http://www.ck12.org Chapter 12. Stoichiometry
following:
0 .0206 mol O 2 ×(^32 1 mol O.00 g O 22 ) = 0 .659 g O 2
1.00 grams of Mg reacts completely with 0.659 grams of O 2 to produce 1.66 grams of MgO.
Limiting Reactant
So far, we have either assumed that all reactants will be completely consumed in a chemical reaction, or we have been
told that one reactant is present in excess. In the previous example, 1 gram of magnesium reacted in the presence of
excess oxygen. However, we may sometimes be presented with initial amounts of multiple reactants without being
told which one will run out first. In such a scenario, thelimiting reactant(sometimes called a limiting reagent)
will be the reactant that is completely consumed. After the limiting reactant runs out, there may still be some of
the excess reactants left over, but the reaction can no longer proceed, because one of the ingredients is missing. To
determine which reactant is limiting in a chemical reaction, we need to look at how many moles of each are present.
These values must then be compared to the coefficients in the balanced equation, which tell us the ratios in which
various reactants combine.
Before working with chemical reactions, it may help to explain the concept of limiting reactants in a more familiar
context. For example, let’s say that you want to make as many cheese sandwiches as possible with the bread and
cheese that is available.
Example 12.8
You have 16 slices of bread and 10 slices of cheese. If each sandwich requires two slices of bread and one slice
of cheese, how many sandwiches can you make, and what ingredients will be left over? For this "reaction," which
reactant is limiting, and which one is present in excess?
Answer:
For this example, we could simply start subtracting bread and cheese as each sandwich is made. After making 8
sandwiches, we would find that we have run out of bread, but there are two slices of cheese left over. Thus, bread
is the limiting reactant, and cheese is present in excess. Notice that we actually have more slices of bread than of
cheese, but because it gets used up twice as fast, bread runs out first (it is limiting).
An alternative way to look at this problem would be to write this "reaction" out as a chemical equation.
2 Bread + Cheese→Sandwich
We cannot directly compare the amounts of bread and cheese, because they are not used in a 1:1 ratio. However, if
we divide each amount by the coefficient from the balanced equation, we get the following:
16 slices bread
2= 8
10 slices cheese
1= 10
These values can be directly compared. After dividing each amount by its coefficient from the balanced equation,
the smallest number corresponds to the ingredient that will run out first. In this case, the limiting reactant is bread,
because 8 <10. To determine how much of the excess reactant is left over, we need to determine how much will be
used up by the limiting reactant. In order to use up all of the limiting reactant (16 slices of bread), we would need:
16 slices bread×(1 slice cheese2 slices bread) =8 slices cheese
After using 8 slices of cheese, all the bread will be used up, and the "reaction" stops. Subtracting this amount from
our original 10 slices of cheese, we can see that there will be two slices left over.