What is a Limiting Reactant?

Published on Author Donna Blakeney

A limiting reactant is the reactant that limits the amount of product that can be produced. The reaction will stop when all the limiting reactant is consumed.

In contrast to this there is an excess reactant, which is the chemical that remains when the reaction stops because the limiting reactant is completely used.

 

Let us consider tricycles:

We have 4 tricycle frames and 56 tires. The most tricycles we can produce is 4. The frames are limiting us from producing any additional tricycles. It does not matter how many more tires we have those are in excess.

 

 

How do I Calculate using the limiting reactant?

The same procedures would be followed as previously discussed. You would just need to complete all calculates on the limiting reactant since that is the chemical that will limit the products from being formed.

Here is an example of a limiting reactant calculation:

  • 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:

4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)
 

  • Next, we can use stoichiometry to calculate how much product is produced by each reactant. NOTE: It does not matter which product is chosen, but the same product must be used for both reactants so that the amounts can be compared.


 

  • The reactant that produces the lesser amount of product in this case is the oxygen, which is the limiting reactant. This will also provide the answer of how much was produced.
  • Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant or in this case ammonia actually did react with the limiting reactant (oxygen).


 

  • We are not finished yet; 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.

 

These same steps should be followed with all limiting reactant problems. Please note, these are also the same steps we followed during the conversions and stoichiometry problems we already completed.