Limiting Reactant


Limiting reactant (or limiting reagent) of a chemical reaction is the substance that is completely consumed when the chemical reaction is completed.

  • This means that the reaction stops as soon as the limiting reactant is completely consumed.
  • As it’s name implies, the limiting reactant limits the amount of product formed.

Examples to help you understand:

Consider a chemical reaction: $\text{C} + \text{O}_{2} \rightarrow \text{CO}_{2}$.

  • Recall that this means that 1 mole of C will react with 1 mole of $\text{O}_{2}$ to form 1 mole of $\text{CO}_{2}$.
  • IF there is 2 moles of C and 1 mole of $\text{O}_{2}$ present, 1 mole of C will react with 1 mole of $\text{O}_{2}$ to form 1 mole of $\text{CO}_{2}$. This will leave behind 1 mole of C and 1 mole of $\text{CO}_{2}$.
  • In this case, $\text{O}_{2}$ will be the limiting reactant.

Consider a slightly more complicated chemical reaction: $\text{N}_{2} + 3 \text{H}_{2} \rightarrow 2 \text{NH}_{3}$

  • This means that 1 mole of $\text{N}_{2}$ will react with 3 moles of $\text{H}_{2}$ to form 2 moles of $\text{NH}_{3}$
  • IF there is 2 moles of $\text{N}_{2}$ and 4 moles of $\text{H}_{2}$ present, there is insufficient $\text{H}_{2}$ present to completely react with $\text{N}_{2}$. Hence, $\text{H}_{2}$ is the limiting reactant
  • Since, 1 mole of $\text{H}_{2}$ will react with $\frac{1}{3}$ mole of $\text{N}_{2}$ to give $\frac{2}{3}$ moles of $\text{NH}_{3}$, 4 moles of $\text{H}_{2}$ will react with $\frac{4}{3}$ moles of $\text{N}_{2}$ to give $\frac{8}{3}$ moles of $\text{NH}_{3}$.
  • This means that $2-\frac{4}{3} = \frac{2}{3}$ moles of $\text{N}_{2}$ will remain unreacted.

Normally, limiting reactant of a chemical reaction can be found by using simple mental calculations. However, it would be good to calculate out the numbers of mole of each reactants as shown above. You can also express this (1 mole of $\text{N}_{2}$ will react with 3 moles of $\text{H}_{2}$) as a mole ratio: $\frac{1}{3}$. Finding the number of moles of $\text{N}_{2}$ reacted is just multiplying the mole ratio by number of moles of $\text{H}_{2}$ available (limiting reactant).

 


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