3.10 Atom economy
5 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which type of reaction always has an atom economy of 100%?
Choose one answer.
Correct Not quite: the answer is A reaction with only one product
Mark scheme
A
Model answer
A: an addition reaction has only one product, so all atoms end up in it.
Hydrogen can be made by reacting methane with steam: CH4 + H2O → CO + 3H2
Calculate the atom economy for making hydrogen. (M_\mathrm{r}: CH4 = 16, H2O = 18, H2 = 2)
Mark scheme
- sum of M_\mathrm{r} reactants = 34 (1)
- desired product = 3 × 2 = 6 (1)
- 6 ÷ 34 × 100 = 17.6% (1)
Model answer
Atom economy = (3 × 2) ÷ (16 + 18) × 100 = 6 ÷ 34 × 100 = 17.6%.
Calcium oxide is made by heating calcium carbonate: CaCO3 → CaO + CO2
Calculate the atom economy for making calcium oxide. (M_\mathrm{r}: CaCO3 = 100, CaO = 56)
Mark scheme
- 56 ÷ 100 × 100 (1)
- = 56% (1)
Model answer
Atom economy = 56 ÷ 100 × 100 = 56%.
Give two reasons why chemists try to use reactions with a high atom economy.
Mark scheme
any two from:
- less waste (1)
- uses fewer raw materials / more sustainable (1)
- cheaper / more economical (1)
Model answer
A high atom economy produces less waste and uses raw materials more efficiently, which is more sustainable and cheaper.
Ethanol can be made by fermentation (atom economy 51%) or by hydration of ethene (atom economy 100%).
Suggest one advantage of each method.
Mark scheme
- fermentation: uses a renewable raw material (sugar from plants) / low temperature (1)
- hydration: higher atom economy / less waste / faster / purer product (1)
Model answer
Fermentation uses renewable sugar from plants, while hydration of ethene has a 100% atom economy, so it produces no waste product.
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