Limiting Reactants

A chemical reaction stops when one of the reactants is used up completely. When this happens, we call that reactant the limited reactant. The reactant that still remains when the reaction stops is called the excess reactant.

The amount of product that forms during a reaction depends on the amount of the limiting reactant. This is because the reaction can’t continue without the limited reactant. Also, if you only use half of the limiting reactant, then you will only get half of the product. This applies as long as there are still enough of the other reactants left over (these reactants are in excess).

How to Identify the Limiting Reactant in a System

Some questions will specify which reactant is in excess. However, sometimes it is implied and you will have to work out which reactant is limited.

To figure out which reactant is the “limiting reactant” in a chemical reaction, you can follow these steps:

1. Calculate the number of moles of each reactant using the equation moles=\dfrac{mass}{m_{r}}

2. Write out the chemical reaction and balance it

3. Compare the number of moles of each reactant

Example

For example, if 18.4 g of sodium reacts with 8.0 g of oxygen to produce sodium oxide, we can figure out which reactant is the limiting reactant and which one is in excess by following these steps:

1. Calculate the number of moles of each reactant using the equation moles=\dfrac{mass}{m_{r}}

Moles of Na = \dfrac{18.4}{23}=0.8

Moles of O2 = \dfrac{8.0}{32}=0.25

2. Write out the chemical reaction and balance it to determine the molar ratio.

4Na + O2 → 2Na2O

  • The molar ratio is 4:1 (Na to O2)

3. Compare the number of moles

To react completely, 0.8 moles of Na requires 0.2 moles of O2 (0.8 ÷ 4). However, there are 0.25 moles of O2, which is more than enough to react with all of the Na. Therefore, O2 is in excess and Na is the limiting reactant.

This means that once all the Na has reacted, the reaction will stop even though there is still some O2 left over.

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