AQA GCSE Chemistry Foundation (8462), Foundation tier · Quantitative chemistry › Yield and atom economy
Practise Atom economy. 6 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.
Atom economy measures how much of the starting materials ends up in the useful product. You calculate it from the balanced equation. This is in GCSE Chemistry only, not Combined Science.
Key facts
% atom economy = Mr of desired product ÷ total Mr of reactants × 100
Use the balancing numbers: multiply each Mr by them
Total Mr of reactants = total Mr of products
Only one product → 100% atom economy
High atom economy → less waste, more sustainable, cheaper
Notes
What atom economy is
Atom economy is a measure of the amount of starting materials that end up as useful products.
percentage atom economy = (Mr of desired product from the reaction ÷ sum of Mr of all reactants) × 100
Use the balanced equation, and multiply each Mr by its balancing number.
Mass is conserved, so the total Mr of all the products equals the total Mr of all the reactants: a useful check.
A reaction with only one product has an atom economy of 100%.
Worked example
diagram
Hydrogen can be made by reacting methane with steam: CH4 + H2O → CO + 3H2.
Sum of reactants: 16 + 18 = 34. Desired product: 3H2 = 3 × 2 = 6.
Only the blue part (hydrogen, the desired product) counts as useful; the rest of the mass is waste.
Why it matters
Reactions with a high atom economy are important for sustainable development and for economic reasons: they use fewer resources and make less waste.
A low atom economy means a lot of waste product, which must be disposed of unless it can be used or sold.
Don’t lose marks
Forgetting the balancing numbers, e.g. using 56 for iron instead of 2 × 56 = 112.
Putting only one reactant on the bottom, or the waste product on top.
Confusing atom economy with percentage yield.
Saying a high atom economy means a fast reaction or a high yield. It tells you nothing about either.
More tips
Memory tricks
Wanted over total: the desired product on top, all the reactants on the bottom.
Check your bottom number by adding up all the products: the total must be the same.
One product → 100%. Two or more products → less than 100%.
Atom economy needs only the equation; percentage yield needs measured masses.
Exam technique
Write out the sum of the Mr values on the bottom so each part can earn a mark.
In 'evaluate' questions, compare both routes for each factor, use the data given, and end with a justified conclusion.
Give the answer to the significant figures asked for, with a % sign.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State what atom economy measuresThe amount of the starting materials that ends up as the useful (desired) product.
Grade 5
Calculate atom economy from an equationAtom economy = Mr of desired product ÷ sum of Mr of all reactants × 100.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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.
100%
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