(^78) › STEP 4. Review the Knowledge You Need to Score High
reactions. For example, suppose Cr(NO 3 ) 3 is dissolved in water. The Cr^3 + cation attracts
water molecules to form the complex ion Cr(H 2 O) 63 +. In this complex ion, water acts
as the ligand. If ammonia is added to this solution, the ammonia can displace the water
molecules from the complex:
[Cr(H 2 O) 6 ]^3 +(aq) + 6 NH 3 (aq) ⇆ [Cr(NH 3 ) 6 ]^3 +(aq) + 6 H 2 O(l)
In reactions involving coordination compounds, the metal acts as the Lewis acid
(electron-pair acceptor), while the ligand acts as a Lewis base (electron-pair donor). In
the reaction above, the ammonia ligand displaced the water ligand from the chromium
complex because nitrogen is a better electron-pair donor (less electronegative) than oxygen.
The nitrogen in the ammonia and the oxygen in the water are the donor atoms. They
are the atoms that actually donate the electrons to the Lewis acid. The coordination
number is the number of donor atoms that surround the central atom. As seen above,
the coordination number for Cr^3 + is 6. Coordination numbers are usually 2, 4, or 6, but
other values can be possible. Silver (Ag+) commonly forms complexes with a coordination
number of 2; zinc (Zn^2 +), copper (Cu^2 +), nickel (Ni^2 +), and platinum (Pt^2 +) commonly
form complexes with a coordination number of 4; most other central ions have a coordination
number of 6.
AgCl(s)2NH(aq) Ag(NH) (aq) Cl(aq)
Zn(OH)(s)2OH(aq) Zn(OH) (aq)
Fe(aq)6CN(aq) Fe(CN) (aq)
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2
3
6
3
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+→ +
+→
+→
+ −− −
+− −Acid–Base Reactions
Acids and bases are extremely common, as are the reactions between acids and bases.
The driving force is often the hydronium ion reacting with the hydroxide ion to form
water. Chapter 15, Equilibrium, describes the equilibrium reactions of acids and bases, as
well as some information concerning acid–base titration. After you finish this section, you
may want to review the acid–base part of the Equilibrium chapter.Properties of Acids, Bases, and Salts
At the macroscopic level, acids taste sour, may be damaging to the skin, and react with bases
to yield salts. Bases taste bitter, feel slippery, and react with acids to form salts.
At the microscopic level, acids are defined as proton (H+) donors (Brønsted–Lowry
theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors
(Brønsted–Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase
reaction between hydrogen chloride and ammonia:HCl(g)+→:NH( 33 g) HNHC+ l(−−s)(orNHC+ 4 l(s))HCl is the acid, because it is donating an H+ and the H+ will accept an electron pair
from ammonia. Ammonia is the base, accepting the H+ and furnishing an electron pair
that the H+ will bond with via coordinate covalent bonding. Coordinate covalent bonds
are covalent bonds in which one of the atoms furnishes both of the electrons for the bond.
After the bond is formed, it is identical to a covalent bond formed by donation of one
electron by both of the bonding atoms.