Reaction 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.

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%.

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.

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.

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.

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.

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.