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6.11: Chemical Properties

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  • [ "article:topic", "ChemPrime", "chemical property", "authorname:chemprime", "showtoc:no" ]

    The most important chemical characteristic of ionic compounds is that each ion 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 atoms in a Cl2 molecule. You eat a considerable quantity of Na+ and Cl ions in table salt 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 aqueous 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 acid, 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

    \[\text{Ba}^{2+}(aq) +  \text{ H}_{\text{2}}\text{SO}_4(aq)\rightarrow \text{ BaSO}_{\text{4}}(s)   +  \text{2H}^{+}(aq)\]

    Below is a video of this reaction.

    The solution of BaCl2 is clear and colorless, but when H2SO4 is added through the the thin glass tube, the contents become white and opaque, as insoluble 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

    \[\text{Ag}^{+}(aq) +  \text{Cl}^{-}(aq)\rightarrow\text{AgCl}(s)\]

    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:

    \[\text{Ag}^{+}(aq) +  \text{Cl}^{-}(aq)\rightarrow\text{AgCl}(s)\]

    Many binary ionic solids not only dissolve in water, they also react with it. When the compound contains an anion such as N3–, O2–, or S2–, which has more than one negative charge, the reaction with water produces hydroxide ions, OH:

    \[\text{O}^{2-} +  \text{ H}_{\text{2}}\text{O}\rightarrow\text{OH}^{-}(aq)   +  \text{OH}^{-}(aq)\]

    \[\text{S}^{2-} +  \text{ H}_{\text{2}}\text{O}\rightarrow\text{HS}^{-}(aq)   +  \text{OH}^{-}(aq)\]

    \[\text{N}^{3-} +  \text{ 3H}_{\text{2}}\text{O}\rightarrow\text{ NH}_{\text{3}}(aq)  +  \text{3OH}^{-}(aq)\]

    Thus, when sodium oxide, Na2O, is added to water, the resulting solution contains sodium ions and hydroxide ions but no oxide ions:

    \[\text{ Na}_{\text{2}}\text{O} + \text{ H}_{\text{2}}\text{O}\rightarrow\text{2Na}^{+}(aq)   +  \text{2OH}^{-}(aq)\]

    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:

    \[\text{LiH}(s) + \text{ H}_{\text{2}}\text{O}\rightarrow\text{Li}^{+}(aq) +  \text{OH}^{-}(aq) + \text{ H}_{\text{2}}(g)\]

    Note that hydrogen gas 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

    \[\text{F}^{-} +  \text{ H}_{\text{2}}\text{O}\rightarrow\text{HF}   +  \text{OH}^{-}\]

    With sodium chloride, by contrast, no such reaction occurs.