REDOX REACTIONS
REDOX REACTIONS
• Reduction/Oxidation = REDOX
• Involve Transfer of Electrons
Between Chemical Species
• e.g.
• A LOSES Electrons (gains +ve
Charge)
∴is A is OXIDIZED
• B GAINS Electrons (gains –ve
Charge)
∴ B is REDUCED
“OXIDATION” Originally
Referred ONLY to
Reactions With Oxygen
• e.g. 4Al + 3O2 → 2Al2O3
(Alumina)
REDOX REACTION RULES
1. OXN. NO: For Atoms in
Neutral Molecule Must Add Up
To 0
* In Ion, Oxn. No. Must Add
Up To Charge On Ion
2 Oxn.Numbers By Group:
i.e. Gp. I = 1
Gp. II = 2
Gp. III = 3 (Usually)
REDOX REACTION RULES
(contd.)
3 a). All Halogen Oxn. Nos. =
-1
(i.e. F, Cl, Br, I)
Except in Compounds With O
or
Compounds with Other
Halogens
3 b). F Oxn. No. ALWAYS -1
4. H Oxn. No. = +1
(Except e.g. LiH, where H
Oxn. No. = -1)
REDOX REACTION RULES
(contd.)
•5. O Oxn. No. = -2 (In
Compounds, not as
Element)
Except: (In Compounds With
F, Where
Rule No. 3 Applies, or In
Compounds with
O-O Bonds, where Rules # 2
& 4 Apply)
In Peroxides: e.g. H2O2,
Na2O2
Oxn. No. = -1
In Superoxides: e.g. KO2 Oxn.
No. = - ½
SOME OXIDATION
NUMBER EXAMPLES
• NaOCl:
Na= +1 (Gp. I Elements Oxn. No.
= +1)
O = -2 (O is always –2; except in
compounds with F, where F is
always (–1) and in
peroxides (e.g. H2O2, where O= 1), and
superoxides (e.g. KO2 ,where O =
-½)
Cl = +1 (Cl is usually –1; except
as
here, in compounds with O and
with other
halogens, where it can have a
positive Oxn, No.)
FURTHER OXIDATION
NUMBER EXAMPLES
• Na2SO4:
Na = +1 (Gp. I element)
So Na2 = 2 x (+1) = (+2)
O = -2 (Basic Oxygen
oxidation No. value)
So O4 = 4 x (-2) = (-8)
_ S Oxidation No. (to ensure
neutral
molecule has net oxidation
No. of 0) must
be -8 +2 = +6
EVEN MORE OXIDATION
NUMBER EXAMPLES
• SO4 21. O oxidation No. = -2
So 4 x (-2) = -8
2. Add –2 for the 2 electrons
on the doubly charged Anion
= -10
So,
3. To give the anion a net
charge of –2, must S still have
an oxidation number of (+6).
{i.e. (-8) + (+6) = (-2)}.
EVEN MORE Exciting
OXIDATION NUMBER
STUFF
• Note that S can sometimes
have a negative Oxn. No:
i.e. Na2S:
Na will be (+1), so Total Oxn.
No.for Na2 = 2 x (+1) = +2
_ So, to make net Oxn. No.
for the molecule equal to 0, S
oxn. no. must be (-2) !