4.3
Reaction Yield
The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The theor…
In a chemical reaction, the theoretical yield is the amount of product that would form with one hundred percent conversion of the limiting reactant to the product.
Consider this example: Suppose 80 kernels are heated to make popcorn. Assuming all kernels will expand, the theoretical yield is 80 pieces of popcorn. However, if only 50 kernels pop, the actual yield is only 50 pieces. The actual yield is thus the amount of product that forms.
The ratio of the actual yield to the theoretical yield multiplied by one hundred gives the percent yield, which, in this case, is 62.5%
For many chemical reactions, the theoretical yield, which is based on stoichiometry, is greater than the yield that is actually obtained. Usually, some amount of the reactants is lost to side reactions, some of the product is lost to reversible reactions, or the product is difficult to collect without some loss.
Consider the burning of magnesium. When a piece of magnesium metal is ignited, it reacts with oxygen in the air to form magnesium oxide. The chemical reaction involves 2 moles of magnesium and one mole of oxygen to form 2 moles of magnesium oxide.
Supposing 63.4 grams of magnesium and 50.7 grams of oxygen, how much magnesium oxide could be produced?
To calculate the theoretical yield, first, the mass of each reactant is converted – using the molar masses – into the corresponding amounts in moles. Then, using mole ratios, the number of moles of the reactants — magnesium and oxygen — are converted to moles of product, magnesium oxide.
Since magnesium makes the least amount of product, it is the limiting reactant, and the theoretical yield is 2.6 moles. In terms of mass, the theoretical yield is 105 grams.
Once the reaction is complete, the actual yield of the product is 80.0 grams. Thus, the percent yield of the production of magnesium oxide is 76.2%.
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Q1: What is the difference between theoretical yield and actual yield?
Theoretical yield is the amount of product that would form with 100% conversion of the limiting reactant, calculated using reaction stoichiometry. Actual yield is the amount of product that actually forms when the reaction is performed. The actual yield is typically less than theoretical yield due to side reactions, reversible reactions, or product loss during collection and purification.
Q2: How do you calculate percent yield in a chemical reaction?
Percent yield is calculated by dividing the actual yield by the theoretical yield and multiplying by 100. The formula is: (actual yield / theoretical yield) × 100. For example, if a reaction produces 80 grams of product but the theoretical yield is 105 grams, the percent yield is 76.2%.
Q3: Why is actual yield often less than theoretical yield?
Actual yield is often less than theoretical yield because reactions are naturally inefficient. Some reactants are lost to side reactions that produce undesired products. Reversible reactions may not go to completion due to equilibrium. Additionally, product loss occurs during purification techniques such as crystallization, distillation, filtration, and chromatography.
Q4: What role does the limiting reactant play in determining theoretical yield?
The limiting reactant determines the theoretical yield because it is the reactant that runs out first and restricts the amount of product formed. Using mole ratios from the balanced equation, the limiting reactant is converted to moles of product. The reactant producing the least amount of product is identified as the limiting reactant, and its conversion determines the maximum theoretical yield.
Q5: How do you determine the limiting reactant when calculating theoretical yield?
To find the limiting reactant, convert the mass of each reactant to moles using molar masses. Then use mole ratios from the balanced equation to calculate how many moles of product each reactant would produce. The reactant that produces the least amount of product is the limiting reactant and determines the theoretical yield.
Q6: Can percent yield ever exceed 100 percent?
Percent yield very rarely exceeds 100 percent. This would require the actual yield to be greater than the theoretical yield, which is theoretically impossible based on stoichiometry. If percent yield appears to exceed 100%, it typically indicates measurement errors, impurities in the product, or incomplete removal of solvent from the final product.
Q7: What units can be used to express theoretical and actual yields?
Theoretical and actual yields can be expressed as masses, molar amounts, or other appropriate properties such as volume for gaseous products. As long as both yields are expressed using the same units, those units will cancel when percent yield is calculated, resulting in a unitless percentage.