Question

Why is the entropy change is greater for the dissolution of NaI compared to the dissolution of NaBr?

Iodide has weaker ion-dipole interactions in water than Bromide

BUT, can you explain why this is so using concepts from intermolecular forces (ion-dipole, polarizability, etc.)?

Answer 1

Entropy is a measure of the randomness present in the system. If No. of ions formed are more entropy value will be higher . This is because of the forces of attraction between oppositely charged ions.

Sodium iodide dissociates as sodium ion & I^- ion

NaI -------> Na⁺ + I^-

^{Sodium bromide dissociates as}

NaBr-------> Na⁺   + Br^-

I ^- ion is highly polarizable due to increase in its size than Br^- ion . Larged sized I ^- ion is less solvated by the solvent. than Br^-i.e. less restiction for the motion. Hence, entropy change for of NaI is greater as it dissociates readily than NaBr.

& less solvated.