Topic: Intro to Redox: Assigning
Oxidation Number
Do Now:
What is oxidation number for the following
Metals in the ionic compounds below
1. Fe2O3
2. FeO
3. AgCl
4. K2S
5. Mn2O7
Redox Reactions = oxidation-reduction
reaction
• Electrons transferred from 1 atom to another
• Redox reactions:
– All single-replacement rxns
– All combustion rxns
• Double Replace is never Redox
OIL RIG - memorize 
O – Oxidation
I - is
L - loss of electron
R - Reduction
I - is
G – gain of electron
REDOX
Oxidation & Reduction reactions are
complementary reactions
loses
• Something is oxidized - __________
electrons
gains
• While something is reduced - ________
electrons
• They occur every day
– Respiration
– Photosynthesis
– combustion
To understand these Redox Rxns we
must first understand Oxidation
numbers
Rules to Assigning Oxidation Numbers
( oxidation # =what an atoms charge
would be if it owned all it’s electron
exclusively (not sharing)
#1: individual atom (monatomic,
diatomic, polyatomic , but only 1
type of atom) = 0
#2
for an ion
oxidation number = charge
Ca+2 = +2
Cl-1 = -1
Al+3 = +3
#3
• When Fluorine is in a compound F is
always ____________
-1
CF4 C= +4
___ F = _-1_
OF2 O=+2
___ F = _-1_
#4
• When in a Oxygen is in a compound, O is
-2
AMLOST always ________
– EXCEPT when with Fluorine
• OF2 O= ____
+2 F = _-1_
– Except when O2 is bonded with group 1 or 2
metal
Peroxide
• Then it’s a ____________________
–K2O2 = K = +1
____O = -1
____
#5
H is nearly always +1
• except if bonded to metal then -1
• H2O H = ____O
+1 = ____
-2
+1
-2
• H2SO4 H = ____SO
4 = ____
-1
• LiH
Li = ____H
+1 = ____
• CaH2 Ca = +2
____H = -1
____
• NH3 N = ____H
-3 = ____
+1
#6
• For neutral compounds, sum of oxidation
numbers = 0
• H2O
=0
• H2SO4
• LiH
=0
• CaH2
2H = +1 = +2
1O= -2 = -2
2H = +1 = +2
1Li = +1 = +1
1SO4= -2 = -2 +2-2 =0
1H= -1 = -1
+1-1
1Ca = +2 = +2
2H= -1 = -2
+2-2
+2-2 =0
#7
• For polyatomic ions, sum of oxidation numbers =
charge of ion
+6
-2
• SO4
-2
+6
+6
-8
-2 = ____
1S = ___= ___ 4O= ___
+6 + ___
-8 =-2
___
+5 -2
• NO3-1
+5 ___
+5 3O= ___
-2 = ____
-6
1N = ___=
-6
+5
___ + ___ =-1
#8
• If covalent compound – both
elements are nonmetal, like to gain
electrons (both negative?! – NO)
–Assign negative to the more
electronegative element
–SiCl4: Si = +4, Cl = -1
Assign Oxidation #’s
• KCl
• CaBr2
• CO
• CO2
•
•
•
•
•
K = +1, Cl = -1
Ca = +2, Br = -1
C = +2, O = -2
C = +4, O = -2
Al(NO3)3
Al = +3, O = -2, N = +5
Na3PO4
Na = +1, O = -2, P = +5
H2S
H = +1, S = -2
+1
NH4
N = -3, H = +1
SO3-2
S = +4, O = -2