Chemistry homework help with Concept of Limiting Reagent

Concept of Limiting Reagent

In reactions involving more than one reactant, one has to identify first, of all the reactant, which is completely consumed (limiting reagent), one can identify the limiting reagent as follow

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If N2 is the limiting reactant moles of NH3 produced = 10. If H2 is the limiting reactant moles of NH3 produced =limiting reactant moles of NH3 produced

The reactant producing the least number of moles of the product is the limiting reactant, hence H2 is the limiting reactant.

Limiting reagent

What if a person needs to have 4 bread every day to keep his stomach full, but I provide him only with two and a half bread daily. Well his stomach won’t be full yet he would eat those two and a half bread, to keep his stomach filled with something. We always say that “something is better than nothing”, so is said by chemistry.

If in a reaction one of the reactants is consumed completely, yet another reactant is left un-reacted; such reagent is called as the “limiting reagent”. Any such substance which gets consumed completely before completion of the reaction is known to be the limiting reagent.

Consequences of limiting reagent:

  1. Excess of reactants unreacted is left out.
  2. Products obtained may react with the unreacted reactant to form a undesired product.
  3. Wastage of resources.

Branch of chemistry dealing with Limiting Reagent: limiting reagent comes in the category of topics under stoichiometry. Just by looking at the stoichiometric coefficient of a reaction one can easily identify the limiting reagent in simpler reactions. Once the limiting reagent is determined accordingly the calculations could be made. The amount of product formed could be determined and the reactant left unreacted could also be known.

How limiting reagent could be found out:

The limiting reagent must be identified in order to calculate the percentage yield of a reaction since the theoretical yield is defined as the amount of product obtained when the limiting reagent reacts completely. Given the balanced chemical equation, which describes the reaction, there are several equivalent ways to identify the limiting reagent and evaluate the excess quantities of other reagents.


Method 1: Comparing reactant concentration or moles: This method is most useful when there are only two reactants. One reactant (A) is chosen, and the balanced chemical equation is used to determine the amount of the other reactant (B) necessary to react with A. If the amount of B actually present exceeds the amount required, then B is in excess and A is the limiting reagent. If the amount of B present is less than required, then B is the limiting reagent.

Example: for the following chemical equation, determine which one is the limiting reagent?

Given: a mass of Al=3.56 gram; the mass of Cl2= 5.36gram.
Equation: 2Al(s)+3Cl2(g)→2AlCl3(s)
Solution: First we will calculate the masses of reactants to moles
So for Al number of moles would be 3.56/26.98= 0.131 moles of Al
And for Cl2 number of moles would be 5.36/70.90=0.075 moles of Cl2
then we would find out limiting reagent using stoichiometric ratio
according to the equation the stoichiometric ratio is (coefficient of Al)/(coefficient of Cl2) = 2/3
hence the stoichiometric ratio would be 0.67
and the molar ratio would be ((0.131 moles of Al))/(0.075 moles of Cl2) = 1.74
which is greater than the stoichiometric ratio, representing that the limiting reagent in the given reaction is Cl2

Method 2: Comparison of product amounts which can be formed from each reactant

In this method, the chemical equation is used to calculate the amount of one product which can be formed from each reactant in the amount present. The limiting reactant is the one which can form the smallest amount of the product considered. This method can be extended to any number of reactants more easily than the first method.

Example: For the following equation determine the limiting reagent.

Given: mass of Fe2O3 = 48.6 gram
Mass of Al = 21.96 gram
Equation: Fe2O3(s)+2Al(s)→2Fe(s)+Al2O3(l)

Solution: moles of Fe from Fe2O3= (grams of Fe2O3)/(molar mass of Fe2O3)=48.6/159.7=0.30
Hence moles of Fe = 0.30× 2moles of Fe=0.60
Moles produced of Fe from reactant Al
Mole of Al=21.96/26.98=0.813
Moles of Fe produced = 0.813×2/2
= 0.813 moles.

Hence 8.31 moles may be produced but only 0.30 moles from Fe2O3 could be reacted, hence 0.813 moles of Fe could not be produced. This implies that the limiting reagent for the above-given reaction is Fe2O3.

Please note that the value of moles of Fe (0.813 moles) that might have produced if Fe2O3 won’t have been the limiting reagent, is called as the theoretical value or the theoretical yield.

Hence after reading the above concepts, all the doubts regarding limiting reagent would have been cleared away. The concept of limiting reagent is an important objective of chemistry and should be understood quite well. The theoretical yield has an application in predicting the amount of product that might be obtained if a reaction proceeds.

Just A trick:

It can be seen from the example above that the amount of product (Fe) formed from each reagent X (Fe2O3 or Al) is proportional to the quantity

(Moles of reagent of X)/(coefficient of reagent X )

This suggests a shortcut which works for any number of reagents. Just calculate this formula for each reagent, and the reagent that has the lowest value of this formula is the limiting reagent.

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