Other Practical Matters in Reaction Stoichiometry

In this section we consider a few additional factors in reaction stoichiometry— both in the laboratory and in the manufacturing plant. First, the calculated outcome of a reaction may not be what is actually observed. Specifically, the amount of product may be, unavoidably, less than expected. Second, the route to producing a desired chemical may require several reactions carried out in sequence. And third, in some cases two or more reactions may occur simultaneously. 

Theoretical Yield, Actual Yield, and Percent Yield 

The theoretical yield of a reaction is the calculated quantity of product expected from given quantities of reactants. The quantity of product that is actually produced is called the actual yield. The percent yield is defined as 

In many reactions the actual yield almost exactly equals the theoretical yield, and the reactions are said to be quantitative. Such reactions can be used in quantitative chemical analyses. In other reactions the actual yield is less than the theoretical yield, and the percent yield is less than 100%. The reduced yield may occur for a variety of reasons. (1) The product of a reaction rarely appears in a pure form, and some product may be lost during the necessary purification steps, which reduces the yield. (2) In many cases the reactants may participate in reactions other than the one of central interest. These are called side reactions, and the unintended products are called by-products. To the extent that side reactions occur, the yield of the main product is reduced. (3) If a reverse reaction occurs, some of the expected product may react to re-form the reactants, and again the yield is less than expected. 

In Example 4-13, we establish the theoretical, actual, and percent yields of an important industrial process. 

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Other Practical Matters in Reaction Stoichiometry— Stoichiometric calculations sometimes involve additional factors, including the reaction s actual yield, the presence of by-products, and how the reaction or reactions proceed. For example, some reactions yield exactly the quantity of product calculated—the theoretical yield. When the actual yield equals the theoretical yield, the percent yield is 100%. In some reactions, the actual yield is less than the theoretical, in which case the percent yield is less than 100%. Lower yields may result from the formation of by-products, substances that replace some of the desired product because of reactions other than the one of interest, called side reactions. Some stoi-... 

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