A chemical reaction’s efficiency is measured by its yield, which compares the product obtained to the maximum possible amount. Percent yield is the formal expression of this efficiency, indicating how much of the starting material was successfully converted into the desired product. While it is possible to calculate a percent yield greater than 100%, this result never indicates a highly successful reaction. Instead, a calculated yield over 100% signals a significant error occurred during the experimental process or measurement.
Defining Percent Yield and Its Components
Understanding a reaction’s outcome requires defining the components of the yield calculation. The Theoretical Yield represents the maximum mass of product that could be formed, assuming the reaction goes to completion without any loss. This value is determined mathematically using the balanced chemical equation and the stoichiometry of the limiting reactant. The Actual Yield is the mass of the purified product physically measured in the laboratory after the reaction is complete.
The percent yield is calculated by dividing the actual yield by the theoretical yield and multiplying by 100. A 100% yield signifies perfect efficiency, meaning the actual amount collected equals the maximum amount predicted by stoichiometry.
The Reality of Over 100% Yield
From a chemical perspective, a percent yield can never truly exceed 100% because this would violate the Law of Conservation of Mass. This law states that mass is neither created nor destroyed in a chemical reaction. Therefore, it is impossible to physically produce more mass of product than the mass of the starting materials allowed.
Any calculated result above 100% is a mathematical artifact, meaning the “Actual Yield” used in the calculation was artificially inflated. This inflated number suggests that something other than the pure, intended product was weighed, mistakenly adding to the measured mass. Consequently, a yield greater than 100% is a clear indication of a severe measurement or purification flaw.
Common Sources of Error Leading to High Yields
The most frequent reason for an inflated actual yield is the presence of impurities. These impurities contribute extra mass to the final weighed sample, artificially boosting the actual yield number.
Residual Solvent or Moisture
If the synthesized compound is not completely dried before being weighed, the mass of trapped water or organic solvent is included in the final measurement. This extra mass falsely increases the actual yield value, leading to a calculation over 100%.
Incomplete Separation
Incomplete separation of the desired product from the reaction mixture means the measured mass may include unreacted starting material or unwanted side products from competing reactions. Both of these are considered impurities. The lack of thorough purification steps, such as washing or recrystallization, allows these contaminants to remain.
Calculation and Weighing Errors
Human error in the laboratory can also contribute to this result. This includes misreading the analytical balance (weighing error) or a mistake in the initial stoichiometry calculation for the theoretical yield. If the theoretical yield is calculated too low, even a slightly inaccurate actual yield measurement can push the percentage over the 100% mark.
Strategies for Accurate Measurement
To prevent the calculation of an impossible yield, careful attention must be paid to the purification and drying steps of the product.
Proper Drying Techniques
Proper drying techniques are paramount to ensure all residual solvent has evaporated. This can include placing the product in a vacuum desiccator or heating it in a controlled oven. The mass should be re-measured after a period of drying to confirm the weight has stabilized, indicating dryness.
Thorough Purification
Thorough purification is important to remove any remaining impurities. Techniques such as recrystallization or careful washing with a suitable solvent can isolate the pure product from unreacted starting materials or side products. A pure sample provides a true measure of the actual yield.
Calculation Verification
Double-checking the initial calculation of the theoretical yield, specifically the molar masses and mole ratios, also eliminates a potential source of mathematical error.