Limiting Reactant:
“The reactant which is earlier consumed in a chemical reaction” is called limiting reactant.
Or:
“The reactant which is completely consumed in a reaction to produce product when the reaction is over” is called limiting reactant.
Limiting reactant is also known as limiting reagent.
Excess Reactant:
The substance whose some amount is left unreacted when the reaction is over is called substance in excess.
Or:
The reactant which is not used up completely and its some amount remains unused after the chemical reaction is termed as excess reactant.
Excess reactant is also known as excess reagent.
Determining the Limiting reactant and Excess Reactant:
Consider a chemical reaction (irreversible reaction) :
N2(g) + 3H2(g) → 2NH3(g)
Molar ratio according to equation
1 mole 3 moles 2 moles
e.g. Let 4 moles of nitrogen are made to react with 6 moles of hydrogen to get ammonia.
Suppose 4 moles of N2 and 6 moles of H2 are made to react, then
Moles present before reaction
4 moles 6 moles 0 mole
Moles present after reaction
2 moles 0 mole 4 moles
As H2 is consumed completely so it is limiting reactant while 2 moles of N2 are left unreacted so it is substance in excess. Thus product will be calculated by H2 which is the limiting reactant.
Consider a chemical reaction
2H2(g) + O2(g) → 2H2O(g)
Formation of water (2H2O) is a balanced chemical equation .Therefore, no reactant is limiting or in excess.
However, when 1mole of each H2 and O2 react together ,then they form a chemical equation:
2 mol H2 gives = 2 mol H2O
1 mol H2 gives = 2/2 mol H2O
1 mol H2 gives = 1 mol H2O
Similarly :
1 mol O2 gives = 2 mol H2O
As one mole of hydrogen (H2) gives the least amount of product ( 1 mole of H2O).
Hence , H2 is a limiting reactant while O2 is an excess reactant .
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