Wize High School Grade 12 Chemistry Textbook > Electrochemistry
Oxidation States
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Oxidation States
Oxidation state: A way to describe the degree of oxidation of a chemical
Wize Tip
Tips to Determine Oxidation State:
1. Free elements: oxidation state = 0
Examples: Cl2(g) has an oxidation state of 0, Na(s) has an oxidation state of 0
2. Monoatomic ions: oxidation state = ionic charge
Examples: Cl-(aq) has an oxidation state of -1, Na+(aq) has an oxidation state of +1.
3. Hydrogen: oxidation state = +1
a. Exception: hydrogen in hydrides (e.g., NaH) – oxidation state = -1
4. Oxygen: oxidation state = -2
a. Exception: oxygen in peroxides (e.g., H2O2) – oxidation state = -1
5. For neutral molecules, oxidation numbers add up to zero.
For polyatomic ions, oxidation numbers add up to the ion charge.
Examples: Cl2(g) has an oxidation state of 0, NaCl oxidation numbers add up to 0, SO42- oxidation numbers add up to 2-.
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Oxidation States - Additional Information
Other Low Yield Tips to Help Determine Oxidation States
- Alkali metals: oxidation state= +1
- Alkaline earth metals: oxidation state= +2
- Nitrogen Group: oxidation state= -3
- Oxygen family: oxidation state= -2
- Halogens: oxidation state= -1
Note on Transition Metals
Transition metals can have different charges (ex. Pb2+ and Pb4+)
As a result, transition metals can also have different oxidation states!
Example:
What is the oxidation state of Cu in CuCl?
+1
What is the oxidation state of copper in Cu(NO3)2?
2+
When naming compounds involving transition metals, the oxidation state of the transition metal is written in Roman numerals and put in brackets:
- copper(I) chloride: CuCl
- copper(II) nitrate: Cu(NO3)2
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Example: Oxidation State Practice
What is the oxidation state of:
S in S2O3:
+3
O in S2O3:
-2
What about Pb in Pb2O3?
This one is different because this compound is a mixed oxidation state compound, so although Pb should have 2+ or 4+, here, we get an oxidation state of
3+
and that is ok because both Pb2+ and Pb4+ are present in the compound.
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Example: What are the oxidation numbers of each atom?
Na:
0
H2O
H oxidation #:
+1
O oxidation #:
-2
Overall charge:
0
SO22-
O oxidation #:
-2
S oxidation #:
+2
Overall charge: -2
NH4Cl
Cl oxidation #:
-1
H oxidation #:
+1
N oxidation #:
-3
Overall charge:
0
What is the oxidation state of Br in BrO3- ?
What is the oxidation state for the following: (enter answer as -1 or +2 for example)
a) P in P2O5:
b) Cr in Na2Cr2O7:
c) Al in Al2(SO4)3:
d) Na in Na2O2:
e) O in Na2O2:
f) N in NH3:
Review of Oxidation/Reduction:
- Redox reaction: involve changes in oxidation states or numbers.
- Oxidation AND reduction must occur in order for it to be called a redox reaction!
Oxidation
- Involves losing/gaining electrons:
- Increases/decreases oxidation #:
- More/less bonds to O:
- More/less bonds to H:
- Oxidizing/reducing agent: agent (aka reductant)
Reduction
- Involves losing/gaining electrons:
- Increases/decreases oxidation #:
- More/less bonds to O:
- More/less bonds to H:
- Oxidizing/reducing agent: agent (aka oxidant)