The most important chemical characteristic of ionic compounds is that each ionAn atom or covalently bonded set of atoms that carries an overall net charge. has its own properties. Such properties are different from those of the atom from which the ion was derived. In other words, an Na+ ion is quite different from an Na atom, and a Cl– ion is unlike an isolated Cl atom or either of the Cl atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. in a Cl2 moleculeA set of atoms joined by covalent bonds and having no net charge.. You eat a considerable quantity of Na+ and Cl– ions in table saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation. every day, but Na atoms or Cl2 molecules would be quite detrimental to your health. The unique chemical properties of each type of ion are quite evident in aqueousDescribing a solution in which the solvent is water. solutions. Most of the reactions of BaCl2(aq), for example, can be classified as reactions of the Ba2+ (aq) ion or the Cl–(aq) ion. If sulfuric acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond., H2SO4, is added to a solution of BaCl2, the solution turns milky and very fine crystals of BaSO4(s) eventually settle out. The reaction can be written as
Ba2+(aq) + H2SO4(aq) → BaSO4(s) + 2H+(aq)
Below is a video of this reaction.
The solution of BaCl2 is clear and colorless, but when H2SO4 is added through the the thin glassA solid material that does not have the long-range order of a crystal lattice; an amorphous solid. A glass melts over a range of temperatures instead of having the definite melting temperature characteristic of crystalline solids. tube, the contents become white and opaque, as insolubleUnable to dissolve appreciably in a solvent. BaSO4(s) come out of solution.
This reaction is characteristic of the barium ion. It will also occur if H2SO4 is added to solutions such as BaI2(aq) or BaBr2(aq) which contain barium ions but no chloride ions.
By contrast, if a solution of silver nitrate, AgNO3, [which contains silver ions, Ag+(aq)] is added to a BaCl2 solution, a reaction characteristic of the chloride ion occurs. A white curdy precipitate of AgCl(s) forms according to the equation
Ag+(aq) + Cl–(aq) → AgCl(s)
Other ionic solutions containing chloride ions, such as LiCl(aq), NaCl(aq), or MgCl2(aq), give an identical reaction. Below is a video of the reaction of a sodium chloride solution with a silver nitrate solution.
Both the NaCl(aq) solution and the AgNO3(aq) solution begin clear and colorless. When the NaCl(aq) solution is added to the AgNO3(aq) solution, a cloudy white precipitate of AgCl(s) is formed. The same result would have occurred had BaCl2 been used, as the reaction is only between the Ag+ and Cl– ions, as seen:
Ag+(aq) + Cl–(aq) → AgCl(s)
Many binary ionic solids not only dissolve in water, they also react with it. When the compound contains an anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. such as N3–, O2–, or S2–, which has more than one negative charge, the reaction with water produces hydroxide ions, OH–:
O2– + H2O → OH–(aq) + OH–(aq) (1)
S2– + H2O → HS–(aq) + OH–(aq) (2)
N3– + 3H2O → NH3(aq) + 3OH–(aq) (3)
Thus, when sodium oxide, Na2O, is added to water, the resulting solution contains sodium ions and hydroxide ions but no oxide ions:
Na2O(s) + H2O(l) → 2Na+(aq) + 2OH– (aq) (4)
The hydride ion also reacts with water to form hydroxide ions. When lithium hydride, LiH, is dissolved in water, for example, the following reaction occurs:
LiH(s) + H2O(l) → Li+(aq) + OH–(aq) + H2(g) (5)
Note that hydrogen gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. is evolved in this reaction. Lithium hydride crystals provide a very compact, if somewhat expensive, method for storing hydrogen.
Among the halide ions (F–, Cl–, Br–, I–) only the fluoride ion shows any tendency to react with water, and that only to a limited extent. When sodium fluoride is dissolved in water, for example, faint traces of hydroxide ion can be detected in the solution owing to the reaction
F– + H2O → HF + OH– (6)
With sodium chloride, by contrast, no such reaction occurs.