Page 133 - Instant notes

E1
IONS IN AQUEOUS SOLUTION

Key Notes
Ions are charged chemical species that at low concentrations are
stabilized by an energetically favorable interaction with water. A
hydration shell of coordinated water molecules is formed that
increases the size of the hydrated ion. The general terms for any
solvent are solvation shell and solvated ion respectively and both
specific and general terms are used for water.
Ion hydration is an exothermic process. The addition of ions to a
solution decreases the solvent volume and solvent entropy in the
solvation shell but increases solvent volume and entropy in the
zone between the solvation shell and bulk solution. For small
ions, the former effect dominates the latter, there is an overall
decrease in volume and entropy and the ion is termed structure-
making. For larger ions, termed structure-breaking, the opposite
is true and there is an increase in volume and entropy.

As the concentration of ions in solution increases, electrostatic
ion-ion interactions become more important. An ionic
atmosphere of oppositely charged counterions forms around each
ion, further stabilizing the system. This clustering affects the ion
thermodynamics.
Related topics Enthalpy (B2) Weak intermolecular
interactions (H6)
Free energy (B6)
Thermodynamics of ions in
solution (E2)

Ionic models

Ions are charged chemical species. Positively charged ions are called cations as they
travel to the cathode of an electrolytic cell (see Topic E3). Negatively charged ions are
termed anions as they travel to the anode. A salt is the solid which dissolves to produce
these cations and anions. When a salt (solute) is dispersed in water (solvent) (see Topic
D1) to form an aqueous solution of ions, also termed an ionic or electrolyte solution,
there is an energetically favorable ion-dipole interaction (Fig. 1).