Reactions and Periodicity ‹ 71
Periodic Relationships
The periodic table can give us many clues as to the type of reaction that is taking place.
One general rule, covered in more detail in Chapter 11, Bonding, is that nonmetals react
with other nonmetals to form covalent compounds, and that metals react with nonmetals
to form ionic compounds. If the reaction that is producing the ionic compound is occur-
ring in solution, you will be expected to write the net ionic equation for the reaction.
Also, because of the wonderful arrangement of the periodic table, the members of a family
or group (a vertical grouping) all react essentially in the same fashion. Many times, in
reactions involving the loss of electrons (oxidation), as we proceed from top to bottom in
a family the reaction rate (speed) increases. Conversely, in reactions involving the gain of
electrons (reduction) the reaction rate increases as we move from the bottom of a family to
the top. Recall also that the noble gases (VIIIA) undergo very few reactions. Other specific
periodic aspects will be discussed in the various reaction sections.General Properties of Aqueous Solutions
Many of the reactions that you will study occur in aqueous solution. Water is called the
universal solvent, because it dissolves so many substances. It readily dissolves ionic com-
pounds as well as polar covalent compounds, because of its polar nature. Ionic compounds
that dissolve in water (dissociate) form electrolyte solutions, which conduct electrical cur-
rent owing to the presence of ions. The ions can attract the polar water molecules and form
a bound layer of water molecules around themselves. This process is called solvation. Refer
to Chapter 13, Solutions and Colligative Properties, for an in-depth discussion of solvation.
Even though many ionic compounds dissolve in water, many others do not. If the
attraction of the oppositely charged ions in the solid for each other is greater than the
attraction of the polar water molecules for the ions, then the salt will not dissolve to an
appreciable amount. If solutions containing ions such as these are mixed, precipitation
will occur, because the strong attraction of the ions for each other overcomes the weaker
attraction for the water molecules.
As mentioned before, certain covalent compounds, like alcohols, readily dissolve in
water because they are polar. Since water is polar, and these covalent compounds are also
polar, water will act as a solvent for them (general rule of solubility: “Like dissolves like”).
Compounds like alcohols are nonelectrolytes—substances that do not conduct an electri-
cal current when dissolved in water. However, certain covalent compounds, like acids, will
ionize in water, that is, form ions:HCl(aq)H→++−(aq) Cl(aq)There are several ways of representing reactions that occur in water. Suppose, for example,
that we were writing the equation to describe the mixing of a lead(II) nitrate solution with
a sodium sulfate solution and showing the resulting formation of solid lead(II) sulfate. One
type of equation that can be written is the molecular equation, in which both the reactants
and products are shown in the undissociated form:Pb(NO 32 )(aq)N+→aS 24 O(aq)PbSO( 43 s)+2NaNO(aq)Molecular equations are quite useful when doing reaction stoichiometry problems
(see Chapter 7).