A reactant that controls the amount of the product formed in a chemical reaction due to its smaller amount is called limiting reactant.
Excess reactant:
A reactant which is present in large amount in a chemical reaction & some of its amount left after the completion of chemical reaction is called excess reactant.
Explanation:
Often, in experimental work, one or more reactants is / are deliberately used in excess quantity. The quantity exceeds the amount required by the reaction’s stoichiometry. This is done, to ensure that all of the other expensive reactant is completely used up in the chemical reaction. Sometimes, this strategy is employed to make reactions occur faster. For example, we know that a large quantity of oxygen in a chemical reaction makes things burn more rapidly. In this way, excess of oxygen is left behind at the end of reaction and the other reactant is consumed earlier. This reactant which is consumed earlier is called a limiting reactant. In this way, the amount of product that forms is limited by the reactant that is completely used.
Example No. 1:
The concept of limiting reactant is analogous to the relationship between the number of “Kababs” and the “Slices” to prepare “Sandwiches”. If we have 30 “Kababs” and five breads having 58 slices, then we can only prepare 29 sandwiches. One Kabab will be extra (excess reactant) and slices will be limiting reactant. It is a practical problem that we cannot purchase exactly sixty slices for 30 Kababs to prepare 30 sandwiches.
Example No. 2:
Reaction of hydrogen and oxygen to form water.
2H2(g) + O2(g) → 2H20(l)
When we take 2 moles of hydrogen (g) and allow it to real with 2 moles of oxygen (64g), then we will get only 2 moles (36g) of water. Actually, we will get 2 moles (36g) of water because 2 moles (4g) of hydrogen react with 1 mole (32g) of oxygen according to balanced equation. Since less hydrogen is present as compared to oxygen, so hydrogen is a limiting reactant. If we would have reacted 4 moles (8g) of hydrogen with 2 moles (64g) of hydrogen, we would have obtained 4 moles (72g) of water.
HOW TO FIND LIMITING REACTANT
To find out a limiting reactant, the steps involved are,
- 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 limiting reactant.
- Differentiate between limiting reactant and excess reactant.
