electrochemistry
DESCRIPTION
ElectrochemistryTRANSCRIPT
Dr. S. M. Condren
Chapter 18
Electrochemistry
Dr. S. M. Condren
Redox ReactionsOxidation
• loss of electrons
Reduction
• gain of electrons
oxidizing agent
• substance that cause oxidation by being reduced
reducing agent
• substance that cause oxidation by being reduced
Dr. S. M. Condren
Electrochemistry
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.
Dr. S. M. Condren
Dr. S. M. Condren
Voltaic Cells
• harnessed chemical reaction which produces an electric current
Dr. S. M. Condren
Voltaic 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)
Dr. S. M. Condren
Voltaic Cells
• copper electrode dipped into a solution of copper(II) sulfate
• zinc electrode dipped into a solution of zinc sulfate
Dr. S. M. Condren
Voltaic Cells
Dr. S. M. Condren
Hydrogen 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
Dr. S. M. Condren
Dr. S. M. Condren
Dr. S. M. Condren
Standard Reduction Potentials
• 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
Dr. S. M. Condren
Some Standard Reduction Potentials 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
Dr. S. M. Condren
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
Dr. S. M. Condren
Cell Potential
• the potential difference, in volts, between the electrodes of an electrochemical cell
• Direction of Oxidation-Reduction Reactions
• positive value indicates a spontaneous reaction
Dr. S. M. Condren
Standard 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
Dr. S. M. Condren
Cell 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
Dr. S. M. Condren
Metal Displacement Reactions
• 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
Dr. S. M. Condren
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
Dr. S. M. Condren
Internet Report
• Due end of day, 5:00 pm, Dec. 1, 2001
Dr. S. M. Condren
Gibbs Free Energyand Cell Potential
G = - nFE
where n => number of electrons changed
F => Faraday’s constant
E => cell potential
Dr. S. M. Condren
Applications of Electrochemical 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
Dr. S. M. Condren
Applications of Electrochemical Cells
Batteries
Primary Cells
"dry" cell & alkaline cell 1.5 v/cell
mercury cell 1.34 v/cell
fuel cell 1.23v/cell
Secondary Cells
lead-acid (automobile battery) 2 v/cell
NiCad 1.25 v/cell
Dr. S. M. Condren
“Dry” Cell
Dr. S. M. Condren
“Dry” Cell
Dr. S. M. Condren
“Flash 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)
Dr. S. M. Condren
“New” 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.
Dr. S. M. Condren
Dr. S. M. Condren
Lead-Acid(Automobile Battery)
Dr. S. M. Condren
Lead-Acid(Automobile Battery)
Pb(s) + PbO2(s) + 2 H2SO4 = 2 PbSO4(s) + 2 H2O
2 v/cell
Dr. S. M. Condren
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
Dr. S. M. Condren
Dr. S. M. Condren
Automobile Oxygen Sensor
Air, constant [O ]2
porous Pt electrodes
migrating O ions2-
measured potential difference
exhaust gas, unknown [O ]2
ZrO / CaO2
Dr. S. M. Condren
Automobile Oxygen Sensor
• see Oxygen Sensor Movie from Solid-State Resources CD-ROM
Dr. S. M. Condren
pH = (Eglass electrode - constant)/0.0592
pH Meter
Dr. S. M. Condren
Effect of Concentration on Cell Voltage: The Nernst Equation
Ecell = Eocell - (RT/nF)ln Q
Ecell = Eocell - (0.0592/n)log Q
where Q => reaction quotient
Q = [products]/[reactants]
Dr. S. M. Condren
EXAMPLE: What is the cell potential for the Daniel's cell when the [Zn+2] = 10 [Cu+2] ?
Q = ([Zn+2]/[Cu+2] = (10 [Cu+2])/[Cu+2] = 10
Eo = (0.34 V)Cu couple + (-(-0.76 V)Zn couple
n = 2, 2 electron change Ecell = Eocell - (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
Dr. S. M. Condren
Nernst 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
Dr. S. M. Condren
Electrolysis
• non-spontaneous reaction is caused by the passage of an electric current through a solution
Dr. S. M. Condren
Dr. S. M. Condren
Electrolysis
Electrolysis of Sodium chloride
(chlor-alkali process)
molten reactants => liquid sodium and chlorine gas
aqueous reactants => caustic soda (sodium hydroxide) and chlorine gas
Dr. S. M. Condren
Electrolysis
Preparation of Aluminum (Hall process)
Dr. S. M. Condren
Electrolytic Refining of Copper
Cu(s) + Cu+2(aq) --> Cu+2
(aq) + Cu(s)
impure pure anode cathode
impurities: anode mud; Ag, Au, Pb
Dr. S. M. Condren
Dr. S. M. Condren
Quantitative Aspects of Electrolysis
• 1 coulomb = 1 amp sec
• 1 mole e- = 96,500 coulombs
Dr. S. M. Condren
Electroplating
EXAMPLE: How many grams of chromium can be plated from a Cr+6 solution in 45 minutes at a 25 amp current?
Dr. S. M. Condren
Electroplating
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 = ------------
Dr. S. M. Condren
Electroplating
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)
Dr. S. M. Condren
Electroplating
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)
Dr. S. M. Condren
Electroplating
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)
Dr. S. M. Condren
Electroplating
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)
Dr. S. M. Condren
Electroplating
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-)
Dr. S. M. Condren
Electroplating
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
Dr. S. M. Condren
Dr. S. M. Condren
Corrosion
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
Dr. S. M. Condren
Preventing Corrosion
painting
galvanizing
sacrificial anode