Why the yield of a reaction is never the full theoretical amount, calculating percentage yield and atom economy, and why industry cares about both. Chemistry only content.
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1.What is the theoretical yield?
The maximum mass of product that could be made, calculated from the balanced equation.
2.What is the actual yield?
The mass of product actually obtained from the reaction.
3.State the equation for percentage yield.
percentage yield = (mass of product actually made divided by maximum theoretical mass) x 100
4.Give three reasons the actual yield is less than the theoretical yield.
The reaction may not go to completion, some product is lost when it is separated or purified, and there may be unexpected side reactions producing other products.
5.A reaction has a theoretical yield of 20 g and produces 15 g. Calculate the percentage yield.
15 divided by 20 = 0.75. 0.75 x 100 = 75 per cent.
6.A reaction produces 4.2 g out of a possible 6 g. Calculate the percentage yield.
4.2 divided by 6 = 0.7, so 70 per cent.
7.Can percentage yield ever be more than 100 per cent?
No. If it appears to be, the product is impure or still wet, so its measured mass is too high.
8.Why does a reversible reaction reduce yield?
The products turn back into reactants, so the reaction never fully converts everything.
9.What is atom economy?
A measure of the proportion of the mass of the reactants that ends up as the useful product.
10.State the equation for atom economy.
atom economy = (relative formula mass of the desired product divided by the sum of the relative formula masses of all reactants) x 100
11.Why is atom economy important to industry?
A low atom economy means most of the raw material becomes waste, which wastes money and creates disposal and environmental problems.
12.What atom economy does a reaction with only one product have?
100 per cent, because every atom from the reactants ends up in the desired product.
13.Calculate the atom economy if the desired product has Mr 44 and total reactant Mr is 100.
44 divided by 100 = 0.44, so 44 per cent.
14.What is the difference between percentage yield and atom economy?
Percentage yield measures how much of the possible product you actually obtained. Atom economy measures how much of the reactant mass could ever become the useful product.
15.Why might a company choose a reaction with lower atom economy?
The other products may be sellable, the reaction may be faster or cheaper, or the raw materials may be more readily available.
16.Give three factors a company considers when choosing a reaction pathway.
The atom economy, the percentage yield, the rate of reaction, whether the reaction is reversible, and whether the by-products are useful or need disposal.
17.Why is a high atom economy better for the environment?
Less material becomes waste, so fewer raw materials are extracted and less has to be disposed of.
18.If the by-products can be sold, how does that change things?
The waste becomes a second useful product, which improves the economics even though the atom economy for the main product is unchanged.
19.Which measure would tell you that product was lost during filtering?
Percentage yield, because the theoretical maximum is unchanged but the mass obtained is lower.
20.Which measure is fixed by the equation alone?
Atom economy. It depends only on the formulae in the balanced equation, not on how the reaction is carried out.
21.Why might a percentage yield be low even with a perfect method?
The reaction may be reversible and reach equilibrium before all the reactants are converted.
22.How could a company improve its percentage yield?
By reducing losses during transfer and purification, and by adjusting conditions to push a reversible reaction further towards the products.
23.A reaction makes 9 g of product from a theoretical 12 g. What is the percentage yield?
9 divided by 12 = 0.75, so 75 per cent.
24.Why is percentage yield never used on its own to judge a process?
A reaction can have a high yield but a poor atom economy, so most of the starting material still ends up as waste.
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