Chemical reactions transform reactants into products, and the term “yield” refers to the amount of product obtained. Scientists calculate an expectation of product based on the initial amounts of reactants used. This maximum, perfect amount, which assumes the reaction proceeds perfectly, is called the theoretical yield.
Defining Theoretical Yield and Its Calculation
The theoretical yield represents an ideal scenario where a chemical reaction proceeds to completion with perfect efficiency and zero loss. This calculated value is the maximum quantity of product that can be created from the initial reactants. To determine this amount, a chemist must first have a correctly balanced chemical equation that establishes the precise ratio of reactants to products.
Calculating the theoretical yield relies on stoichiometry, the quantitative relationship between substances in a reaction. The process involves converting the mass of the starting material into moles, using the mole ratio from the balanced equation to predict product moles, and finally converting this amount back into a measurable quantity, usually grams. This calculation assumes every reactant molecule is successfully converted into the desired product.
An important concept in this calculation is the “limiting reactant,” also known as the limiting reagent. In most reactions, reactants are not present in the exact stoichiometric ratio required by the balanced equation. One reactant will be entirely consumed before the others, effectively stopping the reaction and limiting the total amount of product that can be formed.
The theoretical yield is exclusively determined by the amount of this limiting reactant. For example, if a recipe requires two slices of bread for every slice of cheese, and you have 10 slices of bread and 10 slices of cheese, the bread is the limiting reactant. You can only make five sandwiches before running out of bread, capping the maximum possible product yield.
Actual Yield and Percent Yield
While the theoretical yield is a calculated ideal, the actual yield is a value determined by real-world measurement. The actual yield is the physically measured amount of product that is isolated at the end of an experiment. This quantity is always found through physical measurements, such as weighing the purified product on a balance, not by mathematical calculation.
The actual yield is almost always smaller than the theoretical yield due to inevitable losses and imperfections. Chemists use the percent yield metric to evaluate reaction efficiency by comparing these two values. Percent yield communicates how much product was recovered compared to the maximum amount possible.
The percent yield is calculated using a simple ratio: the actual yield is divided by the theoretical yield, and the result is multiplied by 100%. This calculation provides a percentage that reflects the success of the experiment. A percent yield close to 100% indicates that the reaction was highly efficient, with minimal product loss.
A percentage significantly below 100% signals that a large portion of the potential product was not recovered. Although rarely seen, a percent yield slightly greater than 100% is possible, but this typically indicates the recovered product is impure and still contains residual solvent or unreacted starting material, which artificially inflates the measured mass.
Factors That Reduce Experimental Yield
The difference between the theoretical maximum and the measured actual result stems from practical laboratory challenges. One common factor is incomplete reactions, where not all of the limiting reactant is converted into the desired product. This often happens because many reactions are reversible and reach chemical equilibrium before all reactants are consumed.
Another factor that lowers the actual yield is the presence of side reactions. These are parallel, unintended reactions that occur simultaneously and consume the starting material to form unwanted byproducts instead of the primary product. These competing reactions effectively steal reactants away from the desired chemical pathway.
Physical losses during the purification and handling stages also account for a significant reduction in yield. Every time a substance is transferred between containers, filtered, or purified, a small amount of material is left behind as residue on the glassware or filter paper. These numerous small transfer losses cumulatively reduce the final recovered quantity. Furthermore, if the initial reactants were not completely pure, the starting mass used in the theoretical yield calculation was inflated, leading to a lower actual yield than anticipated.