Q.Assertion: Toxic metal ions are removed by the chelating ligands.
Reason: Chelate complexes tend to be more stable.

The key idea is stability of chelate complexes — chelating ligands form multiple bonds with a metal ion, producing a more stable complex than a comparable monodentate ligand.

1. Assertion is true: Toxic metal ions (e.g., $\text{Hg}^{2+}$, $\text{Pb}^{2+}$) are removed from the body using chelating ligands like EDTA or dimercaprol, which bind the metal and allow excretion.
2. Reason is true: Chelate complexes are thermodynamically more stable due to the chelate effect (entropy gain from displacing many monodentate ligands). …

The Concept: Why Chelation Works for Detoxification

The key idea here is the chelate effect — a thermodynamic principle that makes multidentate ligands form far more stable complexes than their monodentate counterparts. But the reason chelating ligands are used to remove toxic metal ions isn't just "they're stable." It's that they bind selectively and irreversibly enough to prevent the metal from interacting with biological molecules, while still allowing the complex to be excreted.

Let's break this down step by step.

1. Understanding the Assertion

Toxic metal ions like $\text{Pb}^{2+}$, $\text{Hg}^{2+}$, $\text{Cd}^{2+}$, and $\text{As}^{3+}$ cause damage by binding to enzymes and proteins, disrupting their function. Chelating ligands (e.g., EDTA, dimercaprol, penicillamine) are administered as antidotes. They wrap around the metal ion like a claw (from Greek chele = claw), forming a stable, water-soluble complex that can be excreted via urine.

2. Understanding the Reason

Chelate complexes are indeed more stable than analogous complexes with monodentate ligands. This is the chelate effect, quantified by comparing the equilibrium constants.

For example, consider the reaction of $\text{Ni}^{2+}$ with ammonia (monodentate) vs. ethylenediamine (bidentate):

The chelate complex is about $10^{10}$ times more stable! This stability comes from two factors:

• Entropy gain: One chelating ligand replaces several monodentate ligands, increasing the number of free particles in solution (favourable entropy).
• Enthalpy contribution: The bite angle of the chelate ring often matches the metal's preferred geometry, giving stronger bonds.

So the reason is true — chelate complexes are more stable.

3. Connecting Assertion and Reason — The Critical Point

Does the stability of chelate complexes explain why they remove toxic metal ions? Yes — directly. …

Method: Assertion–Reason Analysis (Fact + Logic Check)

This is the standard approach for CBSE / competitive exam Assertion–Reason questions. The steps are:

Step 1: Check the Assertion for truth

• Assertion: Toxic metal ions are removed by the chelating ligands.
• Fact: Yes — chelating ligands (like EDTA, DMSA) bind strongly to toxic metal ions (e.g., $\text{Pb}^{2+}$, $\text{Hg}^{2+}$, $\text{Cd}^{2+}$) and help excrete them from the body. This is the basis of chelation therapy.
• ✓ Assertion is true.

Step 2: Check the Reason for truth

• Reason: Chelate complexes tend to be more stable.
• Fact: Yes — chelate complexes are more stable than complexes with monodentate ligands due to the chelate effect (entropy-driven stability).
• ✓ Reason is true.

Step 3: Check if Reason correctly explains the Assertion

• The reason (greater stability of chelate complexes) is exactly why chelating ligands can out-compete the toxic ion's biological binding sites and lock it into an inert, excretable complex. …

Common Mistakes & How to Avoid Them

Mistake 1: Thinking a General Property Can't Also Be the Explanation

Why students make this mistake:

Students read the reason ("chelate complexes tend to be more stable") as just a general fact about chelates, disconnected from the specific claim in the assertion about removing toxic ions.

How to avoid:

Ask: why does forming a more stable complex remove a toxic ion from circulation? Because the chelating ligand binds the metal ion more strongly (forms a MORE STABLE complex) than the weaker bonds the metal ion would otherwise form with biological molecules (enzymes, proteins). This higher stability is exactly what lets the chelator out-compete and sequester the toxic ion so it can be safely excreted.

✓ Correct approach:

• Assertion: True
• Reason: True
• The reason (greater stability of the chelate) is the correct explanation → Option (i)

Mistake 2: Thinking the Assertion is False

Why students make this mistake:

Some students confuse "removal" with "neutralization" or think chelating ligands only bind, not remove. They forget that chelation therapy (e.g., EDTA for lead poisoning) is a standard medical procedure.

How to avoid:

Recall a real example:

• EDTA (a hexadentate ligand) binds Pb²⁺, Ca²⁺, etc.
• The complex is water-soluble and excreted via urine.
• This is a fact in coordination chemistry and medicine.

✓ Key takeaway: Chelating ligands do remove toxic metal ions from the body.

Mistake 3: Thinking the Reason is False

Why students make this mistake:

Some students think "stability" is vague or that chelate complexes are not always more stable than monodentate complexes. They forget the chelate effect — entropy gain makes chelates more stable.

How to avoid:

Remember the chelate effect:

• A bidentate ligand replaces two monodentate ligands.
• Number of particles increases → entropy increases → ΔG becomes more negative → complex is more stable. …