Transcript 28-Electrochemistry

Balance Redox Rxns:
Fe(OH)3 +
-1
[Cr(OH)4]
Fe(OH)2 +
-2
CrO4
Electrochemistry
Metallic Conduction
•The flow of electrons
through a metal
Ionic Conduction
•The movement of
ions (electrolytes)
through a solution
•Electrolytic Conduct.
Electrode
•The surface or point
in which oxidation or
reduction takes place
Anode
•The electrode
where oxidation
takes place
•An Ox (-)
Cathode
•The electrode
where reduction
takes place
•Red Cat (+)
Voltaic or
Galvanic Cell
Electrochemical
Cell in which:
a spontaneous
oxidation-reduction
reaction produces
electrical energy
Voltaic or
Galvanic Cell
Batteries are
made up of VCs
Half-Cell
•A cell where
either oxidation
or reduction
takes place
•A half-cell will
not work by itself
•Both half-cells
are required
•An electrochemical
cell must have two
half-cells
connected by a salt
bridge
Salt Bridge
1) Allows electrical
contact between
the two half-cells
2) Prevents
mixing
of the two halfcell solutions
3) Allows ions to
flow maintaining
electrical
neutrality
Draw a
Voltaic Cell
made up of
two half-cells
Drill: Define Each
• Oxidation
• Reduction
• Anode
• Cathode
Determining the
Redox Rxn &
Voltage of an
Electrochemical
Cell
1) List all species
(molecules,
elements, & ions)
(reactants) that
exist in each cell
From the Redox
Tables write all
possible halfreactions that could
occur in the system
2a)
2b ) Record
the
voltage for each
half-rxn. If rxn is
reversed, change
sign.
3) Label the
oxidation halfrxn that has the
highest voltage
4) Label the
reduction halfrxn that has the
highest voltage
5) Balance the
electrons
between the two
half-rxns
6a) Add the two
half-rxns to
obtain the full
electrochemical
reaction
6b) Add the
voltage of each
half-rxn to obtain
the std. voltage
required
Determine
o
E
Zn(s) + 2
+1
Ag
2 Ag(s) +
+2
Zn
(aq)
(aq)
REDOX Shorthand
+2
+1
•Zn|Zn ||Ag |Ag
ox
red
+2
+1
•Zn||Zn|Zn ||Ag |Ag||Ag
an
ox
red
cat
Drill: Determine
Shorthand Rxn &
+1
voltage when Cu
is reacts with solid
potassium
Voltaic Cell
Problems
Determine all when a
cell with a Cu electrode
in CuCl2(aq) is
connected to a cell with
a Zn electrode in
ZnBr2(aq)
Drill: Determine all
species that could react
when a cell with an Fe
electrode in FeCl3(aq) is
connected to a cell with a
Mn electrode in MnCl2(aq)
Determine all when a
cell with a Fe electrode
in FeCl3(aq) is connected
to a cell with a Mn
electrode in MnCl2(aq)
Drill: Determine all
species that could react
when a cell with an Fe
electrode in FeCl2(aq) is
connected to a cell with a
Mg electrode in MgCl2(aq)
Determine all when a
cell with a Mg
electrode in MgCl2(aq)
is connected to a cell
with a Au electrode in
AuCl3(aq)
Determine all when a
cell with a Cd electrode
in CdCl2(aq) is
connected to a cell with
a Cu electrode in
CuI(aq)
What could
happen if you
dissolve AuCl3 in
water?
Drill: A voltaic cell is made up
of a iron electrode in an aqueous
of FeI2 in one chamber & a
copper electrode in an aqueous
CuBr2. Determine all of the
substances that could be reactants
in this system.
A voltaic cell is made up
of a iron electrode in an
aqueous of FeI2 in one
chamber & a copper
electrode in an aqueous
CuBr2. Determine all in
this system.
Drill: Determine all
species that could react
when a cell with an Cr
electrode in CrBr3(aq) is
connected to a cell with a
Sn electrode in SnI2(aq)
Determine all when a
cell with an chromium
electrode in CrBr3(aq) is
connected to a cell
with a tin electrode in
SnI2(aq)
Using the standard
Reduction Potential Table,
determine the element that
is the strongest reducing
agent, & the one that ic the
strongest oxidizing agent.
Balance Redox Rxn:
SnO2 + S8
SnO + SO2
in acid
Balance Redox Rxn:
N2O3 + K2CrO4
+3
KNO3 + Cr
in base
Balance Redox Rxn:
SO + H2Cr2O7
H2SO4 +
+2
Cr
Drill: What is the
best reducing agent
and the best
oxidizing agent on
the chart?
Extremely
Important
Electrochemical
Reactions
Lead Sulfate Battery
Pb + SO4-2 
PbSO4 + 2e- Eo = 1.7 V
PbO2 + 4H+ + 2e- 
PbSO4 + H2O
Eo = 0.3 V
Pb + PbO2 + 4H+ SO4-2 
2 PbSO4 + H2O Eo = 2.0 V
Iron Rusting
2Fe  2Fe+2 + 4e-
O2 + 2H2O +
4e

4OH
2Fe + O2 + 2H2O  2Fe+2 + 4OH-
Relating Equations
o
DG
o
DH
o
TDS
=
o
DG = -RTlnKeq
o
o
DG = -nFE
o
E,
Determine rxn,
o
DG , & Keq for a
voltaic cell with halfcells containing Ni(s)
in NiCl2(aq) & Sn(s)
in SnCl2(aq).
Nernst Equation
o
E = E - (RT/nF)lnQ
for non-standard
conditions
• Determine the voltage
of a cell with a silver
electrode in 1.0 M
AgNO3 & a zinc
electrode in 0.010 M
o
ZnCl2 at 27 C
Drill: Determine the
voltage of a cell with
an aluminum electrode
in 1.0 M AlCl3 & a zinc
electrode in 0.010 M
o
ZnCl2 at 27 C
Determine the voltage
of a cell with an
calcium electrode in
1.0 M CaCl2 & a silver
electrode in 0.010 M
o
AgBr at 27 C
Typical Dry
Cell Battery
Electrolysis
•Using electricity to
force a nonspontaneous
electrochemical rxn
Electrolytic Cell
•Chemical cell
where electrolysis
is being performed
How to
determine
everything in an
electrolytic cell
1) List all species
(molecules,
elements, & ions)
(reactants) that
exist in each cell
From the Redox
Tables write all
possible halfreactions that could
occur in the system
2a)
2b ) Record
the
voltage for each
half-rxn. If rxn is
reversed, change
sign.
3) Label the
oxidation halfrxn that has the
highest voltage
4) Label the
reduction halfrxn that has the
highest voltage
5) Balance the
electrons
between the two
half-rxns
6a) Add the two
half-rxns to
obtain the full
electrochemical
reaction
6b) Add the
voltage of each
half-rxn to obtain
the std. voltage
required
• Determine the rxn that
takes place when 1.5 V
is passed through two
Pt electrodes in a
solution containing
MgI2(aq) & ZnCl2(aq)
• Determine the rxn that
takes place when
4.0 V is passed through
two Pt electrode in a
solution of NaCl(aq)
Determine the rxn that
takes place when
electricity is passed
through two Pt
electrode in molten
NaCl
Drill: Determine all
species that could react
when electricity is passed
through two Pt electrode
in a solution containing
CaCl2(aq) & FeF2(aq)
More
Electrolytic
Problems
Determine the rxns that
take place when 2.0 V of
electricity is passed
through two Pt electrode
in a solution containing
CaCl2(aq) & FeF2(aq)
Determine the rxns that
takes place when 1.8 V
of electricity is passed
through two Pt
electrodes in ZnCl2(aq)
Determine the rxns that
takes place when 2.0 V
of electricity is passed
through two Pt
electrodes in ZnCl2(aq)
• Determine the voltage
of a cell with a silver
electrode in 1.0 M
AgNO3 & an iron
electrode in 0.10 M
o
FeCl2 at 27 C
Drill: Determine all
species that could react
when electricity is passed
through two Pt electrode
in a solution containing
CaCl2(aq) & MgF2(aq)
Determine all the
reactions that take place
when electricity is passed
through two Pt electrode
in a solution containing
CaCl2(aq) & MgF2(aq)
Electroplating &
Electro-purifying
Electrolysis
• During electrolysis, oxidation
& degradation would occur at
the anode while reduction &
electroplating would occur at
the cathode
Power Supply
Metal salt solution
Impure Metal
Anode
Pure Metal
Cathode
Standard Unit of
Electricity
•Amphere (A)
•1 Amp = 1 coulomb/sec
Unit of Electric Charge
• Coulomb (C)
• The amount of any
electroplating can be
determined from coulombs
because the charge of an
electron is known
Faraday’s Constant
• The charge of 1 mole
of electrons
• ~96500 C
Electroplating
Formula
•Charge = current x time
•Mass can be determined
from the charge
• Determine the mass of
copper plated onto the
cathode when 9.65 mA is
passed for 2.5 Hrs
through two Cu
electrodes in a solution
containing CuCl2(aq)
• Determine the voltage
of a cell with a copper
electrode in 0.10 M
CuI & a zinc electrode
o
in 1.0 M ZnCl2 at 27 C
• Determine the voltage
of a cell with a silver
electrode in 0.10 M
AgNO3 & a zinc
electrode in 1.0 M
o
ZnCl2 at 27 C
The test on
electrochemistry
will be on
____day.
Current Formula
• Current = charge/unit time
• Amps = coul/sec
• Amount (mass, volume,
moles, etc) can be
determined from the charge
Calculate the mass of
copper plated onto the
cathode when a 9.65
mAmp current is applied
to a solution of CuSO4
for 5.0 minutes.
Calculate the years
required to plate 216 kg
of silver onto the
cathode when a 96.5
mAmp current is applied
to a solution of AgNO3
Drill: Calculate the
current required to
purify 510 kg of
aluminum oxide in
5.0 hours
Balance the Rxn
KMnO4 + HCl
MnO2 + KClO2
Calculate the time
required to electroplate
19.7 mg of gold onto a
plate by passing 965 mA
current through a
solution of Au(NO3)3
• Determine the voltage
of a cell with a silver
electrode in 5.0 M
AgNO3 & an zinc
electrode in 0.25 M
o
ZnCl2 at 27 C
Determine the rxn that
takes place when 1.0 V
is passed through two
Pt electrodes in a soln
containing NaI(aq) &
CoCl2(aq).
Calculate the time required
to purify a 204 kg of ore
that is 60.0 % Al2O3 by
applying a 965 kA current
through molten ore sample:
Drill: Aluminum ore is
purified by electrolysis.
Calculate the time required
to purify a 51 kg of ore that
is 75.0 % Al2O3 by
applying a 9.65 kA current
through molten ore sample:
Calculate the time required
to gold plate a 0.20 mm
layer onto a plate
2
(SA = 750 cm ) by passing
965 mA current through a
solution of AuCl3
3
(DAu = 20 g/cm )
Current, Mass,
Time Formula:
Saul’s Rule
nFm = MWIt
A voltaic cell with a silver
+
electrode in 0.10 M Ag & a
+2
zinc electrode in 1.0 M Zn
o
at 27 C is allowed to react for
5.0 mins at 9.65 A.
o
o
Calculate: E , E, DG , & mass
increase of the cathode.
A voltaic cell with a gold
+3
electrode in 0.0010 M Au &
a zinc electrode in 10.0 M
+2
o
Zn at 27 C is allowed to
react for 5.0 hrs at 9.65 A.
o
o
Calculate: E , E, DG , & mass
increase of the cathode.