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:

N₂ + 3 H₂ ==> 2 NH₃

Recall:

• The ratio of N₂ to H₂ is 1:3
• This ratio means that for every 1 N₂ mole(cule) reacted, 3 H₂ mole(cules) react
• For every 1 N₂ mole(cule) that reacts, 2 NH₃ mole(cules) are produced
• For every 3 H₂ mole(cules) that react, 2 NH₃ mole(cules) are produced

What if I carried out this reaction by adding 5 N₂ molecules and 15 H₂ molecules to a reaction flask?

Answer: All of the N₂ and H₂ molecules would react producing 10 NH₃ molecules (as shown in the picture below).




Notice that all of the N₂ and all of the H₂ is converted to NH₃ 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.