What is a limiting reagent?
A limiting reagent is a reactant in a chemical process which will dictate how much of the products can be formed.
Here is a video which uses the analogy of making sandwiches to explain this concept. The video uses a cool simulation from PhET.
Here is a video that describes this concept in greater detail.
videos from: Noel Pauller
Consider a combustion reaction (of say methane):
There is lots of oxygen in the atmosphere; there is limited methane in your gas bottle. Clearly, here methane is the limiting reagent.
Many problems require you to identify the limiting reagent. If you have a stoichiometric equation, most of the time the limiting reagent is easily designated.
The reactant which runs out first.
The limiting reagent is the reactant which runs out first. After it runs out, the reactant cannot proceed further. It "limits" the reaction. All other reactants are in "excess".
When calculating how much product can be made, always start calculations with the limiting reagent because it determines how much product can be made.
This is an analogy which helped me:
Suppose we are making sandwiches. For each sandwich, we need 2 slices of bread, 3 tablespoons of peanut butter, and 1 tablespoon of jam.
If we have the ingredients in that exact ratio, we can make sandwiches with no leftover "reactants".
Suppose we only have 4 slices of bread. You could have all the peanut butter and jam in the world, but you can only make 2 sandwiches because there are only 4 slices. In this case, the bread would be the limiting reagent.
The limiting reagent is not always the one with the least amount (grams or liters). It is based on mole ratios from the balanced reaction equation.
Note: limiting reagent is also sometimes referred to as limiting reactant.