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Yield

Key Concepts

  • 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:

N2(g) + 3H2(g) 2NH3(g)

Calculate the percentage yield of ammonia.

  1. Actual yield of ammonia (NH3) = 40.8g

  2. Theoretical yield of ammonia (NH3) is calculated using the equation:
    moles = mass ÷ molar mass

    From the balanced chemical equation the mole ratio N2:NH3 is 1:2
    moles NH3 = 2 x moles N2
    moles NH3 = 2 x (mass N2 ÷ molar mass N2) = 2 x 112 ÷ 28 = 8 moles

    theoretical yield NH3 = predicted mass NH3
    predicted mass NH3 = moles NH3 x molar mass NH3 = 8 x 17 = 136g

  3. Percentage yield = (actual yield ÷ theoretical yield) x 100
    percentage yield NH3 = (40.8 ÷ 136) x 100 = 30%

Example 2: Calculating Mass of Product from Yield

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

N2(g) + 3H2(g) 2NH3(g)

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

  1. Percentage yield = 45%

  2. Calculate the theoretical yield of NH3:
    From the balanced chemical equation the mole ratio N2:NH3 is 1:2
    moles NH3 = 2 x moles N2
    moles NH3 = 2 x (mass N2 ÷ molar mass N2) = 2 x 168 ÷ 28 = 12 moles

    theoretical yield NH3 = predicted mass NH3
    predicted mass NH3 = moles NH3 x molar mass NH3 = 12 x 17 = 204 g

  3. 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 NH3 = 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.

Factor Conditions Actual Yield % Yield
reactant concentration increase   increases increases

reactant concentration decrease   decreases decreases

temperature increase exothermic reaction decreases decreases
endothermic reaction increases increases

temperature decrease exothermic reaction increases increases
endothermic reaction decreases decreases

gas pressure increase mol reactant(gas) > mol product(gas) increases increases
mol reactant(gas) < mol product(gas) decreases decreases

gas pressure decrease mol reactant(gas) > mol product(gas) decreases decreases
mol reactant(gas) < mol product(gas) increases increases


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