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4.5: Reaction Yields

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    188841

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    Learning Objectives

    • Calculate the yield for a reaction under specified conditions.
    • Identify the limiting reactant in a chemical equation.  

    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 module, more realistic situations are considered, in which reactants are not present in stoichiometric amounts.

    Limiting Reactant

    Consider another food analogy, making grilled cheese sandwiches (Figure \(\PageIndex{1}\)):

    \[\text{1 slice of cheese} + \text{2 slices of bread} \rightarrow \text{1 sandwich} \label{4.5.A}\]

    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.

    This figure has three rows showing the ingredients needed to make a sandwich. The first row reads, “1 sandwich = 2 slices of bread + 1 slice of cheese.” Two slices of bread and one slice of cheese are shown. The second row reads, “Provided with: 28 slices of bread + 11 slices of cheese.” There are 28 slices of bread and 11 slices of cheese shown. The third row reads, “We can make: 11 sandwiches + 6 slices of bread left over.” 11 sandwiches are shown with six extra slices of bread.
    Figure \(\PageIndex{1}\): Sandwich making can illustrate the concepts of limiting and excess reactants.

    Consider this concept now with regard to a chemical process, the reaction of hydrogen with chlorine to yield hydrogen chloride:

    \[\ce{Cl2}(g)\rightarrow \ce{2HCl}(g)\]

    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 nonreacted.

    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

    \[\mathrm{mol\: HCl\: produced=3\: mol\:H_2\times \dfrac{2\: mol\: HCl}{1\: mol\:H_2}=6\: mol\: HCl}\]

    Complete reaction of the provided chlorine would produce

    \[\mathrm{mol\: HCl\: produced=2\: mol\:Cl_2\times \dfrac{2\: mol\: HCl}{1\: mol\:Cl_2}=4\: mol\: HCl}\]

    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 non-reacted hydrogen remaining once this reaction is complete. Chlorine, therefore, is the limiting reactant and hydrogen is the excess reactant (Figure \(\PageIndex{2}\)).

    The figure shows a space-filling molecular models reacting. There is a reaction arrow pointing to the right in the middle. To the left of the reaction arrow there are three molecules each consisting of two green spheres bonded together. There are also five molecules each consisting of two smaller, white spheres bonded together. Above these molecules is the label, “Before reaction,” and below these molecules is the label, “6 H subscript 2 and 4 C l subscript 2.” To the right of the reaction arrow, there are eight molecules each consisting of one green sphere bonded to a smaller white sphere. There are also two molecules each consisting of two white spheres bonded together. Above these molecules is the label, “After reaction,” and below these molecules is the label, “8 H C l and 2 H subscript 2.”
    Figure \(\PageIndex{2}\): 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.

    Example \(\PageIndex{1}\): Identifying the Limiting Reactant

    Silicon nitride is a very hard, high-temperature-resistant ceramic used as a component of turbine blades in jet engines. It is prepared according to the following equation:

    \[\ce{3Si}(s)+\ce{2N2}(g)\rightarrow \ce{Si3N4}(s) \]

    Which is the limiting reactant when 2.00 g of Si and 1.50 g of N2 react?

    Solution

    Compute the provided molar amounts of reactants, and then compare these amounts to the balanced equation to identify the limiting reactant.

    \[\mathrm{mol\: Si=2.00\:\cancel{g\: Si}\times \dfrac{1\: mol\: Si}{28.09\:\cancel{g\: Si}}=0.0712\: mol\: Si} \]

    \[\mathrm{mol\:N_2=1.50\:\cancel{g\:N_2}\times \dfrac{1\: mol\:N_2}{28.02\:\cancel{g\:N_2}}=0.0535\: mol\:N_2} \]

    The provided Si:N2 molar ratio is:

    \[\mathrm{\dfrac{0.0712\: mol\: Si}{0.0535\: mol\:N_2}=\dfrac{1.33\: mol\: Si}{1\: mol\:N_2}} \]

    The stoichiometric Si:N2 ratio is:

    \[\mathrm{\dfrac{3\: mol\: Si}{2\: mol\:N_2}=\dfrac{1.5\: mol\: Si}{1\: mol\:N_2}} \]

    Comparing these ratios shows that Si is provided in a less-than-stoichiometric amount, and so is the limiting reactant.

    Exercise \(\PageIndex{1}\)

    Which is the limiting reactant when 5.00 g of H2 and 10.0 g of O2 react and form water?

    Answer

    O2

     

    Glossary

    actual yield
    amount of product formed in a reaction
    excess reactant
    reactant present in an amount greater than required by the reaction stoichiometry
    limiting reactant
    reactant present in an amount lower than required by the reaction stoichiometry, thus limiting the amount of product generated
    theoretical yield
    amount of product that may be produced from a given amount of reactant(s) according to the reaction stoichiometry

    Contributors

    This page titled 4.5: Reaction Yields is shared under a CC BY license and was authored, remixed, and/or curated by OpenStax.