5.2 Oxidation Numbers

What we need to learn…

• determine the following:– the oxidation number of an atom in a chemical

species– the change in oxidation number an atom undergoes

when it is oxidized or reduced – whether an atom has been oxidized or reduced by its

change in oxidation number• relate change in oxidation number to gain or loss

of electrons

Oxidation Numbers 

HV2O4- V6+

 Is the above equation oxidation or reduction?

Oxidation numbers are used to quickly determine if oxidation or reduction has occurred in a redox reaction.

OXIDATION NUMBER: The charge that an atom would possess if the species containing the atom were made up of ions.

Oxidation numbers are a somewhat fictitious concept, but, by using them, we can create half-reactions and use them in situations where no such half-reactions actually exist

The use of both half-reactions and oxidation numbers are simply procedures which can be used to get us to a balanced REDOX equation; whether the half-reactions or oxidation numbers are real is irrelevant.

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

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

N2

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

N2 0

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

N2 0

Pb

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

N2 0

Pb 0

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

N2 0

Pb 0

Na

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

N2 0

Pb 0

Na 0

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

N2 0

Pb 0

Na 0

O2

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

N2 0

Pb 0

Na 0

O2 0

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

Na+

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

Na+ +1

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

Na+ +1

Br-

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

Na+ +1

Br- -1

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

Na+ +1

Br- -1

MgSO4

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

Na+ +1

Br- -1

MgSO4 +2

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

Na+ +1

Br- -1

MgSO4 +2

Al2O3

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

Na+ +1

Br- -1

MgSO4 +2

Al2O3 -2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2 -2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2 -2

O2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2 -2

O2 0

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2 -2

O2 0

CH3COOH

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2. 

Na2O -2

CO2 -2

O2 0

CH3COOH -2

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2 or inperoxides -1.

 Na2O -2

CO2 -2

O2 0

CH3COOH -2

HOOH

Rules for Oxidation Numbers Oxidation # 3. The oxidation number of O

in a compound is –2 or inperoxides -1.

 Na2O -2

CO2 -2

O2 0

CH3COOH -2

HOOH exception -1

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1. 

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1. 

H2O

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1. 

H2O +1

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 

H2O +1

C12H24O11

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 

H2O +1

C12H24O11 +1

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 

H2O +1

C12H24O11 +1

H2

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 

H2O +1

C12H22O11 +1

H2 0

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 or inmetal hydrides -1.

 H2O +1

C12H24O11 +1

H2 0

NaH

Rules for Oxidation Numbers Oxidation # 4. The oxidation number for H

in a compound is +1 or inmetal hydrides -1.

 H2O +1

C12H24O11 +1

H2 0

NaH -1

Rules for Oxidation Numbers Oxidation # 5.All alkali and Alkali Earth metals within a

chemical species will be their traditional combining capacity (group 1 [= +1] and 2 [+2] on PT)

6. Most often the halogens (group 17) will be their traditional charge within a chemical species (-1)

NaCl (+1 and -1 respectively)

HClO3 (+1, +5* and -2 respectively) *not always -1

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

SO42-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s

SO42-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

SO42-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

SO42-

Total charge

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

SO42-

Total charge -8

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

SO42-

Total charge -8 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

SO42-

Total charge +6 -8 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +6 -2

SO42-

Total charge +6 -8 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +6 -2 +6

SO42-

Total charge +6 -8 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

PO43-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s

PO43-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

PO43-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

PO43-

Total charge

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

PO43-

Total charge -8

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

PO43-

Total charge -8 = -3

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

PO43-

Total charge +5 -8 = -3

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +5 -2

PO43-

Total charge +5 -8 = -3

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +5 -2 +5

PO43-

Total charge +5 -8 = -3

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

Cr2O72-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s

Cr2O72-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Cr2O72-

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Cr2O72-

Total charge

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Cr2O72-

Total charge -14

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Cr2O72-

Total charge -14 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Cr2O72-

Total charge +12 -14 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s -2

Divide by 2 Cr2O72-

Total charge +12 -14 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +6 -2

Cr2O72-

Total charge +12 -14 = -2

Rules for Oxidation Numbers 5. All other oxidation numbers are assigned so that the

sum of all of the oxidation numbers is equal to the charge on the chemical species.

Oxidation #

oxidation #s +6 -2 +6

Cr2O72-

Total charge +12 -14 = -2

Oxidation #

Ca(NO3)2

Oxidation #

+2

Ca(NO3)2

Oxidation #

+2

Ca(NO3)2 +2

Oxidation #

Na2C2O4

Oxidation #

+1 -2

Na2C2O4 +2 -8 = 0

Oxidation #

+1 -2

Na2C2O4 +2 +6 -8 = 0

Oxidation #

+1 +3 -2

Na2C2O4 +3+2 +6 -8 = 0

Oxidation #

CH3COOH

Oxidation #Rewrite

C2H4O2

Oxidation #Rewrite +1 -2 0

C2H4O2 +4 -4 = 0

Oxidation #

Zn(OH)42-

Oxidation #

+2 -2 +1 +2

Zn(OH)42-

+2 -8 +4 = -2

Oxidation #

H2PtCl6

Oxidation #

+1 +4 -1 +4

H2PtCl6 +2 +4 -6 = 0

Oxidation #

Na2O2

Oxidation #

+1 -2

Na2O2 +2 -4 = 0

Exception - does not work, O must be -1

Oxidation #

+1 -1 -1

Na2O2 +2 -2 = 0

Exception - does not work, O must be -1

Oxidation #

CaH2

Oxidation #

+2 +1

CaH2 +2 +2 = 0

Exception - does not work, H must be -1

Oxidation #

+2 -1 -1

CaH2 +2 -2 = 0

Exception - does not work, H must be -1

Oxidation Numbers

Using oxidation numbers in REDOX equations:

HV2O4- V6+

 Is the above equation oxidation or reduction?

Oxidation Numbers

+3  +6HV2O4

- V6+

 Is the above equation oxidation or reduction?

Oxidation Numbers

Increase

+3  +6HV2O4

- V6+

 Is the above equation oxidation or reduction?

If the oxidation number increases it is oxidation.

Homework….

• Read SWB pages 193 -195• Do exercises #3-5 page 194 and # 6 page

195