Yield & Atom Economy (AQA GCSE Chemistry)

Exam Questions

3 hours30 questions
1a4 marks

This question is about the yield of chemical reactions.

In practice, scientists aim for a 100% yield but this is rarely achieved.

Complete the sentences to explain why a 100% yield is not achieved. 

Choose answers from the box. 

side reactions reversible  incomplete
purification transfer

 

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 _______________ .

1b2 marks

There are different types of yield.

The percentage yield compares the actual yield to the theoretical yield. 

Draw one line from each yield to its description.

 
aqa-gcse-3-3e-q1b-yield-definitions-

1c1 mark

What is the correct equation to calculate percentage yield? 

Circle one equation. 

begin mathsize 14px style fraction numerator actual space yield over denominator theoretical space yield end fraction end style                    fraction numerator size 14px actual size 14px space size 14px yield over denominator size 14px theoretical size 14px space size 14px yield end fraction size 14px cross times size 14px 100                    fraction numerator theoretical space yield over denominator actual space yield end fraction cross times 100

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2a1 mark

A student completed an experiment using magnesium to displace copper from copper(II) sulfate.

Complete the symbol equation. 

Mg + CuSO4 → ________ + ________

2b
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1 mark

The student added 4.8 g of magnesium to a solution of copper(II) sulfate.

Calculate the number of moles of magnesium that the student used. 

Relative atomic mass (Ar):     Mg = 24

Use the equation:

begin mathsize 14px style moles equals mass over M subscript straight r end style

2c
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2 marks

Using your answer to part (b), calculate the theoretical yield of copper. 

Relative atomic mass (Ar):     Cu = 63.5 

 
Theoretical yield of copper = ________ g
2d
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2 marks

Another student completes the same experiment using 4.8 g of magnesium.

They obtain 10.16 g of copper.

What is the percentage yield of this student's experiment? 

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3a1 mark

This question is about atom economy.

What does atom economy measure?

3b
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2 marks

Silver nitrate and sodium iodide can react to form silver iodide.

The equation for the reaction is:

AgNO3 (aq) + NaI (aq) → AgI (s) + NaNO3 (aq)

A student looks at the useful silver iodide product of this reaction and concludes that the percentage atom economy is 50%. 

How might the student mistakenly arrive at this conclusion?

3c
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1 mark

Calculate the relative formula mass of silver iodide, AgI. 

Relative atomic masses (Ar):     Ag = 108;     I = 127

3d
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2 marks

Calculate the relative formula mass of sodium nitrate, NaNO3. 

Relative atomic masses (Ar):     N = 14;     Na = 23;     O = 16

3e
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3 marks

Calculate the percentage atom economy for making silver iodide.

Use the equation: 

Percentage atom economy = fraction numerator relative space formula space mass space of space desired space product over denominator total space mass space of space all space products end fraction cross times 100

Give your answer to 3 significant figures.

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4a
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3 marks

Copper sulfate can be made by reacting copper oxide with sulfuric acid.

CuO + H2SO4 ⟶ CuSO4 + H2O

Calculate the percentage atom economy for making copper sulfate. 

Use the equation: 

Percentage atom economy = fraction numerator size 14px relative size 14px space size 14px formula size 14px space size 14px mass size 14px space size 14px of size 14px space size 14px desired size 14px space size 14px product over denominator size 14px total size 14px space size 14px mass size 14px space size 14px of size 14px space size 14px all size 14px space size 14px reactants end fraction size 14px cross times size 14px 100 

Relative formula masses (Mr):     H2SO4 = 98;     CuO = 79.5;     CuSO4 = 159.5

Give your answer to 3 significant figures.

4b1 mark

Reacting copper carbonate, CuCO3, with sulfuric acid forms copper sulfate and two other products.

CuCO3 + H2SO4 → CuSO4 + H2O + CO2 

What type of reaction is this? 

Tick (✓) one box. 

Combustion  
Endothermic  
Neutralisation  
Decomposition  

 

4c
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2 marks

The two methods of forming copper(II) sulfate using sulfuric acid are:

Method 1: CuO + H2SO4 → CuSO4 + H2O

Method 2: CuCO3 + H2SO4 → CuSO4 + H2O + CO2 

Which method has the highest atom economy? Explain your answer. 

Method: _____ 

Explanation: ____________________________________________________

4d5 marks

High atom economy is a desirable attribute in industrial chemical processes.

Sort the following attributes of chemical reactions into the correct columns in Table 1.

  • Produce useful byproducts
  • High percentage yield
  • High energy costs
  • Efficient reactions
  • Use expensive raw materials

Table 1

Desirable attribute  Undesirable attribute
   
   
   

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5a
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1 mark

To produce a sample of hex-1-ene, 10.2 g of hexan-1-ol is dehydrated by heating with an excess of phosphoric(V) acid.

Hexan-1-ol → Hex-1-ene + water

C6H13OH → C6H12 + H2O

After purification, 5.04 g of hex-1-ene was produced.

Calculate the number of moles of hexan-1-ol. 

Relative molecular masses (Mr):     C6H13OH  = 102;       C6H12 = 84

5b
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2 marks

Calculate the mass of hex-1-ene that could be produced.

5c
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2 marks

What is the percentage yield for the production of hex-1-ene from hexan-1-ol in this experiment?

5d
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3 marks

What is the percentage atom economy for the production of hex-1-ene from hexan-1-ol in this experiment?

C6H13OH → C6H12 + H2O

Relative molecular masses (Mr):     C6H12 = 84;     H2O = 18

Give your answer to three significant figures.

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61 mark

Which of these is/are a factor(s) in choosing a reaction pathway?

  Atom economy Percentage yield Rate of reaction
A ✔    
B ✔ ✔  
C   ✔ ✔
D ✔ ✔ ✔

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    1a3 marks

    Nickel oxide is reduced by heating with carbon, producing nickel metal and carbon monoxide. 

    The equation for the reaction is as follows: 

    NiO + C ⟶ Ni + CO

    Calculate the percentage atom economy for the production of nickel.

    Relative atomic masses (Ar): C = 12; Ni = 59

    Relative formula mass (Mr): NiO = 75;   CO = 28

    Give your answer correct to 3 significant figures.

    Percentage atom economy = _____________________ %

    1b1 mark

    Magnesium can be produced by reacting magnesium oxide with silicon.

    The equation for the reaction is as follows: 

    Si (s) + 2MgO (s) → SiO2 (s) + 2Mg (s)

    A company calculated, based on the amount of reactants and the balanced equation, that they would produce 1.2 tonnes of magnesium, but only 0.9 tonnes was made. 

    Calculate the percentage yield of magnesium.

    Percentage yield = __________ %

    1c2 marks

    State two reasons why the calculated yield of magnesium might not be obtained.

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    2a2 marks

    The extraction of titanium from titanium dioxide is carried out in a two-stage industrial process:  

       Stage 1   TiO2 + 2C + 2Cl2 ⟶ TiCl4 + 2CO

       Stage 2   TiCl4 + 4Na ⟶ Ti + 4NaCl

    In the stage 2 reaction the percentage yield obtained by a company was 91.2% 

    The maximum theoretical mass of titanium for this batch was 12.5 kg.

    Calculate the actual mass of titanium produced.

    Mass of titanium = ______________________ kg

    2b3 marks

    Copper carbonate and sulfuric acid react as follows:

     

    CuCO3 + H2SO4 → CuSO4 + H2O + CO2

    Relative formula masses:   CuCO3 = 123.5;   H2SO4 = 98.0;   CuSO4 = 159.5

    Calculate the percentage atom economy for producing copper sulfate from copper carbonate.

    Atom economy = __________________ %

    2c2 marks

    Give two reasons why it is beneficial for the percentage atom economy of a reaction to be as high as possible.

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    3
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    1 mark

    A student wanted to make 15.0 g of zinc chloride. The equation for the reaction is:

    ZnCO3 + 2HCl →  ZnCl2 + CO2 + H2O

    What mass of zinc carbonate should the student add to the hydrochloric acid to make 15.0 g of zinc chloride?

    Relative atomic masses (Ar): C = 12    Zn = 65   O= 16    Cl = 35.5     H= 1

    • 11.0 g

    • 13.8 g

    • 15.0 g

    • 22.0 g

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    4
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    1 mark

    A student made zinc chloride by reacting 15.0 g of zinc carbonate with hydrochloric acid.

    ZnCO3 + 2HCl →  ZnCl2 + CO2 + H2O

    The percentage yield of zinc chloride was 82.4%.

    Calculate the mass of zinc chloride the student actually produced.

    Relative atomic masses (Ar): C = 12    Zn = 65   O= 16    Cl = 35.5     H= 1

    • 13.4 g

    • 15.0 g

    • 16.3 g

    • 12.4 g

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    5a2 marks

    In the Haber process, nitrogen and hydrogen are reacted together in conditions of high pressure and temperature to produce ammonia. 

    The equation for the reaction is as follows: 

    N2 + 3H2 → 2NH3  

    Calculate the percentage atom economy for the production of ammonia.

    Relative formula masses (Mr):  N2 = 28;  H2 = 2;  NH3= 17

    Percentage atom economy = ____________________ %

    5b2 marks

    Figure 1 shows how the percentage yield of ammonia varies with changing conditions of pressure.

     
    Figure 1

    haber-process-graph-single-pressure-aqa-gcse-3-3m-q5b

    Describe the trend shown in Figure 1.

    5c2 marks

    Use Figure 1 to determine the difference in percentage yield at 100 atmospheres pressure and at 250 atmospheres pressure.

    Difference % yield NH3 = ________ %

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    6a3 marks

    A scientist makes zinc iodide by reacting zinc with iodine. 

    The equation for the reaction is: 

    Zn + I2 ⟶ ZnI2 

    Relative atomic masses (Mr): Zn = 65; I = 127 

    14.5 g of zinc iodide is produced in the reaction. 

    The percentage yield obtained is 93.0%.

    What is the maximum theoretical mass of ZnI2 produced in this reaction?

    Maximum theoretical mass = ____________________ g

    6b1 mark

    Suggest a reason why the percentage yield is not 100%.

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    7
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    1 mark

    An equation for the reaction between copper(II) oxide and carbon is:

    2CuO + C  → 2Cu + CO2

    What is the percentage atom economy for the reaction to produce copper metal?

    Relative atomic masses (Ar):     C = 12        Cu = 64

    Relative formula mass (Mr):       CuO = 80

    • 64%

    • 59.3%

    • 74.4%

    • 92%

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    81 mark

    Which of the following does not explain why the yield of a reaction may not be 100%?

    • The reaction may not go to completion because it is reversible

    • Some of the product may be lost when it is separated from the reaction mixture

    • Some of the reactants may react in ways different to the expected reaction

    • The reaction is endothermic and requires too much energy to complete it 

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    9
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    1 mark

    Calcium oxide can be produced by heating calcium carbonate in a rotating kiln.

    CaCO3   →   CaO  +  CO2

    If 120 tonnes of calcium carbonate is heated in the kiln and 53 tonnes of calcium oxide are produced, what is the percentage yield?

    Relative atomic masses (Ar): Ca = 40   C = 12   O= 16

    • 53.7%

    • 62.1%

    • 78.9%

    • 84.3%

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    10a4 marks

    A student investigated the reactions of copper carbonate and copper oxide with dilute hydrochloric acid. 

    In both reactions one of the products is copper chloride.

    A student wanted to make 25.0 g of copper chloride by reacting copper carbonate  with dilute hydrochloric acid.

    The equation for the reaction is as follows:

    CuCO3 + 2HCl → CuCl2 + H2O + CO2

    Relative atomic masses, Ar: H = 1; C = 12; O = 16; Cl = 35.5; Cu = 63.5

    Calculate the mass of CuCO3 the student should react with dilute hydrochloric acid to produce 25.0 g of copper chloride.

    Mass of copper carbonate needed = _________________________ g

    10b2 marks

    The percentage yield of copper chloride was 82.2 %. 

    Calculate the mass of copper chloride produced. 

    Actual mass of CuCl2 produced = ____________ g

    10c3 marks

    The equations for the two reactions are shown below:

       Reaction 1:     CuCO3 (s) + 2HCl (aq) → CuCl2 (aq) + H2O (l) + CO2 (g)

       Reaction 2:     CuO (s) + 2HCl (aq) → CuCl2 (aq) + H2O (l)

       Relative formula masses:   CuO = 79.5;   HCl = 36.5;   CuCl2 = 134.5;   H2O = 18

    Calculate the percentage atom economy for producing copper chloride via the second reaction.

    Percentage atom economy = ________________________ %

    10d2 marks
    The atom economy for reaction 1 is 68.45 %.

     

    Compare the atom economies of both reactions for making copper chloride. 

    State a reason for the difference.

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    11a3 marks

    This question is about the salt zinc sulfate.

    Zinc sulfate is a compound used as a dietary supplement to treat zinc deficiency. 

     

    It is also added to fertilisers, animal feeds and toothpaste. It can be made by two methods. 

    The equations for the two methods are as follows: 

    Method 1: ZnO + H2SO4 ⟶ ZnSO4 + H2O 

    Method 2: ZnCO3 + H2SO4 ⟶ ZnSO4 + H2O + CO2 

    Calculate the percentage atom economy for making ZnSO4 in Method 1 giving your answer to 3 significant figures. 

    Relative formula masses (Mr):  ZnO = 81;  H2SO4 = 98;  ZnSO4 = 161;  ZnCO3 = 125

    Percentage atom economy = ____________________ %

    11b5 marks

    Compare the atom economy of both methods and decide which method is the most desirable.

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    1a
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    5 marks

    In the Haber process, nitrogen and hydrogen are reacted together in conditions of high pressure and temperature to produce ammonia. 

    The equation for the reaction is as follows:

    N2 (g) + 3H2 (g) → 2NH3 (g)

    Hydrogen is produced for the Haber process by reacting methane with steam:

    CH4 (g) + H2O (g) → 3H2 (g) + CO (g)

    Calculate the percentage atom economy for the production of ammonia. Give your answer to three significant figures.

     

    Relative formula masses (Mr):  N2 = 28;  H2 = 2;  NH3= 17;  CH4 = 16;  H2O = 18

    1b2 marks

    Explain why low pressure is used to produce the hydrogen required for the Haber process.

    Give your answer in terms of equilibrium.

    1c1 mark

    Figure 1 shows the percentage yield of ammonia produced at different temperatures and pressures in the Haber process.

    Figure 1

    percentage-yield-vs-pressure-haber-process

    A temperature of 400 °C and a pressure of 200 atmospheres are used in the Haber process.

    Estimate the percentage yield for the Haber Process under these conditions.

    1d
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    4 marks

    The yield of ammonia at 300 oC and 400 atmospheres is 70%. 

    Calculate the mass of nitrogen, in kg, required to produce 1000 kg of ammonia under these conditions.

     N2 (g) + 3H2 (g) → 2NH3 (g) 

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    2a
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    5 marks

    Iron is extracted from iron oxide in the blast furnace as shown.

    2Fe2O3 + 3C → 4Fe + 3CO2 

    1 tonne of iron oxide was reacted with carbon to produce 650 kg of molten iron.

    Calculate the percentage yield for this reaction.

    Relative atomic masses (Ar):     Fe = 56,     O = 16,     C = 12

    2b1 mark

    The temperature inside the blast furnace is so high that carbon monoxide can be used to extract iron instead of carbon.

    Fe2O3 + 3CO → 2Fe + 3CO2

    Explain which reactant is oxidised in this reaction.

    2c
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    3 marks

    Determine if the reaction of iron oxide with carbon has a higher atom economy than the reaction of iron oxide with carbon monoxide.

    Relative atomic masses (Ar):     Fe = 56,     O = 16,     C = 12

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    3a2 marks

    This question is about magnesium sulfate.

    Calculate, to three significant figures, the percentage by mass of oxygen in magnesium sulfate.

    Relative atomic masses (Ar):     Mg = 24,     S = 32,     O = 16

    3b3 marks

    Magnesium sulfate can be produced by reacting magnesium with sulfuric acid.

     
    Mg + H2SO4 → MgSO4 + H2
     

    Write two balanced symbol equations, including state symbols, to show how magnesium sulfate can be produced by neutralisation reactions.

    3c2 marks

    Calculate the atom economy for the formation of magnesium sulfate by the reaction of magnesium and sulfuric acid. 

    Relative atomic masses (Ar):     Mg = 24     S = 32     O = 16

    3d2 marks

    Explain why the reaction of magnesium with sulfuric acid has a larger atom economy compared to the reactions in part (b).

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    4a
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    4 marks

    Rutile is a naturally occurring ore that contains titanium oxide.

    Titanium can be extracted from this ore by two different methods.

    Method 1 involves the electrolysis of the ore.

    TiO2 → Ti + O2 

    Method 2 involves the use of a more reactive metal to displace the titanium.

    TiO2 + 2Mg → Ti + 2MgO

    Calculate the atom economy for each reaction. 

     

    Relative atomic masses (Ar):     Ti = 48,     O = 16,     Mg = 24

    4b2 marks

    Give one advantage and one disadvantage of using Method 1 to extract titanium.

    4c1 mark
    What is the atom economy of Method 1 if the oxygen is collected and sold?
     

    4d3 marks

    Titanium is used for replacement hip joints.

    Give three properties of titanium that make it suitable for this use.

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    5a4 marks

    A student investigated two different methods to produce zinc chloride.

    Method 1 is the reaction of zinc oxide with dilute hydrochloric acid.

    ZnO (s) + 2HCl (aq) → ZnCl2 (aq) + H2O (l)

    Describe how a sample of zinc chloride crystals could be made from zinc oxide and dilute hydrochloric acid.

    5b4 marks

    A student wanted to make 15.0 g of zinc chloride using Method 1. 

    Relative atomic masses, (Ar):     H = 1,     Cl = 35.5,     Zn = 65,     O = 16 

    Calculate the mass of zinc oxide that the student should react with dilute hydrochloric acid to make 15.0 g of zinc chloride.

    5c
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    2 marks

    The percentage yield of the student's reaction to produce zinc chloride using Method 1 was 67.4 %.

    Calculate the mass of zinc chloride the student actually produced.

    5d
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    4 marks
    Method 2 is the reaction of zinc carbonate with dilute hydrochloric acid.

     

    ZnCO3 (s) + 2HCl (aq) → ZnCl2 (aq) + H2O (l) + CO2 (g)

    The atom economy for Method 1 is 88.3%.

    Calculate the atom economy for Method 2.

    Explain the difference in the atom economies for the two methods of making zinc chloride.

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    6
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    1 mark

    A student made zinc chloride by reacting 15.0 g of zinc carbonate with hydrochloric acid.

    ZnCO3 + 2HCl →  ZnCl2 + CO2 + H2O

    The percentage yield of zinc chloride was 82.4%.

    Calculate the mass of zinc chloride the student actually produced.

    Relative atomic masses (Ar): C = 12    Zn = 65   O= 16    Cl = 35.5     H= 1

    • 13.4 g

    • 15.0 g

    • 16.3 g

    • 12.4 g

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