Title: Limiting Reagent Introduction Page 2
Course: Stoichiometry

N_2 + 3 H_2 ==> 2 NH_3

But what if I do not start with the exact amount of needed reactants?

In this case, assume that I add 5 N_2 molecules and 9 H_2 molecules to a reaction flask. I could ask the following questions:

- Which reagent (N_2 or H_2) is the limiting reagent?

- Which reagent (N_2 or H_2) is the excess reagent?

- How many molecules of the excess reagent will be left?

- How many molecules of NH_3 will form?

The common mistake is to say that N_2 is the limiting reagent because the fewest molecules of N_2 was added. But remember, 3 H_2 molecules are needed for every 1 N_2 molecule. Pictorially, the result is shown below.

 | N_2 molecules | H_2 molecules | NH_3 molecules
Before the Reaction | 5 | 9 | 0
After the Reaction | 2 | 0 | 6

The answers to the questions above are:

- H_2 is the limiting reagent

- N_2 is the excess reagent

- 2 molecules of N_2 are remaining

- 6 molecules of NH_3 form

Limiting and excess reagent problems are among the most challenging in beginning chemistry. Make sure to review problems several times, ask questions, work hard, do not skip steps, and do not get frustrated!!

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