Limiting reagent
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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.
[edit] Example
Consider the combustion of benzene:
1.5 mol C6H6 x
This means that 11.25 mol O2 is required to react with 1.5 mol C6H6. Since only 7 mol O2 is present, the oxygen will be consumed before benzene. Therefore, O2 must be the limiting reagent.
This conclusion can be verified by comparing the mole ratio of O2 and C6H6 required by the balanced equation with the mole ratio actually present:
required: =
actual: =
Since the actual ratio is too small, O2 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?.
First, determine how many moles of Fe(l) can be produced from either reactant.
Moles produced of Fe from reactant Fe2O3
Moles produced of Fe from reactant Al
Because the moles Fe produced from Fe2O3(0.254mol) is less than the moles Fe produced from Al(0.297mol), Fe2O3 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.
[edit] Reference
Zumdahl, Steven S. Chemical Principals. 4th ed. New York: Houghton Mifflin Company, 2005.