AQA GCSE Chemistry (8462), Higher tier · Quantitative chemistry › Yield and atom economy
Practise Atom economy. 14 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Ethene reacts with hydrogen to produce ethane (an addition reaction): C2H4 + H2 → C2H6 Relative formula masses (Mr): C2H4 = 28, H2 = 2, C2H6 = 30 Calculate the percentage atom economy for this reaction.
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Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The atom economy of a reaction can be calculated using the equation: % atom economy = (relative formula mass of desired product ÷ sum of relative formula masses of all reactants) × 100
(a) Which type of reaction always has an atom economy of 100%? Tick (✓) one box.[1]
A reaction that has only one product
A reaction that has two products
A reaction that is reversible
A reaction that uses a catalyst
(b) Calcium oxide is made by heating calcium carbonate: CaCO3 → CaO + CO2 Relative formula masses (Mr): CaCO3 = 100, CaO = 56 Calculate the percentage atom economy for making calcium oxide.[2]
(c) Give one reason why chemical companies use reactions with a high atom economy.[1]
Show the answer and mark scheme
(a)Answer: A reaction that has only one product
(b)Answer: 56%
(56 ÷ 100) × 100
56 (%)
(c)
less waste is produced / fewer unwanted products
more sustainable / fewer raw materials used
more economical / lower cost
Question 2Medium6 marks
Hydrogen can be made by reacting methane with steam: CH4 + H2O → CO + 3H2 Relative formula masses (Mr): CH4 = 16, H2O = 18, CO = 28, H2 = 2
(a) Calculate the percentage atom economy for making hydrogen by this reaction. Give your answer to 3 significant figures.[3]
(b) Hydrogen can also be made by the electrolysis of water: 2H2O → 2H2 + O2 Calculate the percentage atom economy for making hydrogen by this reaction. Give your answer to 3 significant figures.[2]
(c) Suggest how a company using the reaction of methane with steam could make better use of its reactants.[1]
Show the answer and mark scheme
(a)Answer: 17.6%
sum of Mr of reactants = 16 + 18 = 34
Mr of desired product = 3 × 2 = 6
(6 ÷ 34) × 100 = 17.6 (%)
(b)Answer: 11.1%
(4 ÷ 36) × 100
11.1 (%)
(c)
find a use for / sell the carbon monoxide (e.g. as a fuel) / react the carbon monoxide further to make more hydrogen
Question 3Hard7 marks
Sodium hydroxide is made by the electrolysis of sodium chloride solution (brine). The overall equation is: 2NaCl + 2H2O → 2NaOH + H2 + Cl2 Relative atomic masses (Ar): H = 1, O = 16, Na = 23, Cl = 35.5
(a) Calculate the percentage atom economy for making sodium hydroxide. Give your answer to 3 significant figures.[3]
(b) The hydrogen and chlorine produced are also sold. Explain why this makes the process more sustainable, even though the atom economy for sodium hydroxide is only 52.3%.[2]
(c) Calculate the mass of chlorine produced when 1.00 tonne of sodium hydroxide is made. Give your answer to 2 significant figures.[2]
Show the answer and mark scheme
(a)Answer: 52.3%
sum of reactants = (2 × 58.5) + (2 × 18) = 153
mass of desired product = 2 × 40 = 80
(80 ÷ 153) × 100 = 52.3 (%)
(b)
all of the products are useful, so there is no waste
hydrogen and chlorine do not need to be made by other processes, so resources / energy are saved (and selling them earns money)
(c)Answer: 0.89 tonnes
80 tonnes of NaOH are produced with 71 tonnes of Cl2 / (1.00 ÷ 80) × 71