Transcript Redox Reactions

CHAPTER 22
“Oxidation-Reduction Reactions”
LEO SAYS GER
or
OIL
REG
Oxidation and Reduction (Redox)
Early chemists saw “_oxidation_”
reactions only as the _combination
of a material with oxygen to produce
an oxide.
• For example, when methane
burns in air, it oxidizes and forms
oxides of _carbon____ and
_hydrogen___.
Oxidation and Reduction (Redox)
But, not all _oxidation processes_
that use oxygen involve burning___:
•Elemental iron slowly oxidizes to
compounds such as iron (III)
oxide, commonly called “_rust__”
•Bleaching stains in fabrics
•Hydrogen peroxide also releases
__oxygen__ when it decomposes.
Oxidation and Reduction (Redox)
A process called “_reduction__” is the
opposite of oxidation, and originally
meant the _loss of oxygen_ from a
compound
Oxidation and reduction _always
occur simultaneously.
The substance gaining oxygen (or
losing electrons) is _oxidized_, while the
substance losing oxygen (or gaining
electrons) is _reduced_.
Oxidation and Reduction (Redox)
Today, many of these reactions may
not even involve oxygen.
Redox currently says that _electrons_
are transferred between reactants.
Mg
+
S→
Mg2+
+
S2-
(MgS)
•The magnesium atom (which has zero charge) changes to a
magnesium ion by _losing 2 electrons, and is _oxidized_ to Mg2+
•The sulfur atom (which has no charge) is changed to a sulfide ion
by _gaining 2 electrons___, and is _reduced_ to S2-
Oxidation and Reduction (Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom _loses_ one electron:
1
0
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
LEO says GER :
- Losing electrons is oxidation, and the
substance that loses the electrons is
called the _reducing agent_.
- Gaining electrons is reduction, and
the substance that gains the electrons is
called the _oxidizing agent_.
Mg is the
reducing
agent
Mg is oxidized: loses e-, becomes a Mg2+ ion
Mg(s) + S(s) → MgS(s)
S is the oxidizing agent
S is reduced: gains e- = S2- ion
Not All Reactions are Redox Reactions
- Reactions in which there has been no
change in oxidation number are NOT
redox reactions.
Examples:
1 5 2
1
1
1
1
1 5 2
Ag N O 3 ( aq )  Na Cl ( aq )  Ag Cl ( s )  Na N O 3 ( 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 )
Corrosion
•Damage done to metal is costly to
prevent and repair
•Iron, a common construction metal often
used in forming steel alloys, corrodes by
being oxidized to ions of iron by oxygen.
•This corrosion is even faster in the
presence of salts and acids, because
these materials make _electrically
conductive solutions_ that make
electron transfer easy
Corrosion
•Luckily, not all metals corrode easily
•Gold and platinum are called _noble
metals_ because they are _resistant_ to
losing their electrons by corrosion
•Other metals may lose their electrons
easily, but are protected from corrosion by
the oxide coating on their surface, such as
__aluminum_.
•Iron has an oxide coating, but it is not
tightly packed, so water and air can
penetrate it easily
Corrosion
•Serious problems can result if bridges,
storage tanks, or hulls of ships _corrode_.
•Can be _prevented_ by a coating of oil,
paint, plastic, or another metal
•If this surface is scratched or worn away,
the protection is lost
•Other methods of prevention involve the
“sacrifice” of one metal to save the second
•Magnesium, chromium, or even zinc
(called galvanized) coatings can be applied
Rules for Assigning Oxidation Numbers
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
Rules for Assigning Oxidation Numbers
3) The oxidation number of _oxygen in
compounds_ is -2, except in
peroxides, such as H2O2 where it is -1.
4) The oxidation number of hydrogen in
compounds is +1, except in metal
hydrides, like NaH, where it is -1.
1
2
H2O
Rules for Assigning Oxidation Numbers
5) The sum of the oxidation numbers of the
atoms in the compound must equal 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
6) The _sum_ of the oxidation numbers in
the formula of a polyatomic ion is equal
to its _ionic charge_.
? 2
N O3

X + 3(-2) = -1
N
O
thus X = +5
? 2
S O4
2
X + 4(-2) = -2
S
O
thus X = +6
Reducing Agents and Oxidizing Agents
• An increase in oxidation number = oxidation_
• A decrease in oxidation number = _reduction_
1
0
Na  Na  e

Sodium is _oxidized_– it is the reducing agent
0

1
Cl  e  Cl
Chlorine is reduced – it is the oxidizing agent