N₂ + 3 H₂ ==> 2 NH₃

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

In this case, assume that I add 5 N₂ molecules and 9 H₂ molecules to a reaction flask. I could ask the following questions:

• Which reagent (N₂ or H₂) is the limiting reagent?


• Which reagent (N₂ or H₂) is the excess reagent?


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


• How many molecules of NH₃ will form?

The common mistake is to say that N₂ is the limiting reagent because the fewest molecules of N₂ was added. But remember, 3 H₂ molecules are needed for every 1 N₂ molecule. Pictorially, the result is shown below.

	N2 molecules	H2 molecules	NH3 molecules
Before the Reaction	5	9	0
After the Reaction	2	0	6

The answers to the questions above are:

• H₂ is the limiting reagent


• N₂ is the excess reagent


• 2 molecules of N₂ are remaining


• 6 molecules of NH₃ 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!!