Coordinate (Dative Covalent) Bonding - Chemistry LibreTexts

The Reaction Between Ammonia and Hydrogen Chloride

If these colorless gases are allowed to mix, a thick white smoke of solid ammonium chloride is formed.

The reaction is

\[ \ce{NH3 (g) + HCl (g) \rightarrow NH4Cl (s)} \nonumber \]

Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion (a proton) from the hydrogen chloride molecule to the lone pair of electrons on the ammonia molecule.

When the ammonium ion, NH4+, is formed, the fourth hydrogen is attached by a dative covalent bond, because only the hydrogen's nucleus is transferred from the chlorine to the nitrogen. The hydrogen's electron is left behind on the chlorine to form a negative chloride ion. Once the ammonium ion has been formed it is impossible to tell any difference between the dative covalent and the ordinary covalent bonds. Although the electrons are shown differently in the diagram, there is no difference between them in reality.

In simple diagrams, a coordinate bond is shown by an arrow. The arrow points from the atom donating the lone pair to the atom accepting it.

Example \(\PageIndex{1}\): Dissolving \(HCl_{(g)}\) in Water to make Hydrochloric Acid

Something similar happens. A hydrogen ion (H+) is transferred from the chlorine to one of the lone pairs on the oxygen atom.

\[ \ce{H_2O + HCl \rightarrow H_3O^{+} + Cl^{-}} \nonumber]

The H3O+ ion is variously called the hydroxonium ion, the hydronium ion or the oxonium ion. In an introductory chemistry course, whenever you have talked about hydrogen ions (for example in acids), you have actually been talking about the hydroxonium ion. A raw hydrogen ion is simply a proton, and is far too reactive to exist on its own in a test tube.

If you write the hydrogen ion as \(\ce{H^{+}(aq)}\), the "\(\ce{(aq)}\)" represents the water molecule that the hydrogen ion is attached to. When it reacts with something (an alkali, for example), the hydrogen ion simply becomes detached from the water molecule again. Note that once the coordinate bond has been set up, all the hydrogens attached to the oxygen are exactly equivalent. When a hydrogen ion breaks away again, it could be any of the three.