: Atom economy = 100 × [(sum of the Mr of desired products) / (sum of the Mr of all the reactants)].

3. Explain why a reaction with only one product must have an atom economy of 100%.

Answer: If there is only one product then all the atoms of the reactants must have formed the product, so the Mr of the desired product is equal to the Mr of all the reactants.

4. Ammonia (NH3 ) is manufactured using the Haber process. The reaction that occurs is N2 + 3H2  →  2NH3

a) When 140 kg of nitrogen are reacted, the maximum theoretical yield of ammonia is 170 kg. One Haber process reactor produces an actual yield of ammonia of 31.7 kg. Calculate the percentage yield for this reaction.

Answer: Percentage yield = 100 × (31.7 ÷ 170) = 18.647%

b) Give three reasons why the percentage yield of this reaction is not 100%.

Answer: The reaction does not go to completion/the reaction is reversible. Some of the product may be lost during separation from the reaction mixture. The reactants may react in a different way to the desired reaction/the reactants may carry out different reactions.

c) Explain why the manufacture of ammonia using the Haber process may be considered sustainable. Give your answer in terms of atom economy.

Answer: The percentage atom economy for the Haber process is 100% as there is only one product. This means there is less waste of the Earth’s resources in undesired products.

5. Ethanol (CH3CH2OH) can be manufactured using two different processes.

Process 1 produces ethanol from the fermentation of glucose, C6H12O6  →  2CH3CH2OH + 2CO2
The glucose is derived from the plant sugar cane. This process has a relatively low energy requirement.

Process 2 produces ethanol by the hydration of ethene, C2H4 + H2O  →  CH3CH2OH
Ethene is a hydrocarbon produced from crude oil. This process has a high energy requirement.

Relative formula masses (Mr): C6H12O6 = 180 ; CH3CH2OH = 46 ; CO2 = 44 ; H2O = 18

a) Calculate the percentage atom economy for the reaction to produce ethanol in Process 1.

Answer: Percentage atom economy = 100 × [(2 × 46) ÷ 180] = 51.111

b) The overall percentage yield for Process 2 is 92.0%. A reactor has a maximum theoretical yield of 24.8 kg. Calculate the actual yield of ethanol for this reactor.

Answer: Actual yield = maximum theoretical yield × percentage yield = 24.8 kg × (92 ÷ 100) = 22.816 kg

c) Compare the sustainability of the two processes for manufacturing ethanol.

Answer: The source of the glucose in Process 1 is plant matter which is renewable so sustainable, while in Process 2 the ethene from crude oil is non-renewable, so not sustainable (as crude oil will eventually run out). Process 2 is more sustainable in terms of atom economy. The atom economy for Process 2 is 100% (as there is only one product) and much greater than for Process 1. This means less waste in Process 2. Process 1 has a low energy requirement compared to Process 2. This means fewer energy resources are required for Process 1, making it more sustainable than Process 2.