PP 28: Electrochemistry
Drill: Balance the following Rxn:
Fe(OH)3 + [Cr(OH)4]-1  Fe(OH)2 + CrO4-2
Metallic Conduction: The flow of electrons through a metal
Ionic Conduction: The movement of ions (electrolytes) through a solution (Electrolytic Conduction)
Electrode: The surface or point in which oxidation or reduction takes place at the interface of
metallic & electrolytic conduction


Anode: The negative electrode where oxidation takes place (An Ox)
Cathode: The positive electrode where reduction takes place (Red Cat)
Voltaic or Galvanic Cell: An electrochemical Cell in which a spontaneous oxidation-reduction
reaction takes place producing electrical energy

Batteries are made up of Voltaic Cells in series
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
o Salt bridge:
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
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Allows electrical contact between the two half-cells
Prevents mixing of the two half-cell solutions
Allows ions to flow maintaining electrical neutrality
Draw a Voltaic Cell:
Drill: Define Each:
Oxidation
Reduction
Anode
Cathode
Demonstrate how to determine everything in a voltaic cell using the following system:

The Voltaic Cell is made up of a solid copper electrode in a solution of aqueous CuBr2 in one half-cell
and a solid aluminum electrode in a solution of aqueous AlCl3 inn the other half-cell.
Steps for determining the Redox Reaction & Voltage of an Electrochemical Cell:
1. List all species (elements, molecules, or ions) that exist in each cell (possible reactants).
2. Using a standard Reduction potential table:
a. Write all the possible ½ reactions that could take place.
b. Record the voltage of each ½ reaction.
3. Separate all of the ½ reactions into either oxidation of reduction reactions.
4. Choose the ½ reaction that has the greatest voltage for both the oxidation & reduction groups.
5. Balance the electrons between the two ½ reactions.
6. Add the two ½ reactions & add the two voltages.
Problem: Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a cadmium
electrode in aqueous CdCl2 in one ½ cell, & a gold electrode in aqueous AuCl3 in the other.
Drill: Determine Reaction that takes place & the voltage generated when Cu+1 reacts with solid potassium.
Problem: Determine Eo for the following reaction: Zn(s) + 2 Ag+1(aq)  2 Ag(s) + Zn+2(aq)
REDOX Shorthand: Using the reaction above:
•
Zn|Zn+2||Ag+1|Ag
ox
red
•
Zn||Zn|Zn+2||Ag+1|Ag||Ag
an ox
red cat
Oxidation reactants | products || Reduction reactants | products
Anode | all that’s right above | Cathode
Voltaic Cell Problems:
•
Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a Cu electrode in
CuCl2(aq) is connected to a cell with a Zn electrode in ZnBr2(aq)
•
Determine the reaction that takes place & the voltage generated in a Voltaic 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 system with 2 Au electrodes in AuCl3(aq).
Voltaic Cell Problems:
•
Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a 2 Au electrodes
in AuCl3(aq) in both half cells:
•
Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a Ba electrode in
BaCl2(aq) is connected to a cell with a Sn electrode in SnI4(aq)
•
Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a iron electrode in
FeI2(aq) is connected to a cell with a copper electrode in CuBr2(aq)
•
Determine the reaction that takes place & the voltage generated in a Voltaic Cell with a Magnesium
electrode in MgCl2(aq) is connected to a cell with a gold electrode in AuCl3(aq)
Drill: A voltaic cell is made up of a calcium electrode in aqueous CaI2 in one chamber & a tin electrode
in aqueous SnBr2. Determine all of the substances that could be reactants in this system.
The class should divide into groups of 3 or 4 & complete a worksheet.
Drill: Determine all species that could react when a cell with a chrome electrode in CrBr3(aq) is
connected to a cell with a lead electrode in PbI2(aq)
Review for the test
•
•
Complete the problem in the drill by solving for everything.
Using the standard Reduction Potential Table, determine the element that is the strongest
reducing agent, & the one that is the strongest oxidizing agent.
•
Balance the following REDOX Reactions:
o SnO2 + S8  SnO + SO2
o N2O3 + K2CrO4  KNO3 + Cr+3
o SO + H2Cr2O7  H2SO4 + Cr+2
(in acid)
(in base)
Drill: Determine all when a cell with a zinc electrode in ZnCl2(aq) is connected to a cell with a silver
electrode in AgNO3(aq)
Extremely Important Electrochemical Reactions:
•
Lead Sulfate Battery
o
Pb + SO4-2  PbSO4 + 2 eo
PbO2 + 4 H+ + 2e-  PbSO4 + H2O
o Pb + PbO2 + 4 H+ + SO4-2  2 PbSO4 + H2O
•
Eo = 1.7 V
Eo = 0.3 V
Eo = 2.0 V
Iron Rusting
o
2 Fe  2 Fe+2 + 4eo
O2 + 2 H2O + 4e-  4 OHo 2 Fe + O2 + 2 H2O  2 Fe+2 + 4 OH-
Inter-relating Equation:
Go = Ho - TSo
Go = - RTlnKeq
Go = - nFEo
Calculate Go for the reaction in the drill: 2 Ag+ + Zn  2 Ag + Zn+2
Eo = +1.56 V
Drill: Determine rxn, Eo, for a voltaic cell with half-cells containing Ni(s) in NiCl2(aq) & Sn(s) in SnCl2(aq).
Determine Go, & Keq for the same system: Sn+2 + Ni  Ni+2 + Sn
Nernst Equation: E = Eo - (RT/nF)lnQ
Eo = +0.11 V
for non-standard conditions
Problem: Determine the voltage of a cell with a silver electrode in 1.0 M AgNO3 & a zinc electrode
in 0.010 M ZnCl2 at 27oC:
Problem: Determine rxn, Eo, for a voltaic cell with half-cells containing Cai(s) in CaCl2(aq) & Ag(s) in AgBr(aq).
Drill: Determine the voltage of a cell with an aluminum electrode in AlCl3(aq) & a zinc electrode in ZnCl2(aq).
Electrolysis: Using electrical energy (electricity) to force a non-spontaneous electrochemical reaction
Electrolytic Cell: An electrochemical cell where electrolysis is being performed
Steps for solving Reactions taking & the voltage required to run electrochemical cells: (same steps)
1. List all species (elements, molecules, or ions) that exist in each cell (possible reactants).
2. Using a standard Reduction potential table:
a. Write all the possible ½ reactions that could take place.
b. Record the voltage of each ½ reaction.
3. Separate all of the ½ reactions into either oxidation of reduction reactions.
4. Choose the ½ reaction that has the greatest voltage for both the oxidation & reduction groups.
5. Balance the electrons between the two ½ reactions.
6. Add the two ½ reactions & add the two voltages.
Electrochemical Cell Problems:
•
•
•
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)
Electrochemical Cell Problems:
•
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 AgNO3(aq) & an iron electrode in FeCl2(aq)
Drill: Determine all species that could react when electricity is passed through two
Pt electrodes in a solution containing CaCl2(aq) & MgF2(aq)
Electroplating & Electro-purifying: Using electrical energy (Electrolysis) to purify or plate an element
Electrolysis: Using electrical energy (electricity) to perform various chemical actions
• During electrolysis, oxidation & degradation will occur at the anode
• During electrolysis, reduction & electroplating will occur at the cathode
Basic Unit of Electricity: Amphere (A)
• 1 Amp = 1 coulomb/sec
Unit of Electric Charge: (C):
• The amount of any electroplating can be determined from coulombs because the
charge of an electron is known
Faraday’s Constant: (F):
• The charge of 1 mole of electrons
• ~96500 C/mole e
Electroplating Formula: I = q/t Where I = current in amps, q = charge coulombs, & t = time is seconds
• Charge = current x time
• C = A/s
• Mass can be determined from the charge
Problem: 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)
•
•
The problem can be worked using the formula (I = q/t) followed by a series of conversion factors.
The problem can also be worked using Saul’s Rule: nFm = MIt
o
o
o
o
o
o
n = the number of electrons transferred
F = Faraday’s Constant: 96497 coulombs / mole of electrons
m = mass in grams
M = molar mass in grams / mole
I = current in Amps
t = time in seconds
Problems:
•
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 :
•
Calculate the current required to purify 51.0 Kg of aluminum oxide in 96.5 hours.
•
Determine the voltage of a cell with a copper electrode in 0.10 M CuI & a zinc electrode
in 1.0M ZnCl2 at 27oC
•
Determine the voltage of a cell with a silver electrode in 0.10 M AgNO3 & a zinc electrode
in 1.0 M ZnCl2 at 27oC
Drill: Calculate the current required to convert 510 kg of aluminum oxide to pure aluminum 5.0 hours:
Problems:
•
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 time required to electroplate 19.7 mg of gold onto a plate by passing 965 mA
current through a solution of Au(NO3)3
•
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: