Percent yield is a fundamental metric in chemistry that quantifies the efficiency of a chemical reaction. It compares the amount of product actually obtained in an experiment (actual yield) to the maximum amount that was theoretically possible (theoretical yield). While every reaction ideally aims for a 100% yield, achieving this perfect outcome is virtually impossible due to various physical and chemical limitations.
Calculating Percent Yield
Percent yield is calculated by taking the ratio of the actual yield to the theoretical yield and multiplying the result by 100. The formula is expressed as: Percent Yield = (Actual Yield / Theoretical Yield) x 100.
The actual yield is the mass of the product measured and collected after the experiment is complete, typically determined by weighing the dried product. The theoretical yield, by contrast, is a calculated value representing the maximum possible amount of product that could be formed. This calculation is based on the balanced chemical equation and assumes the reaction proceeds perfectly with complete conversion of the limiting reactant.
Common Reasons Yield is Not 100%
The difference between the theoretical maximum and the actual quantity obtained stems from limitations inherent to the chemistry and the experimental procedure. One significant factor is the presence of incomplete or reversible reactions. Many chemical processes establish a state of equilibrium, meaning that reactants and products coexist, preventing all starting material from converting into the desired product.
Side reactions are another common issue, where reactants are consumed to form unintended byproducts instead of the target compound. These competing reactions divert the starting materials, effectively lowering the amount available to form the desired product.
Physical losses during the experimental process also contribute significantly to a reduced yield. Product can be lost when transferring material between different pieces of glassware or remain dissolved in the solvent during purification steps like filtration or recrystallization. Even minute amounts of product left clinging to surfaces or spilled can result in a measurable drop in the overall yield.
Context Matters: What Makes a Yield “Good”
What constitutes a “good” percent yield is highly dependent on the context of the chemical work being performed. In general academic or teaching laboratories, a yield in the 70% to 90% range is often viewed as successful. Lower yields are common in this setting, as the primary goal is often to learn the procedure and master the techniques, rather than achieving maximum efficiency.
In novel research, particularly in the synthesis of complex organic molecules, a much lower yield may be considered acceptable. If a compound is structurally challenging or has never been made before, a yield as low as 20% to 40% might be celebrated. The priority often shifts to confirming the compound’s structure and ensuring its high purity, even at the expense of a lower yield.
For industrial or commercial production, the standard for a good yield is significantly higher, typically requiring 90% or more. Manufacturers must maximize output and minimize waste to ensure cost-effectiveness. Even a small increase in yield can translate to substantial savings when scaling a process, especially when starting materials are expensive, such as in the pharmaceutical industry.
High purity is also a major consideration. A yield appearing above 100% usually indicates that the final product is contaminated with unreacted materials or solvent. Therefore, a 95% yield of a highly pure substance is preferable to a 99% yield of a contaminated product, demonstrating that “good” involves a balance between quantity and quality.