Yield is the mass of product formed in a chemical reaction.

Actual yield is the mass of product formed in an experiment or industrial process.

Theoretical yield is the mass of product predicted by the balanced chemical equation for the reaction.

Percentage yield = (actual yield ÷ theoretical yield) x 100

Optimum yield is the best possible yield achieved for a set of given reaction conditions.

For a chemical reaction which goes to completion:

actual yield = theoretical yield
so, percentage yield = 100%

For a chemical reaction at equilibrium:

actual yield < theoretical yield
so, percentage yield < 100%

For a chemical reaction at equilibrium, actual yield can be affected by factors such as:
temperature
concentration
pressure and volume (gaseous systems)

Percentage Yield Calculations

Example 1: Calculating Percentage Yield

112g of nitrogen gas reacts with hydrogen gas to produce 40.8g of ammonia gas according to the equation given below:

N_{2(g)} + 3H_{2(g)} 2NH_{3(g)}

Calculate the percentage yield of ammonia.

Actual yield of ammonia (NH_{3}) = 40.8g

Theoretical yield of ammonia (NH_{3}) is calculated using the equation:

From the balanced chemical equation the mole ratio N_{2}:NH_{3} is 1:2
moles NH_{3} = 2 x moles N_{2} moles NH_{3} = 2 x (mass N_{2} ÷ molar mass N_{2}) = 2 x 112 ÷ 28 = 8 moles

theoretical yield NH_{3} = predicted mass NH_{3} predicted mass NH_{3} = moles NH_{3} x molar mass NH_{3} = 8 x 17 = 136g

Ammonia can be produced from hydrogen gas and nitrogen gas according to the equation below:

N_{2(g)} + 3H_{2(g)} 2NH_{3(g)}

Calculate the mass of ammonia produced if 168 g of nitrogen gas produces a yield of 45%.

Percentage yield = 45%

Calculate the theoretical yield of NH_{3}:
From the balanced chemical equation the mole ratio N_{2}:NH_{3} is 1:2
moles NH_{3} = 2 x moles N_{2} moles NH_{3} = 2 x (mass N_{2} ÷ molar mass N_{2}) = 2 x 168 ÷ 28 = 12 moles

theoretical yield NH_{3} = predicted mass NH_{3} predicted mass NH_{3} = moles NH_{3} x molar mass NH_{3} = 12 x 17 = 204 g

Calculate the actual yield:

percentage yield = (actual yield ÷ theoretical yield) x 100
Re-arranging this equation gives:
actual yield = theoretical yield x percentage yield ÷ 100

actual yield of NH_{3} = 204 x 45 ÷ 100 = 91.8 g

Factors Affecting Actual Yield

Le Chatelier's Principle can be used to predict the affect of changes in temperature, concentration, gas pressure and volume on actual yield.

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