In chemistry, a perfect outcome where all starting materials transform flawlessly into desired products is rarely achieved. Chemical reactions are complex processes influenced by various factors that prevent a complete conversion of reactants to products. Understanding these influences helps explain why the actual amount of product obtained often falls short of the calculated maximum.
Understanding Theoretical and Actual Yield
To quantify a chemical reaction’s efficiency, chemists use percent yield, which relies on theoretical and actual yield. Theoretical yield represents the maximum product formed from reactants under ideal conditions, determined by stoichiometric calculations. Actual yield is the product genuinely isolated and measured in a laboratory, and is almost always less than theoretical. Percent yield is calculated by dividing actual yield by theoretical yield and multiplying by 100%.
Chemical Factors Affecting Yield
One reason for less than 100% yield is that reactions may not go to completion. This occurs if reactant molecules do not collide effectively, possess sufficient energy to react, or are inaccessible due to inadequate mixing or large, solid clumps.
Many chemical reactions are reversible. These reactions eventually reach chemical equilibrium, where forward and reverse reaction rates become equal. At equilibrium, a mixture of both reactants and products exists, preventing 100% conversion to product.
Another factor is side reactions, where reactants participate in unintended chemical pathways. These alternative reactions consume starting materials to form undesired byproducts instead of the target compound. Such side reactions are common when reactants or products are highly reactive and can interact with each other or with elements like air or moisture.
The desired product might also be unstable under reaction conditions and decompose. This decomposition can break down the product into other substances, reducing the final amount isolated. For example, some compounds might degrade when exposed to heat or light during the reaction or purification steps.
Practical Factors Affecting Yield
Beyond inherent chemistry, the experimental process contributes to lower yields. Physical loss of product occurs during transfer and purification. Some product inevitably remains adhered to glassware, filter paper, or other equipment surfaces during transfers between containers.
Purification steps, such as filtration, washing, or recrystallization, are necessary to obtain a pure product but can also lead to material loss. During filtration, for instance, some product might remain in the liquid or pass through the filter. Similarly, during recrystallization, a portion of the product might remain dissolved in the solvent instead of crystallizing out.
The purity of starting materials can also impact actual yield. If reactants are not 100% pure, the actual amount of reactive substance is less than assumed, meaning less product can be formed than calculated. Using impure reactants can effectively lower the theoretical maximum amount of product achievable.
Errors in measurement or experimental technique are practical factors. Inaccuracies in weighing reactants or products, or inconsistencies in maintaining optimal reaction conditions like temperature and pressure, can contribute to a reduced actual yield.
If reactants or products are volatile, some material might evaporate during the reaction or subsequent work-up procedures. This physical loss directly reduces the recovered amount of product.