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
2
0
Na
0
Cl
2
2
+
1
Na
−
1
Cl

Oxidation Reduction Reactions
(Redox)
2
0
Na
0
Cl
2
2
+
1
Na
−
1
Cl
Each sodium atom loses one electron:
0
Na
→
+
1
Na
+ e
−
Each chlorine atom gains one electron:
0
Cl
+ e
− →
−
1
Cl

LEO says GER :
L ose E lectrons = O xidation
0
Na
→
+
1
Na
+ e
−
Sodium is oxidized
0
Cl
+
G ain E lectrons = R eduction e
− →
−
1
Cl Chlorine is reduced

Reducing Agents and Oxidizing Agents
 The substance reduced is the oxidizing agent
 The substance oxidize d is the reducing agent
0
Na
→
+
1
Na
+ e
−
Sodium is oxidized – it is the reducing agent
0
Cl
+ e
− →
−
1
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
H
2
O

Rules for Assigning Oxidation Number
Rule 5
5. The sum of the oxidation numbers in the formula of a compound is 0
1
2 +
2
−
2
+
1
H
2
O Ca ( O H )
2
2(+1) + (-2) = 0
H O
(+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
−
?
−
2
2
−
N O
3
S O
4
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……..
ClO
2
ClO -
N
2
O
H
2
S

Oxidized or Reduced?
2Sr + O
2
 2SrO
2Li + S  Li
2
S
2Cs + Br
2
 2CsBr
3Mg + N
2
 Mg
3
N
2

Half Reactions
Fe 2+ + MnO
4
 Fe 3+ + Mn 2+
Sn 2+ + IO
3
 Sn 4+ + I -
S 2+ NO
3
 S + NO
NH
3
+ NO
2
 N
2
+ H
2
O

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
( )
+
( )
→
( )
+
( )
2
+
1 2 1
+
1 6 2
( )
+
( )
→ +
+
1 6 2
+
1
−
2
( )
+
( )

Identifying Electron Transfer in
Redox-Reactions
4Al(s) + 3O
2
(g)  2Al
2
O
3
(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.