Question 6: (a) Differentiate between a “limiting reagent” and a “reagent in excess”. How will you identify the limiting reagent in a chemical reaction?
(b) How does a limiting reagent control the amount of the product formed? Give an example.
Reagents in excess
1) Reactant which consumes earlier in a chemical reaction is called ‘limiting reagent’.
Reactant which is not used up earlier in a chemical reaction is called ‘reagent in excess’.
2) Limiting reagent controls the reaction.
Reagent in excess does not control the reaction.
3) It is in less amount in the reaction.
It is in greater amount in the reaction.
4) It is consumed first in the chemical reaction.
It does not consume first. It remains in excess after the limiting reagent is consumed.
5) It completely reacts in a chemical reaction.
It does not react completely in a chemical reaction and may remain un-reacted after the reaction is over.
(b) In a chemical reaction, the formation of products depends upon the availability of the reactants. When a reactant consumes completely, the formation of products stops. Since the limiting reagent consumes first in the reaction, therefore, it controls the extent of product formation.
Example: Consider 2H2 + O2 ———–→ 2H2O
2 mol of hydrogen reacts with 1 mol of oxygen to produce 2 mol of water. So it can be said that, 2 mol of H2 ≅ 1 mol of O2
Now suppose, we have 4 mol of H2 and 3 mol of O2. When 4 mol of H2 and 3 mol of O2 are allowed to react, 4 mol of H2 will react with 2 mol of O2. There would be 1 mol of O2 in excess because the given amount of 4 mol of H2 has now been completely consumed. Therefore, the reaction will come to stop despite 1 more mol of O2 is available. Thus the product formation of H2O is limited by the amount of H2 present. Therefore, H2 is called limiting reagent.
Note: O2 is the reagent in excess in this case.