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Wize University 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

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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.



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:
Extra Practice
Electrochemistry: Oxidation State
What is the oxidation state for the following: (enter answer as -1 or +2 for example)
P in P2O5:
+5
Cr in Na2Cr2O7:
+6
Oxidation state and resonance structure
Choose the correct statement.
Resonance
In the resonance hybrid structure of PO43–, the net charge on each O atom is:
Formal charges and oxidation numbers
Assign formal charges and oxidation numbers of all atoms in SOCl2.
Electrochemistry: Oxidation
Which of the following molecules contains a nitrogen atom in the -3 oxidation state?
Electrochemistry: Oxidation State of Carbon in Methane
What is the oxidation state of the central carbon atom in CH4?
Determine the oxidation number of each element in each of the following compounds:
a. H2CO3
b. N2
Oxidation States
Assign the oxidation states of the atoms indicated:
1. C in C2H5OH
2. N in NaN3
Oxidation state
What is the oxidation state of Ta in TaOF63−?
Oxidation state
What is the oxidation state of N in NH2OH?
More Oxidation State
What is the oxidation state for the following:
Note: If you're ever unsure of which element should have a + or - oxidation #, then think of which is more EN.
For our last example, was N or H more EN?
Electrochemistry: Oxidation state
What is the oxidation state of Br in BrO3- ?