DETERMINING LIMITING AND EXCESS REAGENTS
These problems are a more real-world application of the stoichiometry practice,
as typically one reactant will be consumed, leading the reaction to also stop.
Limiting Reagent
The limiting reagent in a chemical reaction is the reactant that will be
consumed completely. Once there is no more of that reactant, the reaction
cannot proceed. Therefor it limits the reaction from continuing.
Excess Reagent
The excess reagent is the reactant that could keep reacting if the other had not
been consumed.
Examples
1. Letโs start with a non-chemistry example that may make more sense.
โข Making a car is a lot more complicated than four wheels and a
steering wheel, but let's say that itโs all it takes.
o You have 400 wheels and 125 steering wheels, which will you run
out of first? How many of the other will be left over?
โข The way that it is best approached is finding out how many cars would
be made based on each โreactant.โ
400 ๐คโ๐๐๐๐ โ1 ๐๐๐
4 ๐คโ๐๐๐๐ =100 ๐๐๐๐
125 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐๐ โ1 ๐๐๐
1 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐ =125 ๐๐๐๐
โข Because there are only enough wheels to create 100 cars, but the
steering wheels can create 125 cars, the wheels are the limiting
reagent. It is important to notice that the limiting reagent can be
the reactant, which had more in the beginning. To find the amount of
the excess (steering wheels), we need to find the amount needed to
create the 100 cars.
100 ๐๐๐๐ โ 1 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐
1 ๐๐๐ =100 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐๐
โข Thus, we need 100 steering wheels to use up all the wheels.
125 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐๐ ๐ก๐ ๐ ๐ก๐๐๐ก โ 100 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐๐ ๐๐๐๐๐๐
=25 ๐ ๐ก๐๐๐๐๐๐ ๐คโ๐๐๐๐ ๐๐ ๐๐ฅ๐๐๐ ๐
