chapter 21. the study of the production of ___________ during chemical rxns and the changes produced...
TRANSCRIPT
ElectrochemistryChapter 21
Electrochemistrythe study of the production of ___________ during
chemical rxns and the changes produced by ___________ ___________.
Electrochemical reactions are __________________ reactions.
Oilrig:
1. Oxidation = loss of electronsa. the substance oxidized is the reducing agent
2. Reduction = gain of electronsa. the substance reduced is the oxidizing agent
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Electrical Conduction ___________ conduct electric currents well.
metallic conduction
Positively charged ions, ___________, move toward the ___________ electrode.
Negatively charged ions, ___________, move toward the ___________ electrode.
There are two kinds electrochemical cells.
___________ ___________ - nonspontaneous chemical reactions
___________ ___________ - spontaneous chemical reactions
The two parts of the reaction are physically separated.
–oxidation occurs at one cell–reduction occurs in the other cell
Conventions for electrodes:
___________ - electrode at which reduction occurs (red cat)
___________ - electrode at which oxidation occurs (an ox)
___________ electrodes do not react with the liquids or products of the electrochemical reaction. Graphite and Platinum are common inert electrodes.
Electrolytic CellsUse electrical energy to force
______________ (non thermodynamically favored) chemical reactions to occur.
Process called ___________. Used in:–plating of jewelry and auto parts–electrolysis of chemical compounds
Electrolytic cells consist of a:–container for reaction mixture–electrodes immersed in the reaction mixture–source of direct current
In all electrolytic cells, ___________ ___________ ___________ ___________ ___________ ___________ ___________
In all ___________ cells the most easily reduced species is reduced and the most easily oxidized species is oxidized.
Faraday’s Law of Electrolysis
The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell. During electrolysis, one ___________ of electricity (96,487 coulombs) reduces and oxidizes, respectively, one equivalent of the oxidizing agent and the reducing agent.
Corresponds to the passage of ___________ _______ through the electrolytic cell.
FaradayAmount of electricity that reduces one
equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode.
1 faraday of electricity = 6.022x1023 e-
1 faraday = 6.022x1023 e- = 96487 coulombs
1 eq. of oxidizing agent= gain of 6.022x1023 e-
1 eq. of reducing agent = loss of 6.022x1023 e-
Ex. 1) Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 3.20 amperes of current through a solution of palladium (II) sulfate for 30.0 minutes. (1 ampere = 1 coulomb per second)
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Ex. 2) Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in Ex. 1.
Voltaic or Galvanic Cells •Electrochemical cells in which a ___________
chemical reaction produces electrical energy.
•In all voltaic cells, ___________ ___________ ___________ ___________ ___________ ___________ ___________ ___________
•Cell halves are physically separated so that electrons (from redox reaction) are forced to travel through wires and creating a potential difference Examples: Car & flashlight batteries
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The Construction of Simple Voltaic Cells
Half-cell contains the ___________ and ___________ forms of an element (or other chemical species) in contact with each other.
Simple cells consist of: two pieces of metal immersed in solutions of their
ions wire to connect the two half-cells salt bridge to
▪ complete circuit▪ maintain neutrality▪ prevent solution mixing
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The Zinc-Copper Cell
Cell components:Cu strip immersed in 1.0 M copper (II) sulfateZn strip immersed in 1.0 M zinc (II) sulfatewire and a salt bridge to complete circuit
Initial voltage is 1.10 volts How do we find the initial voltage?
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Standard Electrode Potential There is an establish ___________
standard to measure potentials of a variety of electrodes
___________ ___________ ___________(SHE) assigned an arbitrary voltage of 0.000000…
V
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The Electromotive (Activity) Series of the Elements
Electrodes that force the SHE to act as an anode are assigned ___________ standard reduction potentials.
Electrodes that force the SHE to act as the cathode are assigned ___________ standard reduction potentials.
Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written.
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Uses of the Electromotive Series
Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written.
For example, the half-reaction for the standard potassium electrode is:
The large ___________ value tells us that this reaction will occur only under extreme conditions.
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Uses of the Electromotive Series
Compare the potassium half-reaction to fluorine’s half-reaction:
The large ___________ value denotes that this reaction occurs readily as written.
Positive E0 values tell us that the reaction tends to occur to the right larger the value, greater tendency to occur
to the right Opposite for negative values
Uses of the Electromotive Series
1. Choose the appropriate half-reactions from a table of standard reduction potentials.
2. Write the equation for the half-reaction with the more positive E0 value first, along with its E0 value.
3. Write the eqn for the other half-reaction as an oxidation with its oxidation potential, reverse the tabulated reduction half-reaction and change the sign of the tabulated E0.
4. Balance the electron transfer. Do NOT change the Eo values
5. Add the reduction and oxidation half-reactions and their potentials. This produces the equation for the reaction for which E0cell is positive, which indicates that the forward reaction is spontaneous.
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The Zinc-Copper Cell – How to find the initial voltage
Step 1) Find Copper and Zinc on the SRP sheet
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The Zinc-Copper Cell – How to find the initial voltage
Steps 2 & 3) What are the two half reactions?
(V)red:
ox:
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The Zinc-Copper Cell – How to find the initial voltage
Steps 4 & 5) Make sure e- are balanced and add the volts together
(V)red: ox:
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The Zinc-Copper Cell
Anode Zn Zn e
Cathode Cu e Cu
Cell rxn. Zn Cu Zn Cu
Spontaneous rxn. E V
0 2+
2+ 0
0 2+ 2+ 0
cell0
2
2
110.
In all ___________, electrons flow spontaneously from the negative electrode (anode) to the positive electrode (cathode).
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The Zinc-Copper Cell
Zn/Zn2+(1.0 M) || Cu2+(1.0 M)/Cu
species (and concentrations)in contact with electrode surfaces
electrode surfaces
salt bridge
The Zinc-Copper Cell
Short hand notation for voltaic cellsZn-Cu cell example
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The Copper - Silver Cell
Cell components:Cu strip immersed in 1.0 M copper (II)
sulfateAg strip immersed in 1.0 M silver (I)
nitratewire and a salt bridge to complete circuit
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The Copper - Silver Cell
Ex. 3) What is the initial voltage of this cell?
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The Copper - Silver Cell
Ex. 4) What is the shorthand notation of this cell?
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The Copper - Silver Cell
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The Copper - Silver Cell
Anode Cu Cu 2e
Cathode 2 Ag + e Ag
Cell rxn. Cu + 2 Ag Cu + 2 Ag
spontaneous rxn. E V
0 2
+ - 0
0 + 2+ 0
cell0
0 46. Compare the Zn-Cu cell to the Cu-Ag cell
Cu electrode is cathode in Zn-Cu cellCu electrode is anode in Cu-Ag cell
Whether a particular electrode behaves as an anode or as a cathode depends on what the other electrode of the cell is.
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The Copper - Silver Cell
Demonstrates that Cu2+ is a stronger oxidizing agent than Zn2+
Cu2+ oxidizes metallic Zn to Zn2+
Ag+ is is a stronger oxidizing agent than Cu2+
Ag+ oxidizes metallic Cu to Cu2+
Arrange these species in order of increasing strengths
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Uses of the Electromotive Series
Use standard electrode potentials to predict whether an electrochemical reaction at standard state conditions will occur spontaneously.
Ex. 5) Will silver ions, Ag+, oxidize metallic zinc to Zn2+ ions, or will Zn2+ ions oxidize metallic Ag to Ag+ ions? What is the overall value for Eo ?
E0 values are not multiplied by any stoichiometric relationships in this procedure.
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Electrode Potentials for Other Half-Reactions
Ex.6) Will tin(IV) ions oxidize iron (II) ions to iron (III) ions, or will iron (III) ions oxidize tin(II) ions to tin(IV) ions in acidic solution? What is the overall value for rxn?
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Effect of Conc. (or Partial Pressures) on Electrode Potentials - Nernst Equation
Standard electrode potentials are determined at thermodynamic standard conditions.1 M solutions1 atm of pressure for gasesliquids and solids in their standard statestemperature of 250 C
Potentials change if conditions are nonstandard.
___________ equation describes the electrode potentials at nonstandard conditions.
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Effect of Conc. (or Partial Pressures) on Electrode Potentials - Nernst Equation
E = E -2.303RT
nF log Q
E = potential under condition of interest
E potential under standard conditions
R = universal gas constant = 8.314 Jmol K
T = temperature in K
n = number of electrons transferred
F = the faraday = (96,487 C / mol e 1 JC V
J / V mol e
Q = reaction quotient
0
0
-.
-
)
,96 487
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Substitution of the values of the constants into the Nernst equation at 250 C gives:
The Nernst Equation used to calculate electrode potentials
and cell potentials for concentrations other than standard –state values. (pg 877)
Ex. 7) Calculate the cell potential for the following electrochemical cell. If the [Sn2+] = 4.5 x 10-1 M and [Ag+] = 0.110 M
Sn(s) + 2Ag+ Sn2+ + 2Ag(s)
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The Nernst equation can also be used to calculate the potential for a cell that consists of two nonstandard electrodes.
Ex. 8) Calculate the initial potential of a cell that consists of an Fe3+/Fe2+ electrode in which [Fe3+]=1.0 x 10-2 M and [Fe2+]=0.1 M connected to a Sn4+/Sn2+ electrode in which [Sn4+]=1.0 M and [Sn2+]=0.10 M . A wire and salt bridge complete the circuit.
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Relationship of E0cell to DG0
and K From previous chapters we know the
relationship of DG0 and K for a reaction.
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Relationship of E0cell to DG0
and K The relationship between DG0 and E0
cell is also a simple one.
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Relationship of E0cell to DG0
and K You can combine these two
relationships into a single relationship to relate E0
cell to K.
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Relationship of E0cell to DG0
and K Ex. 9) Calculate the standard Gibbs free
energy change, DG0 , at 250C for the following reaction.
Cu PbCuPb 22
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Relationship of E0cell to DG0
and K Calculate E0
cell using the appropriate half-reactions.
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Relationship of E0cell to DG0
and K Now that we know E0
cell , we can calculate DG0 . The negative value tells us that the reaction is
spontaneous as written.
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Relationship of E0cell to DG0
and K Ex. 10) Calculate the thermodynamic
equilibrium constant for the reaction in Ex. 9 at 250C.
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Corrosion
Metallic corrosion is the oxidation-reduction reactions of a metal with atmospheric components such as CO2, O2, and H2O.
4 Fe + 3 O 2 Fe O overall rxn.
Rxn. occurs rapidly at exposed points.
020
2 3
Corrosion = Oxidation of a metal
The oxidation of most metals by oxygen is spontaneous. Many metals develop a thin coating of metal oxide on the outside that prevents further oxidation
The presence of a salt accelerates the corrosion process by increasing the ease with which electrons are conducted from anodic to cathodic regions
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Corrosion Protection Some examples of corrosion protection.
1 Plate a metal with a thin layer of a less active (less easily oxidized) metal.
2. Connect the metal to a sacrificial anode, a piece of a more active metal.
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Corrosion Protection
3 Allow a protective film to form naturally.
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Corrosion Protection4 Galvanizing, coating steel with zinc, a
more active metal.
5. Paint or coat with a polymeric material such as plastic or ceramic.
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Primary Voltaic Cells
As a voltaic cell discharges, its chemicals are consumed.
Once chemicals are consumed, further chemical action is impossible.
Electrodes and electrolytes cannot be regenerated by reversing current flow through cell.
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The Dry Cell (LeClanche’ Cell)
One example is flashlight, radio, etc. batteries. Container is made of zinc
acts as an electrode Graphite rod is in center of cell
acts as the other electrode Space between electrodes is filled with a
mixture of: ammonium chloride, NH4Cl
manganese (IV) oxide, MnO2
zinc chloride, ZnCl2 porous inactive solid
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The Dry Cell (LeClanche’ Cell) As current is produced, Zn dissolves
and goes into solution as Zn2+ ions. Zn electrode is negative (anode).
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Secondary Voltaic Cells
Secondary cells are reversible, rechargeable.
Electrodes can be regenerated One example is the lead storage or car
battery.
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Lead Storage Battery Electrodes are two sets of lead alloy grids (plates). Holes in one grid are filled with lead (IV) oxide,
PbO2. Other holes are filled with spongy lead. Electrolyte is dilute sulfuric acid. When battery is discharging, spongy lead is
oxidized to lead ions and the plate becomes negatively charged.
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Lead Storage Battery
Cell reaction for a discharging lead storage battery is
Anode Pb SO PbSO 2 e
Cathode PbO + 4 H + SO + 2 e PbSO + 2 H O
Cell rxn Pb PbO + 4 H + SO 2 PbSO + 2 H O
s 42-
4 s-
2 s+
42- -
4 s 2
s 2 s+
42-
4 s 2
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Lead Storage Battery This battery can be recharged by passing
electricity through the cell in the opposite direction.
Voltaic cell is converted to galvanic cell.
At lead electrode, lead ions are reduced to lead atoms.
PbSO 2 e Pb SO
cathode - negative during charging
4 s-
s 42-
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The Nickel-Cadmium (Nicad) Cell
Rechargeable cell used in calculators, cameras, watches, etc.
Discharge half-reactions are:
V4.1E
Ni(OH)+Cd(OH) OH 2 + NiO Cdrxn Cell
OH 2+Ni(OH)e 2+OH 2+NiO Cathode
e 2 Cd(OH) OH 2 Cd Anode
0
s2s22s2s
-s2
-2s2
-2
-s
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The Hydrogen-Oxygen Fuel Cell
Fuel cells are batteries that must have their reactants continuously supplied in the presence of appropriate catalysts.
Hydrogen-oxygen fuel cell is used in the space shuttle.
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The Hydrogen-Oxygen Fuel Cell
The reaction combines hydrogen and oxygen to form water. Drinking water supply for astronauts.
Very efficient energy conversion 60-70%
l2g2g2
--2g2
-2
-g2
OH 2 O H 2 rxn Cell
OH 4e 4OH 2O1 Cathode
e 2OH 2OH 2 H2 Anode
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Bonus Questions
1. What are the explosive chemicals in the fuel cell that were aboard Apollo 13? Which tank exploded?
2. Some of the deadliest snakes in the world, for example the cobra, have venoms that are neurotoxins. Neurotoxins have an electrochemical basis. How do neurotoxins disrupt normal chemistry and eventually kill their prey?