NOTES: STOICHIOMETRY PART 2: LIMITING REAGENTS

Limiting Reactant - The reactant in a chemical reaction that limits the amount of product that can be formed. The
reaction will stop when all of the limiting reactant is consumed.

Excess Reactant - The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant
is completely consumed. The excess reactant remains because there is nothing with which it can react.
In the example above, which is the limiting reagent?
In the example above, which is the excess reagent?
No matter how many tires there are, if there are only 8 car bodies, then only 8 cars can be made. Likewise with chemistry, if
there is only a certain amount of one reactant available for a reaction, the reaction must stop when that reactant is consumed
whether or not the other reactant has been used up.
Example Limiting Reactant Calculation:
1. A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen. Which is the limiting reactant and how much excess
reactant remains after the reaction has stopped?

First, we need to create a balanced equation for the reaction:
NH3(g) +
O2(g)
NO(g) +
H2O(g)

Next we can use stoichiometry to calculate how much on one reactant is needed when starting with the
other reactant. NOTE: It doesn’t matter which reactant is chosen, I usually start with the one
furthest left.

Once your stoich calculations are completed, write need above your answer. This is how much of the second
reactant you need in order to completely use all of the first reactant. Now ask yourself if you have as much of this
reactant as you need. If yes, then that reactant is your excess reagent and the other one is your limiting reagent. If
no, then the second reagent is your limiting reactant and the first one is the excess reagent.

NEXT, to find the amount of excess reactant, we must calculate how much of the excess reactant (ammonia)
actually did react with the limiting reactant (oxygen).

We're not finished yet though. 1.70 g is the amount of ammonia that reacted, not what is left over. To find the
amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample.
Limiting Reagent Sample Problem
When copper (II) chloride reacts with sodium nitrate, copper (II) nitrate and sodium chloride are formed.
1) Write the balanced equation for the reaction given above:
2) If 15 grams of copper (II) chloride react with 20 grams of sodium nitrate, how many grams of sodium
chloride can be formed?
Answer:
3) What is the limiting reagent for the reaction in #2?
4) What is the excess reagent for the reaction in #2?
5) How much of the excess reagent is left over in this reaction?
Answer:
Limiting Reagent Key
All of the questions on this worksheet involve the following reaction: When copper (II) chloride reacts with sodium
nitrate, copper (II) nitrate and sodium chloride are formed.
1)
Write the balanced equation for the reaction given above:
CuCl2 + 2 NaNO3  Cu(NO3)2 + 2 NaCl
2)
If 15 grams of copper (II) chloride react with 20 grams of sodium nitrate, how much sodium chloride can be
formed?
3)
What is the limiting reagent for the reaction in #2? copper (II) chloride
4)
How much of the excess reagent is left over in this reaction?
Nonlimiting reagent remaining = 20 grams – 20 grams (13.0 / 13.6)
= 0.88 grams