other words, at times some of one reactant will be left over after the other has
been used up. This is similar to the situation in which two eggs are required to
mix with one cup of flour in a particular recipe, and you have four eggs and four
cups of flour.
Since two eggs require only one cup of flour, four eggs can use only two
cups of flour and two cups of flour will be left over.
A chemical equation is very much like a recipe.
TIP
Remember this recipe analogy to solve limiting- reactant questions!Example
Consider the following reaction:
CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(g)If you are given 15.0 grams of methane (CH 4 ) and 15.0 grams of oxygen(O 2 ), how many grams of carbon dioxide gas can be produced? Which reactant
will be left over? How much of this reactant will not be used?
Once again, dimensional analysis will be used to solve this problem.
However, two equations will be required as it will be necessary to determine
how much carbon dioxide can be produced from each reactant:
Since the oxygen can produce only 10.3 g CO 2 (the lesser of the quantities ofCO 2 shown above), it is referred to as the limiting reactant. There simply is not
enough of oxygen available to make what the methane has the potential to
produce. That smaller amount of CO 2 is referred to as the theoretical yield. A
portion of the CH 4 (the reactant in excess) will be used, however, to produce the
10.3 g of CO 2. To determine that amount, another dimensional analysis is needed.
Since only 3.75 g of CH 4 is needed, the amount of CH 4 in excess can be