MCAT Content / Ions In Solutions / Hydration The Hydronium Ion

Hydration, The Hydronium Ion

Topic: Ions In Solutions

When water is used as the solvent, the dissolving process is called hydration. Hydronium is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water.

Water is a polar molecule with partially-positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water.

In order to dissolve an ionic solid, water molecules must break up the interactions between all of the ions in the solid. To do this, they orient themselves such that they effectively reduce the localized charge on the ions forming dipole ion attractions. This is called hydration. Hydration of ions is a thermodynamically favorable process, and as such can release heat. This is why it is called the “heat of hydration.”

The key factor that determines solubility is the interaction of the ions with the solvent. The electrically-charged ions undergo ion-dipole interactions with water to overcome strong coulombic attraction, and this produces an aqueous solution. The water molecule is polar; it has a partial positive charge on the hydrogens while oxygen bears a partial negative charge. This dipole arises from the disparity in electronegativity present in the O-H bonds within the water molecule. Furthermore, the two lone pairs on the oxygen in water also contribute to the stabilization of any positively charged ions in solution.

Hydronium Ion
Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H30+).

Key Points

• Hydration is the dissolving of an ionic solid into the water where polar water molecules separate the anions and cations due to ion-dipole attractions.

• Water readily ionizes itself to produce hydroxide ions and hydronium ions in an equilibrium

Key Terms

The heat of hydration: The heat that is released by hydration of one mole of ions at a constant pressure. The more the ion is hydrated, the more heat is released.

Ion: An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.

Thermodynamics: The science of the conversions between heat and other forms of energy.

Solution: A homogeneous mixture, which may be a liquid, gas, or solid, formed by dissolving one or more substances.

Solute: Any substance that is dissolved in a liquid solvent to create a solution.

Ionization of water: The process in which water ionizes to hydronium ions and hydroxide ions.

Coulombic attraction: The force of attraction between positive and negative charges.

Hydration shell: More or less continuous cluster of water molecules surrounding and to a degree attached to a hydrophilic substance.

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