Yield and Atom Economy
How to work out the percentage yield of a reaction, why a yield is rarely the full amount, and what atom economy measures and why it matters for cost and waste.
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How much do you really get?
In theory a reaction could turn all its reactants into the product you want. In practice you almost always get less, and some of the reactant mass ends up as waste. Chemists measure this with percentage yield and atom economy. This module shows what each one means and how to work it out.
Yield words to know
Learn these before you do the calculations.
Yield against atom economy
The two measures answer different questions.
Pick the right formula
In the exam, choose the calculation the question needs. For percentage yield, divide the actual yield by the theoretical yield and multiply by 100. For atom economy, divide the mass of the wanted product by the total mass of all reactants and multiply by 100. Always show the numbers and give the answer in percent.
Match the term
- theoretical yield
- actual yield
- percentage yield
- atom economy
- the most that could be made
- the amount really made
- actual as a share of theoretical
- the share of mass that is wanted product
Match the reason for low yield
- the reaction is incomplete
- product is lost when separated
- side reactions occur
- product sticks to the apparatus
- not all reactants turn into product
- some is left behind when filtered
- some reactants make other products
- some cannot be scraped out
Why is yield below the maximum?
Why is the actual yield of a reaction usually less than the theoretical yield?
- Some product is lost or the reaction does not fully complete.
- Because you always make more than expected.
- Because the product changes colour.
- The yield is always exactly the maximum.
Reasons for low yield
Select the TWO reasons a percentage yield is below 100 percent.
- The reaction does not fully complete
- Product is lost when it is separated out
- The reaction happens too quickly
- The lab is warm
Work out the yield
A reaction has a theoretical yield of 50 g but the actual yield is only 40 g. Divide the actual by the theoretical and multiply by 100 to find the percentage yield. What is the answer?
Order the yield calculation
Put the steps of finding a percentage yield in order, earliest first.
- Work out the theoretical yield
- Measure the actual yield
- Divide actual by theoretical
- Multiply by 100 for the percentage
Complete the rules
Percentage yield compares the _____ yield with the theoretical yield, found by dividing one by the other and multiplying by _____. A yield is usually below the maximum because the reaction may be _____. Atom economy measures how much of the reactant mass becomes the _____ product.
Spot the atom-economy points
Tap the TWO statements that are true of a high atom economy.
- less reactant mass is wasted
- the process is more sustainable
- more waste is produced
- the product is always brighter
Choose the measure
Read each question and choose the right measure.
- You want to know how much product you got compared with the most possible. Which measure is this?
- You want to know how much of the reactant mass ends up as useful product and how much is waste. Which measure is this?
- A firm wants a greener process that makes the least waste. Which should be high?
Explain yield and economy
Explain what percentage yield and atom economy each measure, and why a yield is usually below the maximum.
- Say how percentage yield is calculated
- Give two reasons a yield is below the maximum
- Explain what atom economy measures and why a high value is good