Theoretical yield of a chemical reaction is the quantity obtained of a product of a reaction in which the limiting reactant was completely consumed. While chemists learn to balance chemical equations, in practice one reactant will be present in less than stoichiometric amounts. The reactant will limit how much product is possible from the reaction. The method to calculate theoretical yield is straightforward. Applying this calculation in a real world environment is more useful, but more complex.
In the first step of the calculation of theoretical yield, the balanced chemical equation is written and the ratio of moles of each reactant is examined. The amount of each reactant is determined by weighing reagents, measuring concentrations, or using standard solutions. The limiting reactant is found by converting the amount of reactants present into moles of each reactant and determining, based on the ratios of the first step, which reactant will run out before the other reactants are all used. The ratio of moles of product to moles of limiting reactant from the balanced equation is multiplied by the moles of limiting reactant available to find the moles of product. Then using the molecular weight of the product, this answer is converted to grams of product or other suitable measure.
In the laboratory, chemists start with a proposed reaction. The reaction products are predicted and confirmed by experiment. A balanced chemical equation is written using knowledge of the reaction. Given the starting concentrations of each reactant, the limiting reactant is chosen, and the yield is calculated based on that reactant being completely converted to product. In future experiments or sample analysis, the actual yield will be compared to the theoretical yield and causes of product loss determined.
To calculate theoretical yield, it is necessary to know the reactants and products of the reaction. This may be more complex in real industrial environments compared to laboratory conditions. The reaction, for example, might be occurring within an acidic or basic condition, and there may be corrosion of pipes that release metals that may act as catalysts. Laboratory calculations should be backed by samples drawn from the process of interest.
Typically, inorganic reactions, particularly those that produce a solid precipitate or a volatile product, may be conducted under conditions that yield complete reaction of the limiting reactant. These reactions may often yield close to 100% of the theoretical. Organic reactions often produce many more byproducts due to less pure reactant streams and the multiplicity of possible reactions. Industrial processes involving organic reactions in industry seldom yield results approaching the theoretical yield. These processes usually require subsequent separation and purification steps.