Yield (chemistry)
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Yield in chemistry, also known as chemical yield and reaction yield, is the amount of product obtained in a chemical reaction [1]. 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:
The theoretical yield value always relates to 1 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 excellent, yields above 80% very good, yields above about 70% are called good, yields below about 50% are called fair, yields below about 40% 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.
The theoretical yield for a specific reaction depends on the quantity of starting materials, particularly the limiting reagent. The theoretical yield is typically calculated assuming that there is only one reaction involved, that all of the reactant is converted into product, and that all the product is isolated in pure form.
[edit] 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 the weights (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%).
[edit] References
- ^ Vogel's Textbook of Practical Organic Chemistry (5th Edition) (Hardcover) by A.I. Vogel (Author), A.R. Tatchell (Author), B.S. Furnis (Author), A.J. Hannaford (Author), P.W.G. Smith ISBN 0582462363
