Limiting Reagent
Stoichiometry | Some Basic Concepts of Chemistry Class 11
1. Definition & Concept
In a chemical reaction involving more than one reactant, the reactants are rarely present in the exact stoichiometric ratio. One reactant is usually consumed completely before the others.
Excess Reagent (ER): The reactant which is not completely consumed and remains unreacted after the reaction is over.
2. Steps to Identify the Limiting Reagent
Follow these standard steps to avoid errors:
- Balance the chemical equation.
- Convert the given amounts of reactants into Moles.
- Divide the number of moles of each reactant by its respective Stoichiometric Coefficient.
- The reactant with the lowest ratio is the Limiting Reagent.
3. Worked Example
Question: 5 moles of $N_2$ react with 9 moles of $H_2$ to form $NH_3$. Identify the Limiting Reagent and moles of $NH_3$ formed.
Equation: $N_2 + 3H_2 \rightarrow 2NH_3$
Step 1: Calculate Ratio
- For $N_2$: Ratio = $5 / 1 = 5$
- For $H_2$: Ratio = $9 / 3 = 3$
Since 3 < 5, Hydrogen ($H_2$) is the Limiting Reagent.
Step 2: Calculate Product
The product depends on the Limiting Reagent ($H_2$):
$$ 3 \text{ mol } H_2 \rightarrow 2 \text{ mol } NH_3 $$ $$ 9 \text{ mol } H_2 \rightarrow \frac{2}{3} \times 9 = 6 \text{ mol } NH_3 $$Result: 6 moles of Ammonia are formed.
4. A Simple Analogy
If you have 10 slices of bread and 3 cheese slices, and each sandwich requires 2 breads and 1 cheese:
- Bread allows: 5 sandwiches.
- Cheese allows: 3 sandwiches.
You can only make 3 sandwiches. The Cheese is the Limiting Reagent, and bread is in excess.
Practice Quiz
Test your ability to identify the Limiting Reagent.
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