786 CHAPTER 18: Ionic Equilibria I: Acids and Bases
TABLE 18-10 Ionic Radii and Hydrolysis Constants for Some CationsCation Ionic Radius (Å) Hydrated Cation KaLi 0.90 [Li(OH 2 ) 4 ] 1 10 ^14
Be^2 0.59 [Be(OH 2 ) 4 ]^2 1.0 10 ^5Na 1.16 [Na(OH 2 ) 6 ](?) 10 ^14
Mg^2 0.85 [Mg(OH 2 ) 6 ]^2 3.0 10 ^12
Al^3 0.68 [Al(OH 2 ) 6 ]^3 1.2 10 ^5Fe^2 0.76 [Fe(OH 2 ) 6 ]^2 3.0 10 ^10
Fe^3 0.64 [Fe(OH 2 ) 6 ]^3 4.0 10 ^3Co^2 0.74 [Co(OH 2 ) 6 ]^2 5.0 10 ^10
Co^3 0.63 [Co(OH 2 ) 6 ]^3 1.7 10 ^2Cu^2 0.96 [Cu(OH 2 ) 6 ]^2 1.0 10 ^8
Zn^2 0.74 [Zn(OH 2 ) 6 ]^2 2.5 10 ^10
Hg^2 1.10 [Hg(OH 2 ) 6 ]^2 8.3 10 ^7
Bi^3 0.74 [Bi(OH 2 ) 6 ]^3 1.0 10 ^2Key Terms
Amines Derivatives of ammonia in which one or more hydro-
gen atoms has been replaced by organic groups.
Hydrolysis The reaction of a substance with water.
Hydrolysis constant An equilibrium constant for a hydrolysis
reaction.
Indicator An organic compound that exhibits different colors in
solutions of different acidities.
Ion product for water An equilibrium constant for the ioniza-
tion of water,
Kw[H 3 O][OH]1.0 10 ^14 at 25°C
Ionization constant An equilibrium constant for the ionization
of a weak electrolyte.
Monoprotic acid An acid that can form only one hydronium ion
per molecule; may be strong or weak.
pH The negative logarithm of the concentration (mol/L) of the
H 3 O(or H) ion; the commonly used scale ranges from 0 to
14.
pKa The negative logarithm of Ka, the ionization constant for a
weak acid.
pKb The negative logarithm of Kb, the ionization constant for a
weak base.
pKw The negative logarithm of the ion product for water.
pOH The negative logarithm of the concentration (mol/L)
of the OHion; the commonly used scale ranges from 14
to 0.
Polyprotic acid An acid that can form two or more hydronium
ions per molecule.
Solvolysis The reaction of a substance with the solvent in which
it is dissolved.Pepto-Bismol contains
BiO(HOC 4 H 6 COO), bismuth
subsalicylate, a hydrolyzedbismuth
salt. Such salts “coat” polar surfaces
such as glass and the lining of the
stomach.
Smaller, more highly charged cations are stronger acids than larger, less highly charged
cations (Table 18-10). This is because the smaller, more highly charged cations interact
with coordinated water molecules more strongly.
For isoelectronic cations in the same period in the periodic table, the smaller, more
highly charged cation is the stronger acid. (Compare Kavalues for hydrated Liand Be^2
and for hydrated Na, Mg^2 , and Al^3 .) For cations with the same charge from the same
group in the periodic table, the smaller cation hydrolyzes to a greater extent. (Compare
Kavalues for hydrated Be^2 and Mg^2 .) If we compare cations of the same element in
different oxidation states, the smaller, more highly charged cation is the stronger acid.
(Compare Kavalues for hydrated Fe^2 and Fe^3 and for hydrated Co^2 and Co^3 .)