What are oxidation numbers? How to assign oxidation numbers? www.chemistryqa.com

1. Oxidation
A reaction in which the atoms o
f an element lose electrons
Cu0
(s) →Cu2+
(aq) +2 e-

2. Reduction:
A reaction in which the atoms of an
element gain electrons.
Ag+
(aq) + e- → Ag0
(s)

3. Oxidation numbers
Oxidation numbers are simply positive or negative
numbers assigned on the basis of a set of arbitrary
rules. It is important for you to realize that these
are not electric charges. For this reason, we write
an oxidation with the sign preceding the number,
as in +2 or “positive two.” This differs from an ion
charge of 2+ or “two positive” units of electric
charge.

4. Common Oxidation numbers
Rule 1 All elements in uncombined state 0
Rule 2 All monoatomic ions Same as their charge
Rule 3 Hydrogen in all compounds +1
Rule 4 Hydrogen in hydrides( NaH,MgH2) -1
Rule 5 Oxygen in all compounds -2
Rule 6 Oxygen in peroxides (H2O2) -1
Rule 7 All group 1 elements in compounds +1
Rule 8 All group 2 elements in compounds +2
Rule 9 All halogens in group 1 and 2
compounds
-1
Rule10 The sum of oxidation numbers in a
compound
0
Rule 11 The sum of oxidation numbers in a
polyatomic ion
Same as its charge

5. What is the oxidation number of
carbon in methane, CH4?
Assign oxidation number to hydrogen
Find oxidation number of carbon.
CH4
X +1
X+4x(+1)=0 (Hint: Total of oxidation numbers for a compound must be equal to 0)
X+4=0
X=-4

6. What is the oxidation number of
sulfur in sodium sulfate Na2SO4?
Sodium oxidation number=+1
Sulfur oxidation umber=x
Oxygen oxidation number=-2
2(+1)+x+4(-2)=0
2+x-8=0
X-6=0
x=+6

7. What is the oxidation number of
manganese in a permanganate ion,
MnO4
-?
Manganese oxidation number=x
Oxygen oxidation number=-2
X+4(-2)=-1( Hint: total of oxidation
number is equal to charge of the ion)
X-8=-1
X=+7

8. Determine the oxidation number of
S in SO2
X+2(-2)=0
X-4=0
X=+4

9. Determine the oxidation number of
Cl in HClO4
1+x+4(-2)=0
1+x-8=0
X-7=0
X=+7

10. Determine the oxidation number of
S in SO4
2-
X+4(-2)=-2
X-8=-2
X=+6

11. Determine the oxidation number of
Cr in Cr2O7
2-
2x+7(-2)=-2
2x-14=-2
2x=+12
X=+12/2
X=+6

12. Determine the oxidation number of
I in MgI2
2+2x=0
2x=-2
X=-1

13. Determine the oxidation number of
H in CaH2
2+2x=0
2x=-2
X=-1

14. Determine the oxidation number of
nitrogen in N2O
2x+1(-2)=0
2x-2=0
2x=+2
X=+1

15. Determine the oxidation number
of nitrogen in NO
X-2=0
X=+2

16. Determine the oxidation number
of nitrogen in NO2
X+2(-2)=0
X-4=0
X=+4

17. Determine the oxidation number
of nitrogen in NH3
X+3(+1)=0
X+3=0
X=-3

18. Determine the oxidation number
of nitrogen in N2H4
2x+4(+1)=0
2x=-4
X=-2

19. Determine the oxidation number
of nitrogen in NaNO3
1+x+3(-2)=0
1+x-6=0
X-5=0
X=+5

20. Determine the oxidation number
of nitrogen in N2
0
( Hint: all elements
have a oxidation
number of 0)

21. X+4(+1)+1(-1)=0
X+4-1=0
X+3=0
X=-3
Determine the oxidation number
of nitrogen in NH4Cl
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