Limiting Reactants

Limiting Reactant: The reactant in a chemical equation that limits (determines) the amount of product that can be formed.

1. Convert the amount of each reactant to moles.

2. Pick one of the reactants.  Use the coefficients to figure out how much of the OTHER reactant you would need for the reaction.

3. The limiting reactant is the one that is possible, because it requires LESS than you actually have of the other reactant.

For example in the following reaction, any of the following ratios might be observed:

            16 Cu    +   S₈ à    8 Cu₂S

            16 mol       1 mol         8 mol

            32 mol      2 mol        16 mol

            0.8 mol     0.05 mol   0.4 mol

But what if we had 16 mol of copper and 3 mol of sulfur?

            16 Cu    +   S₈ à    8 Cu₂S

            16 mol  +            à

                        +   3 mol à

There are three possibilities:

1. We use up all of the Cu and we have some S₈ left over.

2.We use up all of the S₈ and we have some Cu left over.

3.We have a perfect stoichiometric ratio, and we use up all of both reactants.

Let’s look at possibility #1 first.

We have 16 mol of Cu, which will use up 1 mol of S₈:

            16 Cu    +   S₈ à    8 Cu₂S

            16 mol  +   1 mol à    8 mol

Because we started with 3 mol of S₈ and used 1 mol, we still have 2 mol of S₈ left over when we use up the Cu.  This means we used all of the Cu (i.e., Cu was limiting), and we had some S₈ left over (i.e., S₈ was non-limiting).

What if we looked at possibility #2 first?

We have 3 mol of S₈, which will use up 48 mol of Cu:

            16 Cu        +       S₈ à      8 Cu₂S

             48 mol     +       3 mol  à   24 mol

We can’t do this, because the problem told us that we only had 16 mol of Cu.  This means we don’t have enough Cu to use up all of the S₈.  This means Cu is limiting and S₈ is NOT.
Limiting Reactant Cheeseburger Analogy:

Suppose the “perfect” cheeseburger recipe is:

· 2 slices bread

· ¼ lb. ground beef

· 1 leaf lettuce

· 2 slices cheese

· 2  pickles

· 1 Tbsp. ketchup

In your kitchen, you have 15 slices of bread, 2½ lbs. of ground beef, 12 leaves of lettuce, 9 slices of cheese, 13 pickles, and 32 oz. (64 Tbsp.) of ketchup.

1. How many cheeseburgers can you make before you run out of something?

2. How much of each ingredient will you have left over?