22.2

Oxidation Numbers

Assigning Oxidation Assigning Oxidation NumbersNumbers

• An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

• Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element

Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers1)The oxidation number of any

uncombined element is zero.

2)The oxidation number of a monatomic ion equals its charge.

11

2

00

22

ClNaClNa

Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers3)The oxidation number of oxygen in

compounds is -2, except in peroxides, such as H2O2 where it is -1.

4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.

2

2

1

OH

Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers

5) The sum of the oxidation numbers of the atoms in the compound must equal 0.

2

2

1

OH2(+1) + (-2) = 0 H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers6) The sum of the oxidation numbers in

the formula of a polyatomic ion is equal to its ionic charge.

3

2?

ONX + 3(-2) = -1N O

24

2?

OS

X = +5 X = +6

X + 4(-2) = -2S O

Point to note

• Oxidation numbers are written with the charge first and number second

• Ex. HCl H oxidation number is +1

Cl oxidation nmber is -1

Ionic charge though is written with the charge after the number: H charge is 1+, Cl charge is 1-

Problem:

• What is the oxidation number of each element in SO2

Do oxygen first because you know that is -2.

x -4 = 0 S O2, so, S is +4

S=+4 O=-2

Reducing Agents and Reducing Agents and Oxidizing AgentsOxidizing Agents

• An increase in oxidation number = oxidation

• A decrease in oxidation number = reduction

eNaNa10

10 CleCl

Sodium is oxidized – it is the reducing agent

Chlorine is reduced – it is the oxidizing agent

Trends in Oxidation and ReductionTrends in Oxidation and Reduction

Active metals: Lose electrons easily Are easily oxidized Are strong reducing agents

Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

Problem

• Use oxidation numbers to decide which elements are oxidized and which reduced in the following reaction:

C + O2 ------CO2

0 0 +4 -2

C + O2 ------CO2

Notice C goes from 0 to +4 so it is oxidized ( reducing agent)

O goes from 0 to -2,so it is reduced (oxiding agent)