Final NB 2016-2017 Turley

Theoretical and Actual Yields
Key Terms
(Excess reagent, limiting reagent)
Theoretical and actual yields
Percentage or actual yield
Skills to Develop
Use stoichiometric calculation to determine excess and limiting reagents in a chemical reaction
and explain why.
Calculate theoretical yields of products formed in reactions that involve limiting reagents.
Evaluate percentage or actual yields from known amounts of reactants
Theoretical and Actual Yields
Reactants not completely used up are called excess reagents, and the reactant that completely
reacts is called the limiting reagent. This concept has been illustrated for the reaction:
2Na+Cl2 →2NaCl
Amounts of products calculated from the complete reaction of the limiting reagent are called
theoretical yields, whereas the amount actually produced of a product is the actual yield. The ratio of
actual yield to theoretical yield expressed in percentage is called the percentage yield.
percent yield = actual yield / theoretical yield ×100
Chemical reaction equations give the ideal stoichiometric relationship among reactants and products.
Thus, the theoretical yield can be calculated from reaction stoichiometry. For many chemical
reactions, the actual yield is usually less than the theoretical yield, understandably due to loss in the
process or inefficiency of the chemical reaction.
Example 1
Methyl alcohol can be produced in a high-pressure reaction
CO(g) + 2H2(g) →CH3OH(l)
If 6.1 metric tons of methyl alcohol is obtained from 1.2 metric tons of hydrogen reacting with excess
amount of CO, estimate the theoretical and the percentage yield?
Hint:
To calculate the theoretical yield, consider the reaction
CO(g) + 2H2(g) → CH3OH(l)
28.0 + 4.0 = 32.0 (stoichiometric masses in, g, kg, or tons)
1.2 tons H2 × 32.0 CH3OH = 9.6 tons CH3OH
4.0 H2
Thus, the theoretical yield from 1.2 metric tons (1.2x10 6 g) of hydrogen gas is 9.6 tons. The actual
yield is stated in the problem, 6.1 metric tons. Thus, the percentage yield is
103