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Chapter 3: Problem 36

What does it mean to say a reactant is present “in excess” in a process? Can the limiting reactant be present in excess? Does the presence of an excess of a reactant affect the mass of products expected for a reaction?

Short Answer

Expert verified
A reactant being present "in excess" means there is more of it than needed for complete reaction with other reactants, and it is not consumed completely. The limiting reactant, which cannot be present in excess, is the one determining the maximum amount of product and is entirely consumed during the reaction. The presence of an excess reactant does not affect the mass of products expected, as it is determined by the stoichiometry of the equation and the amount of the limiting reactant. The excess reactant remains unused after the limiting reactant is depleted.

Step by step solution

01

Defining "in excess"

The term "in excess" in a chemical process or reaction means that there is more of a particular reactant than needed for complete reaction with the other reactants. In other words, the excess reactant is a reactant that is not consumed completely during the reaction, and some of it will remain after the reaction has completed. The excess reactant is usually added intentionally to ensure that the other reactant(s) react completely.
02

Understanding the limiting reactant

The limiting reactant (or limiting reagent) is the reactant that is completely consumed during a chemical reaction because it determines the maximum amount of products that can be formed. This reactant is present in the least amount when compared to the stoichiometry of the equation. Once the limiting reactant has been fully consumed, the reaction stops and no more product can be formed.
03

Can the limiting reactant be present in excess?

No, by definition, the limiting reactant cannot be present in excess. The limiting reactant is the one that dictates the amount of product formed and is entirely consumed during the reaction. The term "in excess" applies to other reactants that are not the limiting reactant, as they are not completely consumed during the reaction.
04

The effect of an excess reactant on mass of products

The presence of an excess reactant does not affect the mass of products expected for a reaction because the mass of products is determined by the stoichiometry of the balanced chemical equation and the amount of the limiting reactant present. The excess reactant ensures that the limiting reactant reacts completely, but once the limiting reactant is depleted, no more products will be formed, and the excess reactant will remain unused. Therefore, the mass of products expected for a reaction will remain the same, regardless of the presence of an excess reactant.

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Anabolic steroids are performance enhancement drugs whose use has been banned from most major sporting activities. One anabolic steroid is fluoxymesterone \(\left(\mathrm{C}_{20} \mathrm{H}_{29} \mathrm{FO}_{3}\right)\) . Calculate the percent composition by mass of fluoxymesterone.

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