Limiting Reagent

In every reaction, reactants are not always present in exact stoichiometric amounts.  There  is always reactant that exceeds the required amount and is called excess reagent and the reactant that limits the reaction is called limiting reagent.   

Just like when you want to bake a cake and you buy the ingredients.  Not all ingredients you bought will be used in baking because it will depend on the required amount.  Let see for example you need to use 2 cups of sugar and 4 cups of flour and the available ingredients are 4 cups both ingredients, what is the limiting reagent and the excess reagent.  If we will try to analyze,

        given:     4 cups Sugar
                       4 cups flour

     needed:     2 cups sugar
                       4 cups flour

If we will use all 4 cups of sugar, we need 8 cups of flour and we don't have enough flour to use.  But if we will follow the amount of flour, 4 cups of flour needs only  2 cups of sugar and we still have an excess of 2 cups, therefore the excess reagent is the sugar and the limiting reagent is the flour.  Flour will be the one that will limit the reaction.

Let us apply this in an example:

The reaction between aluminum and iron (III) oxide can generate temperature approaching 3,000 C  and is used in welding metals:

In one process, 120 g of Al are reacted with 600 g of Fe2O3.

a) Calculate the mass in grams of Al2O3 formed.

b) How much of the excess reagent is left at the end of the reaction?

Solution:

In order to determine the limiting reagent, we need to calculate the amount of Fe2O3 if 200 g of Al is used, and calculate the amount of Al if 600 g of Fe2O3 is used.

 From the calculation above, Al is the limiting reagent and the excess reagent is the Fe2O3.

TRY THIS:

1. Nitric oxide (NO) reacts with oxygen gas to form nitrogen dioxide (NO2) a dark brown gas:

In one experiment  0.885 mole of NO is mixed with 0.500 mole of O2.  Calculate which of the two reactants is the limiting reagent.  Calculate also the nunber of moles of NO2 produced.

2.  Urea [NH2)2CO] is prepared by reacting ammonia with carbon dioxide:

 In one process, 637 g of NH3 are treated with 1142 g CO2.

 a.  Which of the two reactants is the limiting reagent?
 b.  Calculate the mass of (NH2)2CO formed.

 c.  How much excess reagent (in grams) is left at the end of the reaction?