Title: Introduction to the Limiting Reagent
Course: Stoichiometry

The Limiting Reagent

Note: for the purposes of this unit the words "reactant" and "reagent" will be used interchangeably. They mean the EXACT SAME THING!!

Consider the following reaction:

N_2 + 3 H_2 ==> 2 NH_3

Recall:
- The ratio of N_2 to H_2 is 1:3
- This ratio means that for every 1 N_2 mole(cule) reacted, 3 H_2 mole(cules) react
- For every 1 N_2 mole(cule) that reacts, 2 NH_3 mole(cules) are produced
- For every 3 H_2 mole(cules) that react, 2 NH_3 mole(cules) are produced

What if I carried out this reaction by adding 5 N_2 molecules and 15 H_2 molecules to a reaction flask?

Answer: All of the N_2 and H_2 molecules would react producing 10 NH_3 molecules (as shown in the picture below).

Notice that all of the N_2 and all of the H_2 is converted to NH_3 in this example. Both reactants (reagents) being consumed entirely is extremely uncommon. Usually one runs out (which causes the reaction to stop) and the other reactant is left over.

- Limiting Reagent - The reagent (reactant) which is used up entirely in a reaction

- Excess Reagent - The reagent (reactant) which is left over after a reaction has stopped.

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