Yield and atom economy of chemical reactions

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Why 100% Yield Is Rarely Achieved

Yield is the amount of product obtained from a reaction.
The reaction may be reversible, so products turn back into reactants and the reaction never goes to completion.
Some product may be lost when it is separated from the reaction mixture, for example during filtration or distillation.
Some reactants may be left behind in the equipment or lost during transfer from one container to another.
Side reactions can occur if a reactant reacts with an impurity or a gas in the air, using up reactants in unexpected ways.

Actual Yield and Theoretical Yield

The actual yield is the mass of product actually obtained in an experiment; it can only be found by experiment.
The theoretical yield is the maximum mass of product that could be formed, calculated from the balanced equation and reacting masses.
The theoretical yield assumes perfect practical and chemical conditions with 100% conversion of reactants to products.
In practice, the actual yield is always less than the theoretical yield.

Calculating Percentage Yield

Percentage yield compares the actual yield to the theoretical yield.
Equation: percentage yield = (actual yield ÷ theoretical yield) × 100.
A percentage yield greater than 100% indicates an error, often from dividing theoretical yield by actual yield instead of the other way round.
For economic reasons, companies aim for the highest possible percentage yield to increase profits and reduce costs and waste.
A well-managed process gives an actual yield close to the theoretical yield.

Calculating Theoretical Mass of Product

This is a higher tier skill: use a balanced equation and a given mass of reactant to find the theoretical mass of product.
Step 1: Write the balanced equation.
Step 2: Convert the given mass of reactant into moles using moles = mass ÷ molar mass.
Step 3: Use the coefficients in the equation to find the moles of product.
Step 4: Convert moles of product into mass using mass = moles × molar mass.
Be careful not to include coefficients when calculating molar masses; molar mass comes from the chemical formula, not the equation.

Atom Economy

Atom economy (atom utilisation) measures the proportion of starting materials that end up as useful products.
Most reactions produce more than one product, and some may not be useful.
Equation: percentage atom economy = (relative formula mass of desired product ÷ total relative formula mass of all reactants) × 100.
You can also use the total relative formula mass of all products instead of reactants, as long as the equation is balanced.
A high atom economy means less waste and is important for sustainable development and economic reasons.
The higher the atom economy, the more sustainable the process.

Comparing Yield and Atom Economy

Percentage yield measures how much product is actually obtained compared to the theoretical maximum.
Atom economy measures how much of the reactants' mass becomes useful product, based on the balanced equation.
A high percentage yield does not necessarily mean a high atom economy; they are different measures.
Both are used to analyse the efficiency of chemical reactions.

Choosing a Reaction Pathway

A reaction pathway is a sequence of reactions used to produce a desired product.
Companies evaluate several factors when choosing a pathway: atom economy, percentage yield, rate of reaction, equilibrium position, and the usefulness of by-products.
High percentage yields and fast reaction rates are desirable attributes in industrial processes.
Low atom economy means using lots of raw materials and producing much waste, which is expensive to dispose of and unsustainable.
In reversible reactions, conditions may be altered to shift the equilibrium towards the products.
If waste products can be sold or reused, the atom economy of the overall process improves.
Alternative methods that produce a more useful by-product may also be considered.

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Practice questions

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1.Which equation correctly calculates percentage yield?
Easy
- A(actual yield / theoretical yield) × 100
- B(theoretical yield / actual yield) × 100
- Cactual yield × theoretical yield × 100
- D(actual yield / theoretical yield) / 100
2.A reaction that is reversible can never have a 100% yield.
Easy
True or false?
3.Which of the following are reasons why the actual yield of a reaction is often less than the theoretical yield? (select all that apply)
Medium
- AThe reaction may be reversible.
- BSome product may be lost during separation and purification.
- CSome reactants may undergo side reactions.
- DThe theoretical yield is always calculated incorrectly.
- EAtoms are destroyed during the reaction.
4.What does atom economy measure?
Easy
- AThe amount of starting materials that end up as useful products
- BThe amount of product actually obtained compared to the theoretical amount
- CThe speed at which a reaction reaches completion
- DThe energy released when a reaction takes place
5.A student says that if a reaction has a 90% yield, its atom economy must also be 90%. Why is this incorrect?
Medium
- AYield compares actual and theoretical product masses, while atom economy compares the mass of desired product to the total mass of reactants.
- BYield is always higher than atom economy.
- CAtom economy is only used for reversible reactions.
- DYield is calculated from the balanced equation, while atom economy is measured experimentally.
6.In the reaction AgNO₃(aq) + NaI(aq) → AgI(s) + NaNO₃(aq), a student concludes the percentage atom economy for silver iodide is 50%. How might the student mistakenly arrive at this conclusion?
Medium
- AThey assumed that because there are two products, each must make up half the total mass.
- BThey used the mass of silver iodide divided by the mass of sodium iodide.
- CThey used the mass of reactants divided by the mass of products.
- DThey forgot to multiply by 100.
7.Which method of forming copper(II) sulfate has the highest atom economy? Method 1: CuO + H₂SO₄ → CuSO₄ + H₂O. Method 2: CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂.
Medium
- AMethod 1, because it produces fewer waste products.
- BMethod 2, because it produces more products.
- CBoth have the same atom economy.
- DMethod 2, because carbon dioxide is useful.
8.Which of these is/are factors in choosing a reaction pathway?
Medium
- AAtom economy only
- BAtom economy and percentage yield
- CAtom economy, percentage yield and rate of reaction
- DPercentage yield and rate of reaction only

More questions

9.In the two-stage industrial process for extracting titanium: Stage 1: TiO₂ + 2C + 2Cl₂ → TiCl₄ + 2CO. Stage 2: TiCl₄ + 4Na → Ti + 4NaCl. In stage 2, the percentage yield was 91.2% and the maximum theoretical mass of titanium was 12.5 kg. Calculate the actual mass of titanium produced.
Medium
- A11.4 kg
- B12.5 kg
- C13.7 kg
- D9.12 kg
10.A student wants to make 15.0 g of zinc chloride. The equation is: ZnCO₃ + 2HCl → ZnCl₂ + CO₂ + H₂O. What mass of zinc carbonate should the student add? Relative atomic masses: C = 12, Zn = 65, O = 16, Cl = 35.5, H = 1.
Medium
- A11.0 g
- B13.8 g
- C15.0 g
- D22.0 g
11.A student made zinc chloride by reacting 15.0 g of zinc carbonate with hydrochloric acid. The percentage yield of zinc chloride was 82.4%. Calculate the mass of zinc chloride actually produced. Relative atomic masses: C = 12, Zn = 65, O = 16, Cl = 35.5, H = 1.
Medium
- A13.4 g
- B15.0 g
- C16.3 g
- D12.4 g
12.Which of the following is a desirable attribute in an industrial chemical process?
Medium
- AHigh percentage yield
- BHigh energy costs
- CUse of expensive raw materials
- DProduction of unwanted byproducts
13.Atom economy is a measure of the amount of starting materials that end up as useful products.
Easy
True or false?
14.Match each term to its correct description.
Medium
- Theoretical yield
- Actual yield
- Percentage yield
- The amount of product obtained in practice
- The maximum amount of product possible from the balanced equation
- The actual yield expressed as a percentage of the theoretical yield
15.Place the steps for calculating the theoretical mass of a product from a given mass of reactant in the correct order.
Medium
- Write the balanced equation.
- Convert the given mass of reactant to moles.
- Use the coefficients to find moles of product.
- Convert moles of product to mass.
16.Which of the following statements about atom economy and percentage yield are correct? (select all that apply)
Hard
- AA reaction can have a high percentage yield but a low atom economy.
- BAtom economy is calculated using the balanced equation.
- CPercentage yield is calculated using the actual and theoretical yields.
- DA high atom economy always means a high percentage yield.
- EAtom economy and percentage yield are the same thing.
17.Which of the following is NOT a reason why the actual yield is less than the theoretical yield?
Medium
- AThe reaction goes to completion.
- BSome product is lost during transfer.
- CSide reactions occur.
- DThe reaction is reversible.
18.Which of the following is an advantage of a high atom economy?
Medium
- AIt reduces waste and is more sustainable.
- BIt always increases the rate of reaction.
- CIt guarantees a 100% yield.
- DIt means the reaction is reversible.
19.The percentage yield can never be greater than 100% unless there is an error or an impurity in the product.
Easy
True or false?
20.Which of the following is the correct definition of theoretical yield?
Medium
- AThe maximum mass of product that could be formed from the reactants, calculated from the balanced equation.
- BThe mass of product actually obtained in an experiment.
- CThe mass of reactant left over after the reaction.
- DThe mass of product lost during purification.
21.In the reaction CH₄(g) + H₂O(g) → CO(g) + 3H₂(g), what is the atom economy for producing hydrogen? Relative atomic masses: C = 12, H = 1, O = 16.
Hard
- A17.6%
- B82.4%
- C6.0%
- D34.0%
22.Which of the following is NOT a factor that companies consider when choosing a reaction pathway?
Medium
- AColour of the product
- BAtom economy
- CPercentage yield
- DRate of reaction
23.Match each term to its correct meaning.
Medium
- Atom economy
- Percentage yield
- Reversible reaction
- A measure of the proportion of reactants that become useful products
- The actual yield as a percentage of the theoretical yield
- A reaction where products can react to re-form reactants
24.Order the steps to calculate percentage yield from a given mass of reactant.
Medium
- Calculate the theoretical yield from the balanced equation.
- Measure the actual yield from the experiment.
- Divide the actual yield by the theoretical yield and multiply by 100.
25.Which of the following are reasons why the actual yield of a chemical reaction is often less than the theoretical yield? (select all that apply)
Medium
- AThe reaction may be reversible and not go to completion.
- BSome product may be lost when it is separated from the reaction mixture.
- CSome reactants may react in ways different from the expected reaction.
- DThe relative formula mass of the product is always greater than that of the reactants.
- EThe reaction always uses a catalyst.
26.In the reaction N₂(g) + 3H₂(g) → 2NH₃(g), what is the percentage atom economy for producing ammonia? Relative atomic masses: N = 14, H = 1.
Medium
- A100%
- B50%
- C34%
- D17%
27.A company can produce a chemical by two different reaction pathways. Pathway A has a high atom economy but a slow reaction rate. Pathway B has a lower atom economy but a fast reaction rate. Which factor is most important for sustainable development?
Hard
- AAtom economy, because it minimises waste and conserves resources.
- BReaction rate, because faster reactions always produce more product.
- CPercentage yield, because it is the only factor that affects profit.
- DThe colour of the product, because it affects marketability.
28.Which of the following are important factors to consider when choosing a reaction pathway for an industrial process? (select all that apply)
Medium
- AAtom economy
- BPercentage yield
- CRate of reaction
- DThe colour of the reactants
- EEquilibrium position
29.In the reaction NiO + C → Ni + CO, what is the percentage atom economy for producing nickel? Relative atomic masses: Ni = 59, C = 12, O = 16.
Hard
- A59%
- B68%
- C41%
- D100%
30.A student reacts 3.2 g of methane with excess oxygen. The equation is: CH₄ + 2O₂ → CO₂ + 2H₂O. They collect 7.2 g of water. What is the percentage yield of water? Relative atomic masses: H = 1, C = 12, O = 16.
Medium
- A80%
- B90%
- C72%
- D100%
31.A company is comparing two reaction pathways to make the same product. Pathway 1 has a 95% yield and an atom economy of 40%. Pathway 2 has an 80% yield and an atom economy of 75%. Which statements are correct? (select all that apply)
Hard
- APathway 2 produces less waste per mole of product.
- BPathway 1 is more sustainable because it has a higher yield.
- CPathway 2 uses a higher proportion of reactant atoms in the desired product.
- DPathway 1 will always be more profitable because yield is higher.
- EPathway 2 has a lower percentage yield but better atom economy.
32.The equation for the formation of ammonia is: N₂ + 3H₂ → 2NH₃. What is the percentage atom economy for ammonia? Relative atomic masses: N = 14, H = 1.
Medium
- A17.6%
- B34%
- C50%
- D100%
33.A reaction with a high percentage yield always has a high atom economy.
Easy
True or false?
34.Place the steps for calculating the percentage yield of a product in the correct order.
Medium
- Calculate the theoretical yield from the balanced equation.
- Measure the actual yield of the product.
- Divide the actual yield by the theoretical yield.
- Multiply by 100 to get a percentage.
35.In a chemical reaction, the theoretical yield is 25 g. The actual yield is 20 g. What is the percentage yield?
Medium
- A80%
- B125%
- C20%
- D25%
36.Which of the following statements about atom economy and sustainable development are correct? (select all that apply)
Hard
- AHigh atom economy means less waste is produced.
- BAtom economy is only important for economic reasons.
- CUsing reactions with high atom economy helps conserve raw materials.
- DA low atom economy always means the process is not economically viable.
- EAtom economy is a measure of the efficiency of a reaction in terms of atoms.
37.A student filters, washes and dries a solid product before weighing it. Which statement best explains why washing the solid before drying improves the accuracy of the percentage yield?
Medium
- AIt removes soluble impurities so the mass recorded is closer to the mass of pure product
- BIt increases the theoretical yield of the reaction
- CIt makes the reaction go further towards completion
- DIt increases the atom economy of the reaction
38.In an experiment, a student heats a reaction mixture in an open beaker and some product is lost as a gas. Which type of error does this represent when calculating percentage yield?
Medium
- AA loss of product during separation, which lowers the actual yield
- BAn increase in the theoretical yield
- CA side reaction that increases the actual yield
- DAn error in the balanced equation
39.A student plans a practical to measure the percentage yield of a solid product. Which of the following practical steps would help to obtain a reliable value? (select all that apply)
Medium
- AFilter the product using a funnel and filter paper
- BWash the residue with a small amount of cold solvent
- CDry the product thoroughly before weighing
- DWeigh the product while it is still wet
- EUse a different equation to calculate the theoretical yield
40.If a student's calculated percentage yield is greater than 100%, the most likely explanation is that the product still contains impurities or is not fully dry.
Easy
True or false?
41.Complete the sentences about why 100% yield is not achieved. Choose from: purification, reversible, incomplete, transfer, side reactions.
Easy
Reactants and products can be left behind during ____ from one container to another. ____ reactions can never have a 100% yield because the products are forming reactants. A reaction might not go to completion because of ____ or the presence of an impurity. These reactions are described as ____.
42.A reaction has a theoretical yield of 2.0 g and an actual yield of 1.6 g. Calculate the percentage yield.
Medium
43.Calculate the percentage atom economy for producing nickel from the reaction NiO + C → Ni + CO. Relative atomic masses: Ni = 59, C = 12, O = 16. Give your answer to 3 significant figures.
Medium
44.A company calculated that they would produce 1.2 tonnes of magnesium, but only 0.9 tonnes was made. Calculate the percentage yield.
Medium
45.Calculate the relative formula mass of silver iodide, AgI. Relative atomic masses: Ag = 108, I = 127.
Medium
46.Calculate the percentage atom economy for making silver iodide in the reaction AgNO₃(aq) + NaI(aq) → AgI(s) + NaNO₃(aq). Relative atomic masses: Ag = 108, I = 127, N = 14, Na = 23, O = 16. Give your answer to 3 significant figures.
Medium
47.Calculate the percentage atom economy for making copper sulfate in the reaction CuO + H₂SO₄ → CuSO₄ + H₂O. Relative formula masses: CuO = 79.5, H₂SO₄ = 98, CuSO₄ = 159.5. Give your answer to 3 significant figures.
Medium
48.Calculate the percentage atom economy for producing copper sulfate from copper carbonate in the reaction CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂. Relative formula masses: CuCO₃ = 123.5, H₂SO₄ = 98.0, CuSO₄ = 159.5. Give your answer to 3 significant figures.
Medium
49.What term describes the amount of product obtained from a reaction?
Medium
50.Complete the equation for percentage atom economy.
Easy
Percentage atom economy = (relative formula mass of ____ product / sum of relative formula masses of all ____ ) × 100
51.In an experiment, 6.5 g of zinc was added to copper(II) sulfate solution. The equation is: Zn(s) + CuSO₄(aq) → Cu(s) + ZnSO₄(aq). The mass of copper obtained was 4.8 g. Calculate the percentage yield of copper. Relative atomic mass: Zn = 65, Cu = 63.5.
Hard
52.In a reaction, the theoretical yield is 5.0 g and the actual yield is 4.0 g. Calculate the percentage yield.
Medium
53.State two reasons why the calculated yield of magnesium might not be obtained in an industrial process.
Hard
54.Give two reasons why it is beneficial for the percentage atom economy of a reaction to be as high as possible.
Hard
55.Explain why a high atom economy is important for sustainable development.
Hard
56.Explain why the actual yield of a chemical reaction is often less than the theoretical yield.
Hard
57.Explain why it is beneficial for the percentage atom economy of a reaction to be as high as possible.
Hard
58.A reaction has a theoretical yield of 2.0 g and an actual yield of 1.6 g. Calculate the percentage yield.
Medium
- A72 %
- B80 %
- C88 %
- D96 %
59.Calculate the percentage atom economy for producing nickel from the reaction NiO + C → Ni + CO. Relative atomic masses: Ni = 59, C = 12, O = 16. Give your answer to 3 significant figures.
Medium
- A74.6 %
- B61 %
- C67.8 %
- D81.4 %
60.A company calculated that they would produce 1.2 tonnes of magnesium, but only 0.9 tonnes was made. Calculate the percentage yield.
Medium
- A67 %
- B83 %
- C75 %
- D91 %
61.Calculate the relative formula mass of silver iodide, AgI. Relative atomic masses: Ag = 108, I = 127.
Medium
- A211
- B259
- C283
- D235
62.Calculate the percentage atom economy for making silver iodide in the reaction AgNO₃(aq) + NaI(aq) → AgI(s) + NaNO₃(aq). Relative atomic masses: Ag = 108, I = 127, N = 14, Na = 23, O = 16. Give your answer to 3 significant figures.
Medium
- A52 %
- B70 %
- C73.4 %
- D64 %
63.Calculate the percentage atom economy for making copper sulfate in the reaction CuO + H₂SO₄ → CuSO₄ + H₂O. Relative formula masses: CuO = 79.5, H₂SO₄ = 98, CuSO₄ = 159.5. Give your answer to 3 significant figures.
Medium
- A107.9 %
- B89.9 %
- C80.9 %
- D98.9 %
64.Calculate the percentage atom economy for producing copper sulfate from copper carbonate in the reaction CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂. Relative formula masses: CuCO₃ = 123.5, H₂SO₄ = 98.0, CuSO₄ = 159.5. Give your answer to 3 significant figures.
Medium
- A86 %
- B79 %
- C72 %
- D65 %
65.In an experiment, 6.5 g of zinc was added to copper(II) sulfate solution. The equation is: Zn(s) + CuSO₄(aq) → Cu(s) + ZnSO₄(aq). The mass of copper obtained was 4.8 g. Calculate the percentage yield of copper. Relative atomic mass: Zn = 65, Cu = 63.5.
Hard
- A76 %
- B68 %
- C92 %
- D84 %
66.In a reaction, the theoretical yield is 5.0 g and the actual yield is 4.0 g. Calculate the percentage yield.
Medium
- A72 %
- B88 %
- C96 %
- D80 %

Flashcards

What is meant by the yield of a chemical reaction?

The yield is the amount of product obtained from a reaction.

What is the actual yield of a reaction?

The actual yield is the recorded amount of product obtained from an experiment.

What is the theoretical yield of a reaction?

The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions, calculated from the balanced equation and reacting masses.

State the equation for calculating percentage yield.

Percentage yield = (actual yield ÷ theoretical yield) × 100.

Why is a 100% yield rarely achieved in a chemical process? Give three reasons.

Some reactants may be left behind in the equipment; the reaction may be reversible so products turn back into reactants; some product may be lost during separation and purification steps such as filtration or distillation.

How can side reactions reduce the yield of a desired product?

Side reactions occur when a substance reacts with a gas in the air or an impurity in a reactant, using up reactants to form unwanted products instead of the desired product.

Why do companies aim for as high a percentage yield as possible?

A high percentage yield increases profits and reduces costs and waste.

A student prepared 1.6 g of dry copper(II) sulfate crystals. The theoretical yield is 2.0 g. Calculate the percentage yield.

Percentage yield = (1.6 ÷ 2.0) × 100 = 80%.

What is a reaction pathway?

A reaction pathway is a sequence of reactions that must occur to produce the required product.

What does atom economy measure?

Atom economy measures the amount of starting materials that end up as useful products.

State the equation for calculating percentage atom economy.

Percentage atom economy = (relative formula mass of desired product ÷ sum of relative formula masses of all reactants) × 100.

Can you calculate atom economy using the total relative formula mass of all products instead of all reactants?

Yes, as long as the equation is fully balanced, the total relative formula mass of all products equals that of all reactants, so either can be used in the denominator.

Why is a high atom economy important for sustainable development?

A high atom economy means less raw material is wasted, so the process uses fewer resources and produces less waste, making it more sustainable.

In the reaction CH₄(g) + H₂O(g) → CO(g) + 3H₂(g), calculate the atom economy for producing hydrogen. (Mr: CH₄=16, H₂O=18, CO=28, H₂=2)

Total Mr of all products = 28 + (3×2) = 34. Mr of desired product (3H₂) = 6. Atom economy = (6 ÷ 34) × 100 = 17.6%.

What factors are considered when choosing a reaction pathway for industrial production?

Atom economy, percentage yield, rate of reaction, equilibrium position, and the usefulness of by-products.

Why are reactions with low atom economies often not economically attractive?

They use up a lot of raw materials and produce a lot of waste, which is expensive to dispose of, so they are not cost-effective.

How can the atom economy of a process be improved if waste products are produced?

If the waste products can be sold or reused in some way, the atom economy is improved.

In a reversible reaction, how can the yield of the desired product be increased?

The position of equilibrium can be changed in favour of the products by altering the reaction conditions.

What is the difference between percentage yield and atom economy?

Percentage yield compares the actual yield to the theoretical yield, while atom economy compares the mass of desired product to the total mass of all reactants, measuring how much of the reactants end up in useful products.

A reaction has a 90% yield and a 50% atom economy. Does the high yield mean the atom economy is also high?

No, a high yield does not imply a high atom economy. They are separate measures: yield is about how much product is obtained, atom economy is about how much of the reactants become useful product.

What is the most common error when calculating percentage yield?

Dividing the theoretical yield by the actual yield instead of the actual yield by the theoretical yield, which can give a percentage yield greater than 100%.

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