Reaction Yields

By the end of this lesson and the next few, you should be able to:

  • Explain the concepts of theoretical yield and limiting reactants/reagents.
  • Derive the theoretical yield for a reaction under specified conditions.
  • Calculate the percent yield for a reaction.

Introducing Reaction Yields

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. All the exercises of the preceding module involved stoichiometric amounts of reactants.

For example, when calculating the amount of product generated from a given amount of reactant, it was assumed that any other reactants required were available in stoichiometric amounts (or greater). In this lesson and the next few, more realistic situations are considered, in which reactants are not present in stoichiometric amounts.

Limiting Reactant

Consider another food analogy, making grilled cheese sandwiches (see figure below):

\(\text{1 slice of cheese} + \text{2 slices of bread} \longrightarrow \text{1 sandwich}\)

Stoichiometric amounts of sandwich ingredients for this recipe are bread and cheese slices in a 2:1 ratio. Provided with 28 slices of bread and 11 slices of cheese, one may prepare 11 sandwiches per the provided recipe, using all the provided cheese and having six slices of bread left over.

In this scenario, the number of sandwiches prepared has been limited by the number of cheese slices, and the bread slices have been provided in excess.

sandwich-analogy

Sandwich making can illustrate the concepts of limiting and excess reactants. Image credit: OpenStax, Chemistry

The balanced equation shows the hydrogen and chlorine react in a 1:1 stoichiometric ratio. If these reactants are provided in any other amounts, one of the reactants will nearly always be entirely consumed, thus limiting the amount of product that may be generated. This substance is the limiting reactant, and the other substance is the excess reactant.

Identifying the limiting and excess reactants for a given situation requires computing the molar amounts of each reactant provided and comparing them to the stoichiometric amounts represented in the balanced chemical equation.

For example, imagine combining 3 moles of H2 and 2 moles of Cl2. This represents a 3:2 (or 1.5:1) ratio of hydrogen to chlorine present for reaction, which is greater than the stoichiometric ratio of 1:1. Hydrogen, therefore, is present in excess, and chlorine is the limiting reactant. Reaction of all the provided chlorine (2 mol) will consume 2 mol of the 3 mol of hydrogen provided, leaving 1 mol of hydrogen unreacted.

An alternative approach to identifying the limiting reactant involves comparing the amount of product expected for the complete reaction of each reactant. Each reactant amount is used to separately calculate the amount of product that would be formed per the reaction’s stoichiometry. The reactant yielding the lesser amount of product is the limiting reactant. For the example in the previous paragraph, complete reaction of the hydrogen would yield

\(\text{mol HCl produced} = \mathrm{3 \; mol \; H_2} × \cfrac{\text{2 mol HCl}}{\mathrm{1 \; mol \; H_2}} = \text{6 mol HCl}\)

Complete reaction of the provided chlorine would produce

\(\text{mol HCl produced} = \mathrm{2 \; mol \; Cl_2} × \cfrac{\text{2 mol HCl}}{\mathrm{1 \; mol \; Cl_2}} = \text{4 mol HCl}\)

The chlorine will be completely consumed once 4 moles of HCl have been produced. Since enough hydrogen was provided to yield 6 moles of HCl, there will be unreacted hydrogen remaining once this reaction is complete. Chlorine, therefore, is the limiting reactant and hydrogen is the excess reactant (see figure below).

limiting-reactant

When H2 and Cl2 are combined in nonstoichiometric amounts, one of these reactants will limit the amount of HCl that can be produced. This illustration shows a reaction in which hydrogen is present in excess and chlorine is the limiting reactant. Image credit: OpenStax, Chemistry

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This is a lesson from the tutorial, Chemical Reactions and Stoichiometry and you are encouraged to log in or register, so that you can track your progress.

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