ChE102 Fall 2011 Class Notes ‐ Electrochemistry
E‐1
E) Electrochemistry
Petrucci
et al.,
10
th
Edition: 3.4, 5.1‐5.2, 5.4‐5.6, 20.1‐20.7
Oxidation States
•
Oxidation state (or number) is related to the number of electrons that an atom gains,
loses, or otherwise uses when combining with other atoms in a compound
•
The concept of oxidation state is arbitrary
•
it does not necessarily have a fundamental meaning
•
it is useful when understanding oxidation‐reduction reactions
NaCl:
Na loses 1 electron
→
Na
+
(Na has an oxidation state +1)
Cl gains 1 electron
→
Cl
‐
(Cl has an oxidation state ‐1)
MgBr
2
: Mg loses 2 electrons
→
Mg
2+
Each Br gains 1 electron
→
2 Br
‐
[see Petrucci et al., front cover for periodic table]
Rules for Assigning Oxidation States (O.S.)
•
The
O.S.
of an individual atom in a free element is 0 (zero)
•
Cl, Cl
2
, N
2
, O
2
, O, etc.
•
The total of the
O.S.
of all atoms in a neutral species is 0 (zero)
•
H
2
O (H: +1, O: ‐2)
•
The total of the
O.S.
of all atoms in an ion equals the charge of the ion
•
Fe
3+
(Fe: +3), MnO
4
‐
(Mn: +7, O: ‐2)
•
Group 1 metals (Li, Na, K, Rb, Cs, Fr) have
O.S. = +
1
•
Group 2 metals (Be, Mg, Ca, Sr, Ba, Ra) have
O.S.
= +2
•
F has
O.S.
= ‐1
•
Cl, Br, I, and At have
O.S.
= ‐1 except when combined with oxygen and other halogens
•
H has
O.S. =
+1, except when bonded to metals when its
O.S.
= ‐1 (LiH, for instance)
•
O has
O.S. = -
2 except for peroxides (H
2
O
2
and N
2
O
2
where
O.S.
= ‐1 for O) or when
combine with F (OF
2
where
O.S.
= +2 for O)
•
In binary compounds with metals:
•
group 17 elements (F, Cl, Br, I, At) have
O.S.
= ‐1
•
group 16 elements (O, S, Se, Te, Po) have
O.S.
= ‐2
•
group 15 elements (N, P, As, Sb, Bi) have
O.S.
= ‐3
•
All other oxidation numbers are selected to make the algebraic sum of the oxidation
numbers equal to the net charge of the molecule or ion
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ChE102 Fall 2011 Class Notes ‐ Electrochemistry
E‐2
Reduction-Oxidation (REDOX) Reactions
•
In a reduction‐oxidation reaction, one of the reagents is
oxidized
while the other is
reduced
•
A chemical substance is oxidized when it loses electrons:
Zn
→
Zn
2+
+ 2e
‐
(
O.S.:
0
→
+2)
•
A chemical substance is reduced when it gains electrons:
Cu
2+
+ 2e
‐
→
Cu (
O.S.:
+2
→
0)
•
Substance that causes the oxidation of another is called the
oxidizing agent
or
______________________.
•
Substance that causes the reduction of another is called the
reducing agent
or
______________________.
Examples of REDOX Reactions
Zn (
s
) + 2 H
+
(
aq
)
→
Zn
2+
(
aq
) + H
2
(
g
)
Zn (
s
)
→
Zn
2+
(
aq
) + ___ e
‐
(reducing agent)
___ H
+
(
aq
) + ___ e
‐
→
H
2
(
g
)
(oxidizing agent)
Zn (
s
) + Cu
2+
(
aq
)
→
Zn
2+
(
aq
) + Cu (
s
)
Zn (
s
)
→
Zn
2+
(
aq
) + ___ e
‐
(reducing agent)
Cu
2+
(
aq
) + ___ e
‐
→
Cu (
s
)
(oxidizing agent)
Cu (
s
) + 2 Ag
+
(
aq
)
→
Cu
2+
(
aq
) + 2 Ag (
s
)
Cu (
s
)
→
Cu
2+
(
aq
) + ___ e
‐
(reducing agent)
__ Ag
+
(
aq
) + ___ e
‐
→
___ Ag (
s
)
(oxidizing agent)
Zn (
s
) + SnCl
2
(
aq
)
→
Zn
2+
(
aq
) + Sn (
s
)
Zn (
s
)
→
Zn
2+
(
aq
) + ___ e
‐
(reducing agent)
Sn
2+
(
aq
) + ___ e
‐
→
Sn (
s
)
(oxidizing agent)
2 Zn (
s
) + O
2
(
g
)
→
2 ZnO (
s
)
___ Zn (
s
)
→
___ Zn
2+
(
s
) + ___ e
‐
(reducing agent)
O
2
(
g
) + ____ e
‐
→
2 O
2‐
(
s
)
(oxidizing agent)
ChE102 Fall 2011 Class Notes ‐ Electrochemistry
E‐3
Hydronium Ion (H
3
O
+
)
Zn (
s
) + 2 H
+
(
aq
)
→
Zn
2+
(
aq
) + H
2
(
g
)
Zn (
s
)
→
Zn
2+
(
aq
) + 2 e
‐
2 H
+
(
aq
) + 2 e
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- Winter '08
- SIMON
- Chemistry, Electrochemistry, Electron
-
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