Hydration

This article deals with the accumulation of water molecules. The term should not be confused with hydration , which is a chemical reaction of water on a substrate, and hydrogenation , which describes the addition of hydrogen .

Under hydration or hydration means the addition of water molecules

• of dissolved ions . This creates a hydrate shell (also called a hydrate sphere ).
• to polar neutral molecules, especially if hydrogen bonds can be formed.
• in solids (minerals) as crystal water for the formation of hydrates .

“Hydration” is the name given to the special case of solvation for the solvent water .

The hydration of calcium oxide (quicklime) , magnesium oxide (burnt magnesia) and cement to calcium hydroxide (slaked lime) , magnesium hydroxide and calcium silicate hydrates is of technical importance in construction .

Water in dissolved ions

Hydration of a sodium ion

The hydration takes place due to the electrostatic forces between the charged ions and the water dipoles (ion-dipole interaction). With the formation of hydrogen bridges to the first hydrate shell, further water molecules can accumulate and thus form another hydrate sphere.

The hydration energy serves as a measure of the tendency of the ions to combine with water molecules . This is the energy that is required to move the ions from the aqueous solution into a non-polar environment.

The number of bound water molecules and the strength of the bond depend on the size and charge of the ions. With the same charge, the smaller ions build up a larger hydrate shell than the larger ones. An aq at the ion expresses that an ion is hydrated in solution:

${\displaystyle \mathrm {NaCl\rightarrow Na_{aq}^{+}+Cl_{aq}^{-}} }$

In the case of small and / or multiply charged cations, the bound water molecules can release protons , one speaks of cation acids (see Lewis acids ). The solvent water may not be removed from solutions containing cation acids:

${\displaystyle \mathrm {AlCl_{3}+3H_{2}O\rightarrow Al{(OH)}_{3}+3HCl} }$

Anions are generally much larger than cations and therefore less hydrated.

In other polar solvents, e.g. B. ammonia , similar effects occur, which are commonly called solvation .

Water on non-polar particles (hydrophobic hydration)

If non-polar and uncharged (i.e. water-hostile) particles are in aqueous solution, so-called hydrophobic effects occur . This also includes hydrophobic (water-avoiding) hydration . The water in the immediate vicinity of the non-polar particle (e.g. a xenon atom) or a non-polar molecular group (e.g. an alkyl group) then has a little more structure, and the translational and rotational mobility of the neighboring water molecules is reduced. Since the hydrophobic effect also takes place in the vicinity of large biomolecules, where water is arranged next to both polar and non-polar groups of molecules, this effect plays an extremely important role in many biochemical processes.

See also

• Solvation
• Hydration energy
• Solution (chemistry)

Individual evidence

1. ↑ Dipl.-Ing. Bonar Marbun: Kinetics of the hydration of CaO and MgO , dissertation, February 2006, Faculty of Natural and Material Sciences, Clausthal University of Technology
2. ↑ W. Blokzijl and JBFN Engberts: Hydrophobic Effects - Views and Facts In: Angew. Chemie 105, 1993, pp. 1610-1648.
3. ^ R. Haselmeier, M. Holz, W. Marbach, H. Weingärtner: Water Dynamics Near a Dissolved Noble Gas. First Direct Experimental Evidence for a Retardation Effect. In: J. Phys. Chem. 99, 1995, pp. 2243-2246.