Chemical yield
This is based on the Wikipedia article, accessed October 27, 2006.
Yield in chemistry, also known as chemical yield and reaction yield, is the amount of product obtained in a chemical reaction. The absolute yield can be given as the weight in gram or in mol (molar yield). The yield is usually also given as a relative yield, which is the actual yield divided by the theoretical yield (the ideal or mathematically calculated yield). The relative yield is usually given as a percentage value, the percent yield:
- <math>\frac{actual\ yield}{theoretical\ yield} \times 100\ = percent\ yield</math>
The theoretical yield value always relates to one of the reactants. This is usually the limiting one, taking into account the molar relation of the reactants and the stoichiometry of the reaction.
The ideal or theoretical yield of a chemical reaction would be 100%, a value that is rarely reached. Yields above about 90% are called very good, yields above about 75% are called good, yields below about 60% are called modest, yields below about 30% are called poor. Yields can also be above 100% when an extraneous chemical from outside of the reaction has found its way into the yield.
Example
This is an example of an esterification reaction where one molecule acetic acid reacts with one molecule ethanol, yielding one molecule ethyl acetate (a bimolecular second-order reaction of the type A + B → C):
- 120 g acetic acid (60 g/mol, 2.0 mol) was reacted with 230 g ethanol (46 g/mol, 5.0 mol), yielding 132 g ethyl acetate (88 g/mol, 1.5 mol). The yield was 75%.
- The molar amount of the reactants is calculated from their weight (acetic acid: 120 g ÷ 60 g/mol = 2.0 mol; ethanol: 230 g ÷ 46 g/mol = 5.0 mol).
- Ethanol is used in a 2.5-fold excess (5.0 mol ÷ 2.0 mol).
- The theoretical molar yield is 2.0 mol (the molar amount of the limiting compound, acetic acid).
- The molar yield of the product is calculated from its weight (132 g ÷ 88 g/mol = 1.5 mol).
- The yield is calculated from the actual molar yield and the theoretical molar yield (1.5 mol ÷ 2.0 mol × 100% = 75%).
