Limiting reagent

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In chemistry, the limiting reagent, or also called the "limiting reactant", 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 fewer mols than the proportion requires is the limiting reagent.

[edit] Example

Consider the combustion of benzene:

1.5 mol $C 6 H 6$ 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 $C 6 H 6$ . 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 $C 6 H 6$ 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.

Consider a typical thermite reaction.

If 20.0 g of Fe₂O₃ are reacted with 8.00 g Al(s) in the thermite reaction, Which reactant is limiting?.

$Fe_2O_3(s) + 2 Al(s) \rightarrow 2 Fe(l) + Al_2O_3(s)\,$

First, determine how many moles of Fe(l) can be produced from either reactant.

Moles produced of $F e$ from reactant $F e 2 O 3$

$mol Fe_2O_3 = \frac{grams Fe_2O_3}{g/mol Fe_2O_3}\,$

$mol Fe_2O_3 = \frac{20.0 g}{157.9 g/mol} = 0.127 mol\,$

$mol Fe = (2)(0.127) = 0.254 mol Fe\,$

Moles produced of $F e$ from reactant $A l$

$mol Al = \frac{grams Al}{g/mol Al}\,$

$mol Al = \frac{8.00 g}{26.98 g/mol} = 0.297 mol\,$

$mol Fe = \frac{(2)(0.297)}{2} = 0.297 mol Fe\,$

Because the moles $F e$ produced from $F e 2 O 3$ ( $0.254 m o l$ ) is less than the moles $F e$ produced from $A l$ ( $0.297 m o l$ ), $F e 2 O 3$ is the limiting reagent.

By looking at chemical equation for the thermite reaction, we can find the limiting reagent based on the ratio of moles of one reactant to another and the total atomic mass of the reactant compounds.