5 Steps to a 5 AP Chemistry

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

numberis 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.

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.

The chapter on 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, acidsare defined as proton (H+) donors (Brønsted–Lowry

theory) or electron-pair acceptors (Lewis theory). Basesare defined as proton (H+) acceptors

(Brønsted–Lowry theory) or electron-pair donors (Lewis theory). Consider the

gas-phase reaction between hydrogen chloride and ammonia:

(or NH 4 +Cl–)

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 bondsare

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 elec-

tron by both of the bonding atoms.

Acids and bases may be strong, dissociating completely, or weak, partially dissociating

and forming an equilibrium system.(See Chapter 15 for the details on weak acids and

bases.) Strong acids include:

1. Hydrochloric, HCl


2. Hydrobromic, HBr


3. Hydroiodic, HI

HCl g() :+→NH g 33 () HNH Cl+−

AgCl s NH aq Ag NH aq Cl aq

Z

()+→() ⎡⎣ ( ) ()⎤⎦ + ()

+ −

(^2332)
nn OH s OH aq Zn OH aq
Fe a

()() () ()()

(

24

2

3

+→ 2 − ⎡⎣ ⎤⎦

−

+ qqCNaq FeCN aq)()()()+→⎡

⎣ ⎤⎦

6 − −

6

3

76  STEP 4. Review the Knowledge You Need to Score High

STRATEGY