Chapter 19 Electrochemistry Mr. Watson HST. Mr. Watson Redox Reactions Oxidation loss of electrons...
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Transcript of Chapter 19 Electrochemistry Mr. Watson HST. Mr. Watson Redox Reactions Oxidation loss of electrons...
![Page 1: Chapter 19 Electrochemistry Mr. Watson HST. Mr. Watson Redox Reactions Oxidation loss of electrons Reduction gain of electrons oxidizing agent substance.](https://reader033.fdocuments.us/reader033/viewer/2022061514/56649e585503460f94b51ff6/html5/thumbnails/1.jpg)
Chapter 19Chapter 19
Electrochemistry
Mr. Watson HST
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Mr. Watson HST
Redox ReactionsRedox ReactionsOxidationloss of electrons
Reductiongain of electrons
oxidizing agentsubstance that cause oxidation by being reduced
reducing agentsubstance that cause oxidation by being oxidized
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Mr. Watson HST
ElectrochemistryElectrochemistry
In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.
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Mr. Watson HST
Oxidation-Reduction Oxidation-Reduction ReactionsReactions
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Mr. Watson HST
Voltaic CellsVoltaic Cells
harnessed chemical reaction which produces an electric current
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Mr. Watson HST
Voltaic CellsVoltaic Cells
Cells and Cell Reactions
Daniel's Cell
Zn(s) + Cu+2(aq) ---> Zn+2
(aq) + Cu(s)
oxidation half reaction
anode Zn(s) ---> Zn+2(aq) + 2 e-
reduction half reaction
cathode Cu+2(aq) + 2 e- ---> Cu(s)
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Mr. Watson HST
Voltaic CellsVoltaic Cells
copper electrode dipped into a solution of copper(II) sulfate
zinc electrode dipped into a solution of zinc sulfate
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Mr. Watson HST
Voltaic CellsVoltaic Cells
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Mr. Watson HST
Hydrogen ElectrodeHydrogen Electrode consists of a platinum
electrode covered with a fine powder of platinum around which H2(g) is bubbled. Its potential is defined as zero volts.
Hydrogen Half-Cell
H2(g) = 2 H+(aq) + 2 e-
reversible reaction
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Mr. Watson HST
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Mr. Watson HST
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Mr. Watson HST
Standard Reduction Standard Reduction PotentialsPotentials
the potential under standard conditions (25oC with all ions at 1 M concentrations and all gases at 1 atm pressure) of a half-reaction in which reduction is occurring
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Mr. Watson HST
Some Standard Reduction Some Standard Reduction Potentials Table 18-1, pg 837Potentials Table 18-1, pg 837
Li+ + e- ---> Li -3.045 v
Zn+2 + 2 e- ---> Zn -0.763v
Fe+2 + 2 e- ---> Fe -0.44v
2 H+(aq) + 2 e- ---> H2(g) 0.00v
Cu+2 + 2 e- ---> Cu +0.337v
O2(g) + 4 H+(aq) + 4 e- ---> 2 H2O(l) +1.229v
F2 + 2e- ---> 2 F- +2.87v
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Mr. Watson HST
If the reduction of mercury (I) in a voltaic cell is desired, the half reaction is:
Which of the following reactions could be used as the anode (oxidation)?
A, B
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Mr. Watson HST
Cell PotentialCell Potential
the potential difference, in volts, between the electrodes of an electrochemical cell
Direction of Oxidation-Reduction Reactionspositive value indicates a spontaneous
reaction
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Mr. Watson HST
Standard Cell PotentialStandard Cell Potential
the potential difference, in volts, between the electrodes of an electrochemical cell when the all concentrations of all solutes is 1 molar, all the partial pressures of any gases are 1 atm, and the temperature at 25oC
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Mr. Watson HST
Cell DiagramCell Diagram
the shorthand representation of an electrochemical cell showing the two half-cells connected by a salt bridge or porous barrier, such as:
Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s)
anode cathode
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Mr. Watson HST
Metal Displacement Metal Displacement ReactionsReactions
solid of more reactive metals will displace ions of a less reactive metal from solution
relative reactivity based on potentials of half reactions
metals with very different potentials react most vigorously
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Mr. Watson HST
Ag+ + e- --->Ag E°= 0.80 V
Cu2+ + 2e- ---> Cu E°= 0.34 V
Will Ag react with Cu2+?
yes, no
Will Cu react with Ag+?
yes, no
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Mr. Watson HST
Gibbs Free EnergyGibbs Free Energyand Cell Potentialand Cell Potential
G = - nFE
where n => number of electrons changed
F => Faraday’s constant
E => cell potential
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Mr. Watson HST
Applications of Applications of Electrochemical CellsElectrochemical Cells
Batteries– device that converts chemical energy into
electricity
Primary Cells– non-reversible electrochemical cell– non-rechargeable cell
Secondary Cells– reversible electrochemical cell– rechargeable cell
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Mr. Watson HST
Applications of Applications of Electrochemical CellsElectrochemical Cells
BatteriesPrimary Cells
"dry" cell & alkaline cell 1.5 v/cellmercury cell 1.34 v/cellfuel cell 1.23v/cell
Secondary Cellslead-acid (automobile battery) 2
v/cellNiCad 1.25 v/cell
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Mr. Watson HST
““Dry” CellDry” Cell
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Mr. Watson HST
““Dry” CellDry” Cell
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Mr. Watson HST
““Flash Light” BatteriesFlash Light” Batteries
"Dry" CellZn(s) + 2 MnO2(s) + 2 NH4
+ ----->
Zn+2(aq) + 2 MnO(OH)(s) + 2 NH3
Alkaline CellZn(s) + 2 MnO2(s) ---> ZnO(s) + Mn2O3(s)
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Mr. Watson HST
““New” Super Iron BatteryNew” Super Iron Battery
Mfe(VI)O4 + 3/2 Zn 1/2 Fe(III)2O3 + 1/2 ZnO + MZnO2
(M = K2 or Ba)Environmentally friendlier than MnO2 containing batteries.
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Mr. Watson HST
Mercury CellMercury Cell
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Mr. Watson HST
Lead-AcidLead-Acid(Automobile Battery)(Automobile Battery)
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Mr. Watson HST
Lead-AcidLead-Acid(Automobile Battery)(Automobile Battery)
Pb(s) + PbO2(s) + 2 H2SO4 = 2 PbSO4(s) + 2 H2O
2 v/cell
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Mr. Watson HST
Nickel-Cadmium (Ni-Cad)Nickel-Cadmium (Ni-Cad)
Cd(s) + 2 Ni(OH)3(s) = Cd(OH)2(s) + 2 Ni(OH)2(s)
NiCad 1.25 v/cell
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Mr. Watson HST
Hydrogen-Oxygen Fuel CellHydrogen-Oxygen Fuel Cell
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Mr. Watson HST
Automobile Oxygen SensorAutomobile Oxygen Sensor
Air, constant [O ]2
porous Pt electrodes
migrating O ions2-
measured potential difference
exhaust gas, unknown [O ]2
ZrO / CaO2
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Mr. Watson HST
Automobile Oxygen SensorAutomobile Oxygen Sensor
see Oxygen Sensor Movie from Solid-State Resources CD-ROM
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Mr. Watson HST
pH = (Eglass electrode - constant)/0.0592
pH MeterpH Meter
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Mr. Watson HST
Effect of Concentration on Cell Effect of Concentration on Cell Voltage: The Nernst EquationVoltage: The Nernst EquationEcell = Eo
cell - (RT/nF)ln Q
Ecell = Eocell - (0.0592/n)log Q
where Q => reaction quotient
Q = [products]/[reactants]
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Mr. Watson HST
EXAMPLE:EXAMPLE: What is the cell potential for What is the cell potential for the Daniel's cell when the the Daniel's cell when the [Zn[Zn+2+2] = 10 [Cu] = 10 [Cu+2+2]] ??
Q = ([ZnQ = ([Zn+2+2]/[Cu]/[Cu+2+2] = (] = (10 [Cu10 [Cu+2+2])/[Cu])/[Cu+2+2]] = 10 = 10
EEoo = (0.34 V) = (0.34 V)Cu coupleCu couple + (-(-0.76 V) + (-(-0.76 V)Zn coupleZn couple
n = 2, 2 electron changen = 2, 2 electron change EEcellcell = E = Eoocellcell - (0.0257/n)ln Q - (0.0257/n)ln Q
thus Ecell = (1.10 - (0.0257/2)ln 10) V
Ecell = (1.10 - (0.0257/2)2.303) V
Ecell = (1.10 - 0.0296) V = 1.07 V
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Mr. Watson HST
Nernst EquationNernst Equation
+0.83 V
salt bridge
H in2
1 atm
voltmeter
Pt electrode
Pt electrodeNaOH
1 M
H in2
1 atm
e– e–
anode (–)
HCl 1 M
cathode (+)
pn+
+
+
+
–
–
–
–
[H+]acid side [H+]base side
E = Eo – RTnF ln Q =
– 2.3 RTF log
[H+]base side[H+]acid side
[h+]p-type side [h+]n-type side
E (in volts) = – 2.3 RT
F log [h+]n-type side [h+]p-type side
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Mr. Watson HST
ElectrolysisElectrolysis
non-spontaneous reaction is caused by the passage of an electric current through a solution
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Mr. Watson HST
Electrolysis of KIElectrolysis of KI(aq)(aq)
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Mr. Watson HST
ElectrolysisElectrolysis
Electrolysis of Sodium chloride
(chlor-alkali process)
molten reactants => liquid sodium and chlorine gas
aqueous reactants => caustic soda (sodium hydroxide) and chlorine gas
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Mr. Watson HST
Aqueous Reactants => Aqueous Reactants => caustic soda (sodium hydroxide) and chlorine caustic soda (sodium hydroxide) and chlorine
gasgas
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Mr. Watson HST
Molten Reactants =>Molten Reactants =>liquid sodium and chlorine gasliquid sodium and chlorine gas
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Mr. Watson HST
ElectrolysisElectrolysis
Preparation of Aluminum (Hall process)
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Mr. Watson HST
Electrolytic Refining of Electrolytic Refining of CopperCopper
Cu(s) + Cu+2(aq) --> Cu+2
(aq) + Cu(s)
impure pure anode cathode
impurities: anode mud; Ag, Au, Pb
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Mr. Watson HST
Copper PurificationCopper Purification
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Mr. Watson HST
Copper PurificationCopper Purification
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Mr. Watson HST
Hall Process for AluminumHall Process for Aluminum
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Mr. Watson HST
Quantitative Aspects of Quantitative Aspects of ElectrolysisElectrolysis
1 coulomb = 1 amp sec1 mole e- = 96,500 coulombs
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
(45 min)#g Cr = ------------
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
definition of minute
(45 min)(60 sec)#g Cr = ---------------------
(1 min)
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
(45) (60 sec) (25 amp)#g Cr = ---------------------------
(1)
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
definition of a coulomb
(45)(60 sec)(25 amp)(1 C)#g Cr = -----------------------------
(1) (1 amp sec)
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
Faraday’s constant
(45)(25)(60)(1 C)(1 mol e-)#g Cr = ----------------------------------
(1)(1)(96,500 C)
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
atomic weight
(45)(60)(25)(1)(1 mol e-)(52 g Cr)#g Cr = -------------------------------------------
(1)(1)(96,500) (6 mol e-)
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Mr. Watson HST
ElectroplatingElectroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
(45)(60)(25)(1)(1 mol e-)(52 g Cr)#g Cr = -------------------------------------------
(1)(1)(96,500)(6 mol e-)
= 58 g Cr
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Mr. Watson HST
Rusting of IronRusting of Iron
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Mr. Watson HST
CorrosionCorrosion
O2(g) + 4 H+(aq) + 4 e- -----> 2 H2O(l)
Eo = 1.23 V
RustingFe(s) -----> Fe+2
(aq) + 2 e- Eo = 0.44 V
O2(g) + 4 H+(aq) + 4 e- -----> 2 H2O(l) Eo = 1.23 V
------------------------------------------- --------------2 Fe(s) + O2(g) + 4 H+
(aq) ----->
2 H2O(l) + Fe+2(aq) Eo = 1.67 V
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Mr. Watson HST
Preventing CorrosionPreventing Corrosion
painting
galvanizing
sacrificial anode