Interconversion of electrical & chemical potential energy · reaction is converted to electrical...
Transcript of Interconversion of electrical & chemical potential energy · reaction is converted to electrical...
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Electrochemistry
Interconversion of electrical
& chemical potential energy
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Electrochemistry
Always involves redox reactions.
1. Energy released by spontaneous reaction is converted to electrical energy (voltaic or galvanic cell)
OR2. Electricity drives nonspontaneous
reaction (“electrolytic cell”)
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Redox Reactions
Mg + 2HCl MgCl2 + H2
For example:
ox # 0 +1 +2 0
• Mg is oxidized (loses e-)• H+ is reduced (gains e-)• Cl- is a spectator
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Redox Reactions
For you to review:
Assigning oxidation numbersActivity series
more active metal oxidationmore active nonmetal reduction
Balancing redox reactions (attached)
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Activity SeriesWill this reaction occur spontaneously?
Mg + Co2+ Mg2+ + Co
Yes!
Mg is more active than Co.
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Redox Balancing: Practice
Cu + HNO3 Cu2+ + NO + H2O
CN- + MnO4- CNO- + MnO2
acid
base
‘Odd’ atoms
O with H2O
H with H+
Charge with e-
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Galvanic (Voltaic) Cell
net: Zn + Cu+2 Zn+2 + Cu
Spontaneous “half reactions” are separated in a galvanic cell(to use the flow of electrons).
Zn + CuSO4 ZnSO4 + Cu
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Voltaic Cell
Zn + Cu+2 Zn+2 + Cu
Oxidation at Zn electrode (anode)
Zn Zn+2 + 2e-
Reduction at Cu electrode (cathode)
Cu+2 + 2e- Cu
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(-) (+)
Voltaic CellOxidation
Zn Zn+2 + 2e-Reduction
Cu+2 + 2e- Cu
Voltaic Cell videohttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
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Electrochemical Cells
Electric current (e-) flows through the wire due to difference in electrical potential, measure by
cell voltage or emf.
Non-reacting cations and anions flow through the salt bridge to maintain electrical neutrality.
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Standard Electrode Potentials
It is impossible to measure the emf(voltage) of a single half cell, so one is arbitrarily set to equal zero.
2H+(1M) + 2e- H2 (1 atm)
Standard hydrogen electrode (SHE)
EoSHE = 0 volt (definition)
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S.H.E.
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Standard Reduction Potential
All other half-cell potentials are measured relative to SHE, and are assigned Eo values if:
solutes are 1M gases are 1.0 atm temperature is 25oC
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Standard Reduction Potential
Eo for: Zn(s) Zn+2(aq) + 2e-
How do you tell it is oxidation?
0.76V
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Standard Reduction Potential
Under these conditions, Zn is found to be the anode (oxidation).
And the voltmeter reads: Eo
cell = 0.76 V
Zn(s) Zn+2(aq) + 2e-
Standard Reduction Potential
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Eocell = Eo
cathode + Eoanode
Eocathode = reduction potential of
reduction half-cell (cathode)
Eoanode = reduction potential of
oxidation half-cell (anode)(switch the sign when
putting into the equation)
switch sign for oxidation
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Standard Reduction Potential
Eocell = Eo
cathode + Eoanode
0.76 V = 0.0 V + EoZn+2/Zn
= EoH+/H2 + Eo
Zn+2/Zn
(reduction) (oxidation)
EoZn+2/Zn = 0.76 V (oxidation)
so reduction potential is -0.76V
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Standard Reduction Potential
See problem set for the standard reduction potentials for many half reactions.
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Some “Rules”
Eo’s are all written as reductions.
Zn+2 + 2e- Zn
(switch Eo sign if oxidation)
Eo’s are listed for standard conditions.
1M, 1 atm., 25oC
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Some “Rules” cont.
A redox reaction is thermodynamically favored in the forward direction if Ecell is positive.
Zn + Cu+2 Zn+2 + Cu
Eocell = +1.10 V
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Some “Rules” cont.
The more positive Eo is, the greater tendency for reduction.
F2 + 2e - 2F-
Greatest tendency to go in forward direction.
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Some “Rules” cont.
Half cells are reversible: any electrode can act as anode (oxidation) or cathode (reduction)
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Some “Rules” cont.
The “Diagonal Rule”
Draw a diagonal from Lio to F2.Under standard conditions, a reaction is spontaneous (+Ecell) if the reactants can be connected by this diagonal.
Don’t forget water at 1.23V & -.83V!
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Diagonal Rule
Diagonal connects reactants of spont. redoxreaction
Runs as reduction
Runs as oxidation
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Some “Rules” cont.
Changing the stoichiometric coefficients of a half-cell reaction, does not change Eo.
I2 + 2e- 2I- Eo = 0.53 V
2I2 + 4e- 4I- Eo = 0.53 V
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Some “Rules” cont.
Eo is a thermodynamic function like H, G & S. Reversing the direction of a reaction changes the sign of Eo.
I2 + 2e- 2I- Eo = 0.53 V
2I- I2 + 2e- Eo = - 0.53 V
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Some “Rules” cont.
These rules apply whether the reactants are separated in a galvanic cell or all mixed together.
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Redox Prediction
Write the net ionic reaction that takes place when Br2 is added to a solution of NaCl and NaI.
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You have:Br2(l) Na+(aq)Cl-(aq)I-(aq)H2O
Look for the longest spontaneous diagonal. It identifies the reactants.
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Redox Prediction
A Cr electrode in 1M Cr(NO3)3 is connected to a silver electrode in 1M AgNO3. What is the anode and cathode?Write the net ionic reaction and calculate Eo
cell.
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You have:CrCr3+
AgAg+
H2O
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Eocell DGo Keq
DGo, the amount of energy
available to do work, is given by:
DGo = -n F Eocell
n = number of moles of e-
F = Farady constant
= 96,500 J/V mol e-
= 96,500 C/mol e-
For spontaneous reaction:(+) (-) (>1)
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Eocell DGo Keq
DGo = -n F Eocell
Also, since
DGo = -RT lnK
then
Eocell = RT
nFlnK
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Eo
DGo KDGo = -RT lnK
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Eocell DGo Keq
DGo K Eo Rxn. at Std. State
- > 1 + Spontaneous
0 = 1 0 At equilibrium
+ < 1 - Non-spontaneous
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Show Your Stuff
1.Calculate Keq for the following reaction at 25oC:
Sn(s) + 2Cu+2 Sn+2 + 2Cu+
2.Calculate DGo for the following
reaction at 25oC:
2Au(s) + 3Ca+2 2Au+3 + 3Ca(s)
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Effect of Concentration on Eo
If concentrations are not standard (1.0M), use LeChatelier’s principle to see the effect on voltage:
e.g. If a reactant concentration is greater than 1.0M, reaction will go in forward direction to a greater extent, so cell voltage will increase.
Effect of Concentration on E
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Co+2(aq) + Fe(s) Co(s) + Fe+2(aq)
Calculate Eocell when: [Co+2] = [Fe+2] = 1.0M
Does Ecell increase or decrease when:
1. [Co+2] = 0.5M2. [Fe+2] = 1.5M3. More Co(s) is added
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Batteries
Portable electrochemical cell
used as a source for electricity.
Dry cellCar battery (Pb)Fuel cell
Read !!!
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Electrolysis
The opposite of a galvanic cell. Find the shortest opposite diagonal.
Use of electrical energy to cause a non-spontaneous chemical reaction to occur. This process is carried out in an electrolytic cell.
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Electrolysis: NaCl(l)
Anode 2Cl- Cl2 + 2e-
Cathode 2Na+ + 2e- 2NaOverall 2Na+ + 2Cl- 2Na + Cl2
Since Eo = -4 V, at least 4V must be supplied to cause the reaction to occur.
Electrolysis: NaCl(aq)
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What are the two half-reactions and the voltage required for this electrolysis?
Use the table of reduction potentials to find shortest nonspontaneous diagonal (electrolytic cell), not the longest spontaneous diagonal (galvanic cell).
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You have:• Na+
• Cl-
• H2O
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Electrolysis: H2O
What are the two half reactions for the electrolysis of acidifiedwater to produce hydrogen gas and oxygen gas?
Find shortest nonspontaneous diagonal.
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You have:• H+
• H2O
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Electrolysis: H2O
Anode 2H2O O2 + 4H+ +4e-
Cathode 4H+ + 4e- 2H2Overall 2H2O 2H2 + O2
Eo = -1.23 V
H2SO4 is added as electrolyte & source of H+.
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Hall Process
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Electrolysis of KI(aq)
Lab
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Quantitative Electrolysis stoichiometry with electrons!
andcurrent = charge/timeampere = coulomb/sec.
1A = 1 C/s
1 F = charge of 1 mol e-
= 96,500 C/mol e-
Electrolysis demohttp://media.pearsoncmg.com/bc/bc_0media_chem/chem_sim/electrolysis_fc1_gm_11-26-12/main.html
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Quantitative Electrolysis
In an electrolytic cell, Ca+2 is converted to Ca at the cathode. How much Ca is produced by 0.452 A (current) in 1.50 hours?
Ca+2 + 2e- Ca
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Quantitative Electrolysis0.452 A = 0.452 C/s
0.452 C1 s
x1.5 hr x 3600shr
x 1 mol e-
96,500 Cx
1 mol Ca2 mol e-
x 40.08 g Ca1 mol Ca
= 0.507 g Ca
Ca+2 + 2e- Ca
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Time to Rock ‘n Roll
In the electrolytic reduction of acidified water, what volume of hydrogen gas at STP is produced using a current of 1.26 A for 7.44 hours?
2H+ + 2e- H2