Limiting reagent

In chemistry, the limiting reagent is the chemical that determines how far the reaction will go before the chemical in question gets used up, causing the reaction to stop. It is determined by working out the balanced equation for the chemical reaction, comparing how many units (mols) of each go into the reaction (in a proportion), and then measuring how many mols of each chemical will be used in that reaction. The chemical of which there are less mols than the proportion requires is the limiting reagent. Note: This process requires Stoichiometry.

Example

Consider the combustion of benzene:

$2C_6H_6 + 15O_2 \rightarrow \ 12CO_2 + 6H_20$

If 1.5 moles of benzene are reacted with 7 moles of oxygen, the limiting reagent can be determined by performing the following calculations:

Since 15 mol $O_2$ reacts with 2 mol $C_6H_6$ (see balanced equation) the number of moles of $O_2$ that will react with 1.5 mol $C_6H_6$ is:

1.5 mol $C_6H_6$ x $\frac{15 mol O_2}{2 mol C_6H_6}=11.25 mol O_2$

This means that 11.25 mol $O_2$ is required to react with 1.5 mol $C_6H_6$ . Since only 7 mol $O_2$ is present, the oxygen will be consumed before benzene. Therefore, $O_2$ must be the limiting reagent.

This conclusion can be verified by comparing the mole ratio of $O_2$ and $C_6H_6$ required by the balanced equation with the mole ratio actually present:

required: $\frac{mol O_2}{mol C_6H_6}$ = $\frac{15 mol O_2}{2 mol C_6H_6}=7.5 mol O_2$

actual: $\frac{mol O_2}{mol C_6H_6}$ = $\frac{7 mol O_2}{1.5 mol C_6H_6}=4.7 mol O_2$

Since the actual ratio is too small, $O_2$ is the limiting reagent.

Reference

Zumdahl, Steven S. Chemical Principals. 4th ed. New York: Houghton Mifflin Company, 2005.

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Reference