12.3. Limiting Reactant and Percent Yield http://www.ck12.org
What happened in this reaction? The chemist started with 3 moles of N 2. You may think of this as being enough to
make 3 batches of the “recipe” (as shown in the balanced equation), since the coefficient for N 2 is 1. However, the 6
moles of H 2 that the chemist started with is only enough for 2 batches, since the coefficient for H 2 is 3, and 3×2 =
- After this reaction is complete, all of the hydrogen gas will be gone, but there will be 1 mole of nitrogen gas left
over. At most, 4 moles of NH 3 (two "batches") can be generated with these amounts of reactants, because, after that
point, one of the "ingredients" has been completely used up. The overall reaction that occurred is the following:
2 mol N 2 + 6 mol H 2 →4 mol NH 3All the amounts are doubled from the original balanced equation.
Thelimiting reactant(orlimiting reagent)is the reactant that determines the amount of product that can be
formed in a chemical reaction. The reaction proceeds until the limiting reactant is completely used up. In our
example above, H 2 is the limiting reactant. Theexcess reactant(orexcess reagent)is any reactant that cannot be
completely consumed by the full reaction of the limiting reactant. In other words, there is always excess reactant left
over after the reaction is complete. In the above example, N 2 is the excess reactant.
Get help with limiting reactant problems at http://www.chem.tamu.edu/class/majors/tutorialnotefiles/limiting.htm.
Watch an animation of a reaction involving a limiting reactant at http://www.dlt.ncssm.edu/core/Chapter6-Stoichiom
etry/Chapter6-Animations/LimitingReactant.html.
Solving Limiting Reactant Problems
In the real world, amounts of reactants and products are typically measured by mass or by volume. It is first necessary
to convert the given quantities of each reactant to moles in order to identify the limiting reactant.
Sample Problem 12.9: Determining the Limiting Reactant
Silver metal reacts with sulfur to form silver sulfide according to the following balanced equation:
2Ag(s) + S(s)→Ag 2 S(s)What is the limiting reactant when 50.0 g Ag is reacted with 10.0 g S?
Step 1: List the known quantities and plan the problem.
Known