Problem 31 - Chemistry: The Central Science

Question

(a) What is meant by the term chelate effect? (b) What thermodynamic factor is generally responsible for the chelate effect? (c) Why are polydentate ligands often called sequestering agents?

Step-by-Step Solution

Answer

(a) The chelate effect refers to the enhanced stability of a complex formed by a polydentate ligand binding to a metal ion, compared to a monodentate ligand, due to the formation of chelate rings. (b) Entropy is the thermodynamic factor responsible for the chelate effect, as the increase in overall entropy due to the release of multiple counter ions makes chelate complex formation more favorable. (c) Polydentate ligands are called sequestering agents because they can capture and form stable complexes with metal ions, rendering them inactive for chemical reactions, which can be used for applications such as removing heavy metal ions from a system.

Step 1: (a) Explaining the term chelate effect

A chelate effect refers to the enhanced stability of a complex, which is formed when a polydentate ligand (a ligand that can bind to a metal ion through more than one place) binds to a metal ion as compared to the stability of a complex formed with a monodentate ligand (a ligand that can bind through only one place). This increased stability is due to the formation of a ring-like structure, called a chelate ring, which involves both the central metal ion and the polydentate ligand.

Step 2: (b) Thermodynamic factor responsible for the chelate effect

The thermodynamic factor responsible for the chelate effect is entropy. When a polydentate ligand replaces monodentate ligands in a complex, there is an increase in the overall entropy of the system due to the release of multiple counter ions. This leads to a more positive (favorable) change in entropy, thus making the formation of the chelate complex thermodynamically more favorable compared to the formation of a complex with monodentate ligands.

Step 3: (c) Polydentate ligands as sequestering agents

Polydentate ligands are often called sequestering agents because they can "sequester," or capture and form stable complexes with metal ions, rendering them inactive or unavailable for chemical reactions. This property makes polydentate ligands useful for various applications, such as the removal of heavy metal ions from a system, as these complexes are thermodynamically stable due to the chelate effect and do not participate in further chemical reactions.