Oxidation-Reduction Reactions
“Redox”
LEO SAYS GER
Oxidation and Reduction Reactions
 Also known as redox or electron transfer
reactions
 One or more elements change oxidation
numbers
 All single replacement & combustion
reactions
 Some synthesis & decomposition
reactions
How can I tell if it is redox?
 If there is an element in the reactants or
products, you know it must be redox.
 In a redox reaction, electrons are lost and
gained.
 The element that loses electrons is oxidized.
The element that gains electrons is reduced.
LEO says GER (OIL RIG)
 If the oxidation numbers change during the
reaction, it must be redox
Oxidation and Reduction (Redox)
 Electrons are transferred
 Spontaneous redox rxns can transfer
energy
 Electrons (electricity)
 Heat
 Non-spontaneous redox rxns can be
made to happen with electricity
 So what are oxidation numbers?
Trends in Oxidation and Reduction
Active metals:
Lose electrons easily
Are easily oxidized
Are strong reducing agents
Active nonmetals:
Gain electrons easily
Are easily reduced
Are strong oxidizing agents
What are oxidation numbers?
 A tool to help determine which element is
oxidized and which is reduced.
 For ionic compounds, the oxidation
number is the element’s charge.
Rules for Assigning Oxidation Numbers
Rules 1 & 2
1. The oxidation number of any uncombined
element is zero
2. The oxidation number of a monatomic ion
equals its charge
0
0
1
1
2 Na  Cl 2  2 Na Cl
Oxidation Reduction Reactions
(Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom loses one electron:
0
1

Na  Na  e
Each chlorine atom gains one electron:
0

1
Cl  e  Cl
LEO says GER :
Lose Electrons = Oxidation
1
0
Na  Na  e

Sodium is oxidized
Gain Electrons = Reduction
0

1
Cl  e  Cl
Chlorine is reduced
Reducing Agents and Oxidizing Agents
 The substance reduced is the oxidizing agent
 The substance oxidized is the reducing agent
0
1

Na  Na  e
Sodium is oxidized – it is the reducing agent
0
1

Cl  e  Cl
Chlorine is reduced – it is the oxidizing agent
Rules for Assigning Oxidation Numbers
Rules 3 & 4
3. The oxidation number of oxygen in
compounds is -2
4. The oxidation number of hydrogen in
compounds is +1
1
2
H2O
Rules for Assigning Oxidation Number
Rule 5
5. The sum of the oxidation numbers
in the formula of a compound is 0
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca(O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers
Rule 6
6. The sum of the oxidation numbers in the
formula of a polyatomic ion is equal to
its charge
? 2
N O3

? 2
S O4
2
X + 3(-2) = -1
N
O
X + 4(-2) = -2
S
O
 X = +5
 X = +6
The Oxidation Number Rules SIMPLIFIED
1. The sum of the oxidation numbers in
ANYTHING is equal to its charge
2. Hydrogen in compounds is +1
3. Oxygen in compounds is -2
4. Charge on uncombined element is zero
Let’s try a few……..
ClO2
ClO-
N2 O
H2S
Oxidized or Reduced?
2Sr + O2  2SrO
2Li + S  Li2S
2Cs + Br2  2CsBr
3Mg + N2  Mg3N2
Half Reactions
Fe2+ + MnO4-  Fe3+ + Mn2+
Sn2+ + IO3-  Sn4+ + IS2- + NO3-  S + NO
NH3 + NO2  N2 + H2O
Not All Reactions are Redox Reactions
Reactions in which there has been no change
in oxidation number are not redox rxns.
Examples:
1 5 2
1
1
1
1
1 5 2
Ag N O3 (aq)  Na Cl (aq)  Ag Cl (s)  Na N O3 (aq)
1 2 1
1
6 2
1
6 2
1
2
2 Na O H (aq)  H 2 S O 4 (aq)   Na 2 S O 4 (aq)  H 2 O(l )
Identifying Electron Transfer in
Redox-Reactions
4Al(s) + 3O2(g)  2Al2O3(s)
 Each Al atom loses 3 electrons (for a total
of 12 electrons)
Each O atom gains 2 electrons (for a total
of 12 electrons)
Oxidation-Reductions Rxns (cont’d)
• Always have both oxidation and reduction
• LEO says GER
Lose e- = oxidized; Gain e- = reduced
• Split reaction into oxidation half-reaction
and a reduction half reaction
• Half reactions include electrons
Oxidation
• Oxidation is the process that occurs when:
• Oxidation number of an element increases
• Element loses electrons
• Compound adds oxygen
• Compound loses hydrogen
• Half reactions have electrons as products
Reduction
• Reduction is the process that occurs when:
• Oxidation number of an element decreases
• Element gains electron
• Compound loses oxygen
• Compound gain hydrogen
• Half reactions have electrons as reactants
Predicting Products of
Metal + Nonmetal Reactions
• Metal + Nonmetal  ionic compound
• Ionic compounds always solid unless
dissolved in water
• In the ionic compounds, the metal is now a
cation
• In the ionic compounds, the nonmetal is now
an anion
Predicting Products of
Metal + Nonmetal Reactions (cont’d)
• To predict direct synthesis of metal +
nonmetal:
1. Determine the charges on the cation and
anion from their positions on the periodic
table.
2. Balance the charges to get the formula of
the compound.
3. Balance the equation.