Calculations of Product (Edexcel International A Level Chemistry)

Revision Note

Test yourself
Richard

Author

Richard

Last updated

Calculating Yield & Atom Economy

Percentage yield

  • In a lot of reactions, not all reactants react to form products which can be due to several factors:
    • Other reactions take place simultaneously
    • The reaction does not go to completion
    • Products are lost during separation and purification

  • The percentage yield shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:

p e r c e n t a g e space y i e l d space equals space fraction numerator a c t u a l space y i e l d over denominator t h e o r e t i c a l space y i e l d end fraction space cross times space 100

    • The actual yield is the number of moles or mass of product obtained experimentally
    • The theoretical yield is the number of moles or mass obtained by a reacting mass calculation

Worked example

In an experiment to displace copper from copper(II) sulfate, 6.5 g of zinc was added to an excess of copper(II) sulfate solution. The resulting copper was filtered off, washed and dried. The mass of copper obtained was 4.8 g.

Calculate the percentage yield of copper.

   Answer:

   Step 1: The balanced symbol equation is:

Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)


   Step 2: Calculate the amount of zinc reacted in moles

n u m b e r space o f space m o l e s space equals space fraction numerator 6.5 space g over denominator 65.4 space g space m o l to the power of negative 1 end exponent end fraction space equals space 0.10 space m o l
Step 3: Calculate the maximum amount of copper that could be formed from the molar ratio:
   
Since the ratio of Zn(s) to Cu(s) is 1:1 a maximum of 0.10 moles can be produced

Step 4: Calculate the maximum mass of copper that could be formed (theoretical yield)
mass =   mol  x  M
mass =   0.10 mol x 63.55 g mol-1
mass =   6.4 g (2 sig figs)


   Step 5: Calculate the percentage yield of copper

p e r c e n t a g e space y i e l d space equals space fraction numerator 4.8 space g over denominator 6.4 space g end fraction space cross times space 100 space equals space 75 percent sign

Examiner Tip

It is possible to calculate a percentage yield that is greater than 100%. This would be an error which could occur when preparing crystals due to: 

  • The crystals may still be dry when weighed
  • There are impurities in the crystals
  • The mass of the filter paper or container could mistakingly be included in the total mass of the product 

Atom economy

  • The atom economy of a reaction shows how many of the atoms used in the reaction become the desired product
    • The rest of the atoms or mass is wasted

  • It is found directly from the balanced equation by calculating the Mr of the desired product

A t o m space e c o n o m y space equals space fraction numerator m o l e c u l a r space m a s s space o f space d e s i r e d space p r o d u c t over denominator s u m space o f space m o l e c u l a r space m a s s e s space o f space A L L space r e a c t a n t s end fraction space cross times space 100

  • In addition reactions, the atom economy will always be 100% because all of the atoms are used to make the desired product
    • Whenever there is only one product, the atom economy will always be 100%
    • For example, in the reaction between ethene and bromine:

CH2=CH2 + Br2 → CH2BrCH2Br

  • The atom economy could also be calculated using mass, instead or Mr
    • In this case, you would divide the mass of the desired product formed by the total mass of all reactants, and then multiply by 100
  • Questions about atom economy often ask in qualitative or quantitative terms

Worked example

Qualitative atom economy
Ethanol can be produced by various reactions, such as:

Hydration of ethene:       C2H4 + H2O → C2H5OH

Substitution of bromoethane:    C2H5Br + NaOH → C2H5OH + NaBr


Explain which reaction has a higher atom economy.

   Answer

Hydration of ethene has a higher atom economy (of 100%) because all of the reactants are converted into products, whereas the substitution of bromoethane produces NaBr as a waste product

   

Worked example

Quantitative atom economy


The blast furnace uses carbon monoxide to reduce iron(III) oxide to iron.

Fe2O3 + 3CO → 2Fe + 3CO2


Calculate the atom economy for this reaction, assuming that iron is the desired product.

(ArMr data: Fe2O3 = 159.6, CO = 28.0, Fe = 55.8, CO2 = 44.0)

   Answer

   Step 1: Write the equation:


A t o m space e c o n o m y space equals space fraction numerator m o l e c u l a r space m a s s space o f space d e s i r e d space p r o d u c t over denominator s u m space o f space m o l e c u l a r space m a s s e s space o f space A L L space r e a c t a n t s end fraction space cross times space 100

   Step 2: Substitute values and evaluate:

A t o m space e c o n o m y equals fraction numerator 2 cross times 55.8 over denominator 159.6 plus left parenthesis 3 cross times 28.0 right parenthesis end fraction cross times 100 equals 45.9 percent sign

Examiner Tip

Careful: Sometimes a question may ask you to show your working when calculating atom economy.

In this case, even if it is an addition reaction and it is obvious that the atom economy is 100%, you will still need to show your working.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.