The Foundations of Chemistry

Other amphoteric metal hydroxides undergo similar reactions.

Table 10-1 contains lists of the common amphoteric hydroxides. Three are hydroxides

of metalloids, As, Sb, and Si, which are located along the line that divides metals and

nonmetals in the periodic table.

STRENGTHS OF ACIDS

Binary Acids

The ease of ionization of binary protonic acids depends on both (1) the ease of breaking

HXX bonds and (2) the stability of the resulting ions in solution. Let us consider the

relative strengths of the Group VIIA hydrohalic acids. Hydrogen fluoride ionizes only

slightly in dilute aqueous solutions.

HF(aq)
H 2 O(
) 34 H 3 O
(aq)
F(aq)

HCl, HBr, and HI, however, ionize completely or nearly completely in dilute aqueous

solutions because the HXX bonds are much weaker.

HX(aq)
H 2 O(
)88nH 3 O
(aq)
X(aq) XCl, Br, I

The order of bond strengthsfor the hydrogen halides is

(strongest bonds) HFHClHBrHI (weakest bonds)

To understand why HF is so much weaker an acid than the other hydrogen halides, let

us consider the following factors.

1.In HF the electronegativity difference is 1.9, compared with 0.9 in HCl, 0.7 in HBr,

and 0.4 in HI (Section 7-8). We might expect the very polar HXF bond in HF to

ionize easily. The fact that HF is the weakestof these acids suggests that this effect

must be of minor importance.

10-7

Generally, elements of intermediate
electronegativity form amphoteric
hydroxides. Those of high and low
electronegativity form acidic and basic
“hydroxides,” respectively.

380 CHAPTER 10: Reactions in Aqueous Solutions I: Acids, Bases, and Salts

TABLE 10-1 Amphoteric Hydroxides

Insoluble Complex Ion Formed

Metal or Amphoteric in an Excess of a

Metalloid ions Hydroxide Strong Base

Be^2 
 Be(OH) 2 [Be(OH) 4 ]^2 

Al^3 
 Al(OH) 3 [Al(OH) 4 ]

Cr^3 
 Cr(OH) 3 [Cr(OH) 4 ]

Zn^2 
 Zn(OH) 2 [Zn(OH) 4 ]^2 

Sn^2 
 Sn(OH) 2 [Sn(OH) 3 ]

Sn^4 
 Sn(OH) 4 [Sn(OH) 6 ]^2 

Pb^2 
 Pb(OH) 2 [Pb(OH) 4 ]^2 

As^3 
 As(OH) 3 [As(OH) 4 ]

Sb^3 
 Sb(OH) 3 [Sb(OH) 4 ]

Si^4 
 Si(OH) 4 SiO 44 and SiO 32 

Co^2 
 Co(OH) 2 [Co(OH) 4 ]^2 

Cu^2 
 Cu(OH) 2 [Cu(OH) 4 ]^2 

A weakacid may be very reactive. For
example, HF dissolves sand and glass.
The equation for its reaction with
sand is

SiO 2 (s)
4HF(g)88n
SiF 4 (g)
2H 2 O(
)

The reaction with glass and other
silicates is similar. These reactions are
notrelated to acid strength; none of
the three stronghydrohalic acids—
HCl, HBr, or HI—undergoes such a
reaction.