What is a limiting reagent? Explain with a suitable example how it determines the amount of product formed.

📘 Step-by-Step Explanation

🔵 What is a Limiting Reagent?

The limiting reagent is the reactant that is completely consumed first during a chemical reaction.

It limits the amount of product that can be formed. Once the limiting reagent is completely used up, the reaction cannot produce any more product, even if some other reactant is still left.

Limiting Reagent → Determines Maximum Product

🧮 Suitable Example

Consider the reaction between hydrogen and oxygen:

2H₂ + O₂ → 2H₂O

According to the balanced equation:

2 mol H₂ + 1 mol O₂ → 2 mol H₂O

Step 1: Given Reactants

4 mol H₂

1 mol O₂

We need to determine which reactant is the limiting reagent.

Step 2: Compare with the Required Mole Ratio

The balanced equation requires:

H₂ : O₂ = 2 : 1

For 1 mol O₂, only 2 mol H₂ are required.

But we have 4 mol H₂. Therefore, hydrogen is present in excess.

2 mol H₂ + 1 mol O₂ → 2 mol H₂O

🎬 Limiting Reagent Animation

H₂ H₂ H₂ H₂ + O₂ → H₂O H₂O

The available 1 mol O₂ reacts with only 2 mol H₂. The remaining 2 mol H₂ is left unreacted.

🚨 Step 3: Identify the Limiting Reagent

1 mol O₂ is completely consumed

Therefore, O₂ is the Limiting Reagent.

Hydrogen is present in excess because 4 mol H₂ were available but only 2 mol H₂ are required to react with 1 mol O₂.

Step 4: Calculate the Amount of Product

From the balanced equation:

1 mol O₂ → 2 mol H₂O

Therefore, 1 mol O₂ produces:

2 mol H₂O

⚖️ Product Mass

Molar mass of H₂O = 18 g mol⁻¹

Mass = Moles × Molar Mass

= 2 × 18

= 36 g H₂O

✅ Final Answer

O₂ is the limiting reagent.

Maximum H₂O formed = 2 mol = 36 g

Thus, the limiting reagent determines the maximum amount of product that can be formed.

🎯 Quick Revision

Limiting Reagent = Reactant consumed first
It determines = Maximum Product

2H₂ + O₂ → 2H₂O

4 mol H₂ + 1 mol O₂ → 2 mol H₂O