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ELECTROCHEMISTRY
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Brief Review
OXIDATIONloss of electron(s) by a species;
increase in oxidation number; increase in oxygen.
REDUCTIONgain of electron(s); decrease in
oxidation number; decrease in oxygen; increasein hydrogen.
OXIDIZING AGENTelectron acceptor; species
is reduced. REDUCING AGENTelectron donor; species is
oxidized.
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Electron Transfer Reactions
Electron transfer reactions are oxidation-
reduction orredox reactions.
Results in the generation of an electric current
(electricity) or be caused by imposing an
electric current.
Therefore, this field of chemistry is often called
ELECTROCHEMISTRY.
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OXIDATION-REDUCTION
REACTIONSDirect Redox Reaction
Oxidizing and reducing agents in direct contact.
Cu(s) + 2 Ag+(aq) ---> Cu2+
(aq) + 2 Ag(s)
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OXIDATION-REDUCTION
REACTIONSIndirect Redox Reaction
A battery functions by transferring electrons through an
external wire from the reducing agent to the oxidizingagent.
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CHEMICAL CHANGEELECTRIC
CURRENT
With time, Cu plates outonto Mg metal strip, and
Mg strip disappears.
Mg is oxidized and is the reducing agent
Mg(s) ---> Mg2+
(aq) + 2e-
Cu2+ is reduced and is the oxidizing agent
Cu2+(aq)
+ 2e- ---> Cu(s)
Mg metal
Cu2+ions
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Anode Cathode
Basic Concepts of Electrochemical Cells
Mg
Mg2+
ions
Cu
Cu2+
ions
wire
salt
bridge
electrons
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CHEMICAL CHANGEELECTRIC
CURRENT
To obtain a useful current,
we separate the oxidizing
and reducing agents so that
electron transfer occurs thruan external wire.
This is accomplished in a GALVANIC orVOLTAIC cell.
A group of such cells is called a battery.
Mg
Mg2+ions
Cu
Cu2+ions
wire
saltbridge
electrons
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Electrons travel thru external wire.
Salt bridge allows anions and cations tomove between electrode
compartments.
Mg
Mg2+
ions
Cu
Cu2+ions
wire
saltbridge
electronsOxidationAnodeNegative
ReductionCathodePositive
Cu2+ + 2e- --> CuMg --> Mg2+ + 2e-
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CELL POTENTIAL, (E0)
For Mg/Cu cell, potential is2.71Vat 25 C
This is theSTANDARD CELL POTENTIAL, Eo
E0 a quantitative measure of the tendency of
reactants to proceed to products when all are in
their standard states at 25 C.
If E0 is positive the reaction is spontaneous on
the otherwise the reaction is non- spontaneous.
Mg
Mg2+ions
Cu
Cu2+ions
wire
saltbridge
electrons
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Calculating Cell Voltage
Balanced half-reactions can be added together to
get overall, balanced equation.
If we know E0 for each half-reaction, we could get
E0 for net reaction.
Cu2+(aq) + 2e- ---> Cu(s)Mg(s) ---> Mg
2+(aq) + 2e
-
--------------------------------------------Cu2+
(aq)
+ Mg(s)
---> Mg2+(aq)
+ Cu(s)
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Note that this is thereduction potential. In
the reaction, Mg
underwent oxidation
such that the sign of
the E0
from -2.73.
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reduction potential E0
of Cu is + 0.34
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Mg/Cu Electrochemical Cell
Cu2+
(aq) + 2e-
---> Cu(s) Eo
= +0.34 VMg(s) ---> Mg2+
(aq) + 2e- Eo = - 2.37 V
---------------------------------------------------------------Cu2+(aq)
+ Zn(s) Zn
2+(aq) + Cu(s) E
o = +2.71 V
Cathode,positive, sinkfor electrons
Anode,negative,source ofelectrons
Mg
Mg2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
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+ -
KI(aq)
KIaq K+1 + I -1
electrolysis
There are three possible
species:
K+1 + e- K (s)
I2 + 2e- 2I-
H2O + 2e- H2(g) + 2OH -
Add: Phenolphthalein base indicator
(pink)
Starch - test for iodine
(blueviolet)
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Which has higher SRP?
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There are three possible species:
K+1 + e- K (s) Eo = -2.925
I2 + 2e- 2I- = + 0.535
H2O + 2e- H2(g) + 2OH
- = - 0.828
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Cathode : H2O + 2e- H2(g) + 2OH
- Eo = - 0.828
Anode : 2I- I2 + 2e- E0 = + 0.535
H2O + 2I- I2 + H2(g) + 2OH
-
E0cell = E0cat - E
0an= (-0.828) - (+ 0.535)
= - 1.363 V
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+ -
K+
e-
Anode Cathode
I-
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More About
Calculating Cell Voltage
Assume I- ion can reduce water.
2 H2O + 2e- ---> H2 + 2 OH- Cathode
2 I- ---> I2 + 2e- Anode
-------------------------------------------------2 I- + 2 H2O
--> I2 + 2 OH- + H2
Assuming reaction occurs as written,
E = Ecat- Ean= (-0.828 V) (+0.535 V) = -1.363 V
Minus E means rxn. occurs in opposite direction
(the connection is backwards or you are recharging the
battery)
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ACd Cd2+ + 2e-Fe2+ + 2e- Fe
B
Fe Fe2+ + 2e-
Cd2+ + 2e- Cd
Eo for a Voltaic Cell
All ingredients are present.Which way does reaction
proceed?
Write the full equation and its net
reduction potential.
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From the table, you see
Fe is a better reducing agent than Cd Cd2+ is a better oxidizing
agent than
Fe2+
Eo for a Voltaic Cell
Cd2+ + 2e- Cd E0 = -0.40Fe Fe2+ + 2e- E0 =
-0.44---------------------------------------------
Fe + Cd2+ Fe+2 + Cd E0 = 0.04
