The reactant which reacts completely in the reaction is called a limiting reactant or limiting reagent.
Excess Reactant
The reactant which is not consumed completely in the reaction is called excess reactant.
In a chemical reaction, the limiting reactant is the reactant that determines the quantity of the products that are produced.
The other reactants present in the reactions are sometimes found to be in excess since there is some leftover quantity of them.
The maximum amount of product that is produced is known as the theoretical yield. The limiting reagent should be identified to calculate the percentage yield of a reaction. Given the balanced chemical equation, that describes the reaction, there are many equivalent ways to identify the limiting reagent and calculate the excess quantities of others.
Consider some examples to understand the concept of limiting reactants.
The concept of limiting reactant is analogous to the relationship between the number of “kababs ” and the “slice” to prepare “sandwiches”. If we have 30 kababs and five pieces of bread having 58 slices, then we can prepare only 29 “sandwiches”. One kabab will be extra (excess reactant) and slices will be the limiting reactant. It is a practical problem that we cannot purchase exactly sixty “slices” for 30 “kababs” to prepare 30 “sandwiches”.
Reaction for the formation of ammonia:
3H2 + N2 –> 2NH3
In the reaction shown above, 3 moles of hydrogen gas are required for the reaction with 1 mole of nitrogen gas for the formation of 2 moles of ammonia. But what if, during the time of the reaction, there are only 2 moles of hydrogen gas available with 1 mole of nitrogen?
In this case, the entire quantity of nitrogen cannot be used since the entirety of nitrogen requires 3 moles of hydrogen gas to react. Therefore, the hydrogen gas is limiting the reaction and is hence called the limiting reagent for this reaction.
How to determine limiting reactant?
To identify a limiting reactant, the following three steps are performed.
Calculate the number of moles from the given amount of reactant.
Find out the number of moles of product with the help of a balanced chemical equation.
Identify the reactant which produces the least amount of product as a limiting reactant.
Solved Examples
Solved problem 1
NH3 gas can be prepared by heating together two solids NH4Cl and Ca(OH)2 . If a mixture containing 100 g of each solid is heated then
Calculate the number of grams of NH3 produced.
Calculate the excess amount of reagent left unreacted.
2NH4 Cl (s) + Ca (OH)2 (s) CaCl2 (s) + 2NH3 (g) + 2H2O(l)
Solution:
Convert given amounts of both reactants into their number of moles.
Mass of NH4Cl = 100 g
Molar mass of NH4Cl = 53.5 g/mol
Moles of NH4Cl = 100/53.5 = 1.87
Mass of Ca (OH) 2 = 100 g
Molar mass of Ca (OH) 2 = 74 g/mol
Moles of Ca (OH) 2 = 100/74 = 1.35
Compare the number of moles of NH4Cl with those of NH3
NH4Cl : NH3
2 : 2
1 : 1
1.87: 1.87
Similarly, compare the number of moles of Ca(OH)2 with those of NH3
Ca (OH) 2 : NH3
1 : 2
1.35 : 2.70
Since the number of moles of NH3 produced by 100g or 1.87 moles of NH4Cl is less, so NH4Cl is the limiting reactant. The other reactant, Ca (OH) 2 is present in excess.
Hence
Mass of NH3 produced = 1.87 moles x 17 g/mol = 31.79 g
Amount of the reagent present in excess
Let us calculate the number of moles of Ca (OH) 2 which will completely react with 1.87 moles of NH4Cl with the help of the equation. For this purpose, compare the number of moles of NH4Cl and Ca (OH) 2.
NH4Cl : Ca (OH) 2
2 : 1
1 : ½
1.87 : 0.935
Hence the number of mole of Ca (OH) 2 which completely react with 1.87 mol of NH4Cl is 0.935 mol.
No of moles of Ca (OH) 2 taken = 1.35
No of moles of Ca (OH) 2 used = 0.935
No of moles of Ca (OH) 2 left behind = 1.35 – 0.935 = 0.415
Mass of Ca (OH) 2 left unreacted = 0.415 x 74 = 30.71
It means that we should have mixed 100 g of NH4Cl with 69.3 g (100 – 30.71) of Ca (OH) 2 to get 1.87 moles of NH3.
How limiting reactants control the amount of product?
Limiting reactants control the amount of product because limiting reactants consumes earlier and stop the reaction.
How do you find the limiting reactant?
One way of finding the limiting reagent is by calculating the amount of product that can be formed by each reactant; the one that produces fewer products is the limiting reagent.
What is a limiting reagent?
Answer: The limiting reagent in a chemical reaction is a reactant that is totally consumed when the chemical reaction is completed. The amount of product formed is limited by this reagent, since the reaction cannot continue without it.
What are limiting and excess reactants?
The reactant that is fully consumed or entirely reacted is known as Limiting Reactant, whereas the reactant that is not wholly consumed is known as an excess reactant.
written by shahzad haider
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