ElectrochemistryLesson 3

Oxidation Numbers

Silver crystals on a copper frame

Ho Ho Ho

Oxidation Numbers 

HV2O4- V6+

 Is the above equation oxidation or reduction?

Oxidation numbers are assigned numbers that are used to determine if oxidation or reduction has occurred.

Rules for Oxidation Numbers Oxidation # 1. All elements are zero:

N2 0

Pb 0

Na 0

O2 0

Rules for Oxidation Numbers Oxidation # 2. Monatomic Ions are their charge

Na+ +1

Br- -1

MgSO4+2

Al2S3

-2

Rules for Oxidation Numbers Oxidation # 3. O in a compound is -2

exception: peroxides -1 

O2 0

Na2O -2CO2 -2

CH3COOH -2exceptionHOOH -1

Rules for Oxidation Numbers Oxidation # 4. H in a compound is +1

exception: metal hydrides -1 

H20

H2O +1

C12H22O11+1

NaH -1

Rules for Oxidation Numbers 5. The sum of the oxidation numbers must equal the

charge

oxidation #s

SO42-

Total charge= -2

-2

-8+6

+6

-2 x 4

oxidation #s

PO43-

Total charge= -3

-2

-8+5

+5

oxidation #s

Cr2O72-

Total charge= -2

-2

-14+12

12 / 2

+6

Ca(NO3)2

+2

Na2C4O8 = 0

-2

-16

+1

+2 +14

+7/2

CH3COOH

Rewrite

C2H4O2 = 0

-2

-4

+1

+40

0

Zn(OH)42-

= -2

+1

+4

-2

-8+2

+2

H2PtCl6 -1

= 0-6

+1 +4

+2 +4

H3PO3 = 0

-2

-6

+1 +3

+3 +3

Al(NO3)3 = 0

-2

-18

+3 +5

+3 +15

Na2O2

Exception - does not work, O must be -1

= 0

+1 -2

+2 -4

-1

-2

CaH2 -1+1

+2 = 0+2

+2

-2

Exception - does not work, H must be -1

Fe(H2O)5(OH)2+

0 -1

Fe(H2O)5(OH)2+

? + 0 -1 = +2

+3 0 -1

Fe(H2O)5(OH)2+

+3 + 0 -1 = +2

Oxidation Numbers

 HV2O4

- V6+

 Is the above equation oxidation or reduction?

+6+3

Increase

If the oxidation number increases it is oxidation.

Using Oxidation Numbers 

If the oxidation number of the central atom increases going from left to right, its oxidation.

ClO2- ClO4

-

Increase is oxidation

oxidation

4 electrons lostreducing agent

+3 +7

Using Oxidation Numbers 

If the oxidation number of the central atom decreases going from left to right, its reduction.

NO3- HNO2

decrease is reduction

reduction

2 electrons gainedoxidizing agent

+5 +3

Label the oxidizing agent and the reducing agent.  +7 +1 +5 0 ReO4

- + IO- IO3- + Re

Reduced Oxidized

Ox Agent Red Agent

Label the oxidizing agent and the reducing agent.  +3 +5 +5 +2 3As2O3 + 4NO3

- + 7H2O + 4H+ 6H3AsO4 + 4NO

oxidized reduced

red agent ox agent

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