The Foundations of Chemistry

Recall the lists of strong acids (Table 4-5) and strong bases (Table 4-7). These acids
and bases are completely or almost completely ionized or dissociated in dilute aqueous
solutions. Other common acids and bases are either insoluble or only slightly ionized or
dissociated. In addition, the solubility guidelines (page 134 and Table 4-8) allow you to
determine which salts are soluble in water. Most salts that are soluble in water are also
strong electrolytes. Exceptions such as lead acetate, Pb(CH 3 COO) 2 , which is soluble but
does not ionize appreciably, will be noted as they are encountered.

The only common substances that should be written as ions in ionic equations are

(1) strong acids, (2) strong bases, and (3) soluble ionic salts.

OXIDATION NUMBERS

The many reactions that involve the transfer of electrons from one species to another are
called oxidation–reduction reactions,or simply, redox reactions.We use oxidation
numbers to keep track of electron transfers. The systematic naming of compounds
(Sections 4-11 and 4-12) also makes use of oxidation numbers.
The oxidation number,or oxidation state,of an element in a simple binaryionic
compound is the number of electrons gained or lost by an atom of that element when it
forms the compound. In the case of a single-atom ion, it corresponds to the actual charge

4-4

Oxidation–reduction and displacement

reactions are discussed in Sections 4-5

and 4-8.

Binary means two. Binary compounds

contain two elements.

4-4 Oxidation Numbers 137

Problem-Solving Tip:Writing Ionic Equations

The following chart will help in deciding which formula units are to be written as sepa-

rate ions in the total ionic equation and which ones are to be written as unchanged

formula units. You must answer two questions about a substance to determine whether

it should be written in ionic form or as a formula unit in the total and net ionic equa-

tions.

1.Does it dissolve in water? If not, write the full formula.

2.(a) If it dissolves, does it ionize (a strong acid)?

(b) If it dissolves, does it dissociate (a strong base or a soluble salt)?

If the answer to either partof the second question is yes, the substance is a soluble strong

electrolyte, and its formula is written in ionic form.

Write in ionic form;

e.g., [H
(aq)
Cl(aq)] (because

HCl is completely ionized)

e.g., [2Na
(aq)
SO 42 ] (because

Na 2 SO 4 is a soluble ionic salt)

Write as full formula;

e.g., PbSO 4 (s)

Is it mostly

either ionized

or dissociated

in H 2 O?

Is it soluble

in water?

Write as full formula;

e.g., CH 3 COOH(aq)

Yes Yes

No No

(^66)
(^66)
(^66)
(^6) g
(^66)
(^66)
g
88888888n 8888n