1. Chapter 19.1

2. 1. Assign oxidation numbers to reactant and
product species.
2. Define oxidation and reduction.
3. Explain what an oxidation-reduction reaction
(redox reaction) is.

3. • The oxidation number assigned to an element in a
molecule is based on the distribution of electrons in
that molecule.
• The rules by which oxidation numbers are assigned
are summarized on the next slide.
chromium (II)
chloride
chromium (III)
chloride
potassium
chromate
potassium
dichromate

6. • Reactions in which the atoms or ions of an element
experience an increase in oxidation state are
oxidation processes.
• A species whose oxidation number increases is
oxidized.

7. • Reactions in which the oxidation state of an element
decreases are reduction processes.
• A species that undergoes a decrease in oxidation state
is reduced.
Aluminum from Aluminum Oxide
Tungsten from Tungsten Oxide

8. • Any chemical process in which elements undergo
changes in oxidation number is an oxidation-
reduction reaction.
• This name is often shortened to redox reaction.
• The part of the reaction involving oxidation or
reduction alone can be written as a half-reaction.

9. • Equations for the reaction between nitric acid and
copper illustrate the relationship between half-
reactions and the overall redox reaction.
2+
+20
Cu Cu + 2e –
+5 2 +1 +4 2 +1
+
23
2
22NO + 2 + 4H 2NO + 2H Oe 
–
–
– –
–
+5
+ 2+
3 2
+20 +4
2Cu + 2NO + 4H Cu + 2NO + 2H O–
(oxidation half-reaction)
(reduction half-reaction)
(redox reaction)

10. • When hydrogen burns in chlorine, a covalent bond
forms from the sharing of two electrons.
• The pair of electrons is more strongly attracted to the
chlorine atom because of its higher electronegativity.
2 2
0 0 +1 1
H + Cl 2HCl
–
• Neither atom has totally lost or totally gained any
electrons.
• Hydrogen has donated a share of its bonding electron to
the chlorine but has not completely transferred that
electron.