Transcript Tong.ch_16.0910 - NordoniaHonorsChemistry

Redox Reactions
Day One
REDOX-OXIDATION STATES
16.2-16.3
Oxidation-Reduction
• Process of gaining or losing electrons
• Oxidation: losing electrons
• Reduction: Gaining Electrons
OIL RIG
How do we Identify if something is
being reduced or oxidized?
• Oxidation States (Oxidation Numbers)
– Computed for each element based on the number
of electrons assigned
• There are FIVE (5) Rules for assigning
Oxidation States
Oxidation State RULES
1. Oxidation state of an atom in a free element
is ZERO (0).
2. Hydrogen always has an oxidation number of
(+1) , except in hydrides
3. Oxygen always has an oxidation number of
(-2), except in peroxides
Oxidation State Rules
4. The sum of oxidation states of all atoms in:
– Molecule = (0)
– Ion = charge of the ion
5. In their compounds,
– Group I metal = (+1)
– Group II metals (+2)
Summary of Rules
1st- There are many that don’t have rules and are exceptions!
• Rule Number One
All free, uncombined elements have an oxidation number of zero.
This includes diatomic elements such as O2 or others like P4 and S8.
• Rule Number Two
Hydrogen, in all its compounds except hydrides, has an oxidation number of
+1
• Rule Number Three
Oxygen, in all its compounds except peroxides, has an oxidation number of -2
• We can also assume in most cases group 1’s are +1
• With polyatomics- look at each ion separate and then assign numbers
• Oxidation #’s are the # of atoms- not after multiplication.
LETS TRY A FEW!!!
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Br2
K+1
LiF
CO2
SO4-2
Na2O2
Br = 0, Br= 0
1+
Li = 1+, F = 1- total = 0
O = -2 each = -4 Total = 0
C = +4
O= -2 ea. = -8
S = +6
Total = -2
Na = +1ea. = +2
O = -2 ea. = -4
Na has priority so it sets the rule so
O will have a -1ea so the total = 0
Identify Redox Reaction
• You can identify a redox by checking to see if
one compound was reduced and the other
was oxidized.
• Remember:
– Oxidation = an increase in oxidation state
– Reduction = a decrease in oxidation state
Day Two
½ REACTIONS
16.4
Balancing Redox Reactions
1. Assign Oxidation Numbers
2. Split reaction into two ½ reactions
3. Balance only the atom undergoing the change
4. Count up the # of electrons transferred
5. Balance the electrons by multiplying the ½ rxn
6. Bring the equation back together
7. Balance the rest of the reaction
Al(s) + Ag+1  Al+3 + Ag(s)
Ag+1 = +1 → Al+3 = +3
Al = 0
Ag= 0
Al → Al +3
Ag + → Ag
0
+1
3+
0
3e-
1eAl →Al +3
3 Ag +→ 3Ag
Al + 3Ag+ → Al +3 + 3Ag
SnCl3 + Fe  SnCl2 + FeCl2
Day Three
½ REACTIONS
ACIDIC AND BASIC SOLN
Balancing Redox Reactions
in Acidic Solutions
1. Assign Oxidation Numbers
2. Split reaction into two ½ reactions
3. Balance only the atom undergoing the change
4. Count up the # of electrons transferred
5. Balance the electrons by multiplying the ½ rxn
6. Bring the equation back together
7. Balance the rest of the reaction
a. Except O’s and H’s
b. Balance O’s by adding H2O
c. Balance H’s by adding H+
HNO3 + H2S  S + NO
Acidic Solution
Balancing Redox Reactions
in Basic Solutions
1. Assign Oxidation Numbers
2. Split reaction into two ½ reactions
3. Balance only the atom undergoing the change
4. Count up the # of electrons transferred
5. Balance the electrons by multiplying the ½ rxn
6. Bring the equation back together
7. Balance the rest of the reaction
a.
b.
c.
d.
e.
f.
Except O’s and H’s
Balance O’s by adding H2O
Balance H’s by adding H+
Add OH to both sides to balance H+
Clean up H+ & OH- (by making water)
Reduce/cancel excess H2O’s
NO2  NO-2 + NO3-1
Basic Soln