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Determining Oxidation Numbers

 

 

Oxidation number is the apparent charge of an atom in a compound.

Rules for assigning oxidation numbers.

Follow my order when you assign the numbers and you never can go wrong.

 

1. Free elements are assigned an oxidation state of zero.

ex. Fe, Ag, Br2

2. The Group 1 metals (Li, Na, K, Rb, and Cs) in compounds are always assigned an oxidation state of +1.

ex NaCl  (Na  +1, Cl -1)

3. The Group 2 metals (Be, Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2.

4.Fluorine in compounds is always assigned an oxidation state of -1. (The most electronegative element.)

5.  Hydrogen in compounds is assigned an oxidation state of +1.  

Exceptions Metal hydrides  (you already assigned group 1, so H must be -1)

NaH   (Na +1,  H-1)

6. Oxygen in compounds is assigned an oxidation state of -2.

Exceptions Peroxides  (you already assigned hydrogen +1, so O must be -1)

H2O2  (H +1,  O-1)

With Fluorine (you already assigned fluorine -1, so O must be +2)

OF2 ( O+2, F-1)

7. The sum of the oxidation states of all that atoms in a species must be equal to the net charge on the species.

SO42-

Since each O is -2 and the ox. numbers add to 2-

 S + 4(-2)=-2

S is +6

 

8. If all else fails, halogen in compounds is assigned an oxidation state of -1.

More examples

+1 -2

+1 -1

+1 -1

+1 -2

0

+6 -2

N2O

LiH

H2O2

KO2

O2

SO42-

 

Game-->Mahjong Chemistry With Oxidation Numbers

 

Past Regents Questions involving Oxidation Numbers

Jan 2007-45 What is the oxidation number of chromium in the chromate ion, CrO42–?
(1) +6   (3) +3
(2) +2   (4) +8

Jan 2006-21 What is the oxidation number assigned to manganese in KMnO4?
(1) +7     (3) +3
(2) +2     (4) +4

Jan 2003-4 In which compound does chlorine have the highest oxidation number?

(1) NaClO         (3) NaClO3
(2) NaClO2        (4) NaClO4

June 2009-44 Given the balanced equation representing a reaction:
Fe2O3 + 2Al--> Al2O3 + 2Fe
During this reaction, the oxidation number of Fe changes from
(1) +2 to 0 as electrons are transferred
(2) +2 to 0 as protons are transferred
(3) +3 to 0 as electrons are transferred
(4) +3 to 0 as protons are transferred

Jan 2009-44 What is the oxidation number of sulfur in Na2S2O3?
(1) -1      (3) +6
(2) +2     (4) +4

More Practice

What is the oxidation number of . . .

Highlight to reveal answers
1) N in NO3 The O is -2 and three of them makes -6. Since -1 is left over, the N must be +5
2) C in CO32 The O is -2 and three of them makes -6. Since -2 is left over, the C must be +4
3) Cr in CrO42 The O is -2 and four of them makes -8. Since -2 is left over, the Cr must be +6
4) Cr in Cr2O72 The O is -2 and seven of them makes -14. Since -2 is left over, the two Cr must be +12.  Each Cr is +6.
5) Fe in Fe2O3 The O is -2 and three of them makes -6. Each Fe must then be +3
6) Pb in PbOH+    +2
7) V in VO2+    +5
8) V in VO2+    +4
9) Mn in MnO4    +7
10) Mn in MnO42    +6

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