Electrochemistry and the Nernst Equation

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Electrochemistry and the Nernst Equation

• Experiment 14

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14 Electrochemistry and the Nernst Equation

• Goals
• To determine reduction potentials of metals
• To measure the effect of concentration on
  reduction potential
• To prepare a Nernst plot to find solubility of
  silver halides (AgX)
• Method
• Use an electrochemical cell and voltmeter

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Redox Chemistry/Electrochemistry

• Thermodynamics of redox reactions
• Chemical / electrical work interchange
• Involves transfer of electrons or electron
  density
• Oxidation
• Loss of electrons
• Reduction
• Gain of electrons

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(No Transcript)
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Redox reactions and spontaneity

• Spontaneity is determined by thermodynamics
• Ex. Cu/Cu2 // Zn/Zn2 system
• What will be oxidized (lose e-)? Cu or Zn
• What will be reduced (gain e-)? Cu2 or Zn2
• Will e- flow from Zn to Cu2 or from Cu to Zn2?
• What will the energy change be?
• Current
• Flow of e-
• Systems attempt to attain equilibrium
• (minimum energy state)

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Zn (s) Cu2(aq) ? Cu (s) Zn2(aq)
2e-
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Electrochemistry/Electrochemical Cells

• Redox reaction produces or uses electrical energy
• Voltaic (galvanic) cell
• spontaneous reaction generates electrical energy
  (battery)
• Electrolytic cell
• absorbs energy from an electrical source to
  drive nonspontaneous reaction (recharge)

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Cell Components

• Electrodes
• conduct electricity between cell and surroundings
• Anode oxidation site AO, an ox
• Cathode reduction site CR, red cat
• Electrolyte
• ion mixture involved in reaction or carrying
  charge
• Salt bridge
• completes circuit (provides charge balance)

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Experimental Set-up

• Electrochemical cell
• Separated ½ reactions
• (ox and red)
• Driving force for
• electron transfer is
• measurable
• What is 1.10?

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Electrochemical Cell Set-up
voltmeter
Mred(s)
Mox(s)
Salt Bridge (KNO3)
My
Mx

Oxidizing agent
Reducing agent


Cathode Ions are reduced My(aq) y e- ? M(s)
Anode Electrode is oxidized M(s) ? Mx(aq) x e-
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Example Set-up
1.10V
Cu(s)
Zn(s)
Salt Bridge (KNO3)
Cu2
Zn2

Oxidizing agent
Reducing agent


Cathode Ions are reduced Cu2(aq) 2e- ? Cu(s)
Anode Electrode is oxidized Zn(s) ? Zn2(aq) 2e-
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Zn (s) Cu2(aq) ? Cu (s) Zn2(aq)

• Zn gives up e- to Cu spontaneously
• Zn pushes harder on e-
• e- on Zn greater potential energy
• greater electrical potential
• 1.10 V is a measure of this

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Electrochemical potential measured voltage

• Voltage
• difference in energy of the e- on the metals or
• relative difference in metals abilities to give
  e-
• different metals ? different e- energy
• ? different push on e-
• Electromotive force (EMF cell potential), Ecell
• Driving force on electrons
• Measured voltage potential difference
• Higher Ecell larger drive

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Thermoelectric bridge work and e- flow

• DG0 free energy change (available work)
• E0 standard cell potential
• n number of moles of e- transferred
• F Faradays constant

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Energy, E0, and Spontaneity

• Cell potential Free Energy
  Spontaneity
• Positive E0cell DG0 lt 0 Spontaneous
• Negative E0cell DG0 gt 0 Not
• Zero E0cell DG0 0 Equilibrium
• DG0 free energy of change
• amount of available (electrical) work

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Standard Reduction Potentials, E0

• E0cell cell potential under standard conditions
• (reference tables)
• elements in standard states s, l, g
• solutions 1 M
• gases 1 atm
• Relative to standard hydrogen electrode, SHE
• 2H(aq) 2 e- ? H2(g) E0cell 0.00 V
• Overall E0cell combine E0s for half-reactions

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Example E0 values

• Reduction reaction E0
• Mg2 2e- ? Mg -2.30 V
• Zn2 2e- ? Zn -0.76 V
• Ni2 2e- ? Ni -0.23 V
• 2H2 2e- ? H2 0.00 V
• Cu2 2e-? ? Cu 0.34 V
• Ag e-? ? Ag 0.80 V
• Au3 3e-? ? Au 1.50 V
• ? More positive E0 greater reduction
  potential
• ?The push on e- relative to H2/2H

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E0 values

• More positive
• Stronger oxidizing agent
• Easier to reduce
• More readily accepts e-
• More negative
• Stronger reducing agent
• More easily oxidized
• More readily gives e-
• In a spontaneous reaction
• Stronger R.A. O.A. ? Weaker R.A. O.A.

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Calculating E0cell

• Reaction Zn (s) Cu2(aq) ? Cu (s)
  Zn2(aq)
• red. Zn(s) ? Zn2(aq) 2 e- E0 0.76 V
• ox. Cu2(aq) 2 e- ? Cu(s) E0 0.34 V
• Zn (s) Cu2(aq)? Cu(s) Zn2(aq) E0 1.10 V
• Assumes 1 M Cu2 and Zn2 solutions under
  standard conditions

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Connection to work DG0, E0, and K
From thermodynamics
From electrochemistry
So
So
At equilibrium DG0 0 and Keq Q
n moles of e- transferred
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Nernst Equation
Nonstandard conditions
So
Q, reaction quotient
Cell potential298K
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Summary of Key Equations
Standard Conditions and at Equilibrium
Non-standard conditions298K

• Remember Ecell is proportional to ?DG

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Concentration Dependence

• Electrical potentials depend on
• type of metal
• solution concentration
• For Zn (s) Cu2(aq) ? Cu (s) Zn2(aq)

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Example

• Gold will plate onto silver (not vice versa)
  why?

Au3(aq)
Ag(aq)
Ag(aq)
e-
Au(s)
Au(s)
Ag(s)
3Ag Au3 ? 3Ag Au
No reaction
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Example Au plating on Ag
Spontaneous reaction 3Ag Au3 ? 3Ag Au
Given (tables) Ag3 e- ? Ag E0 0.80
V Au3 3e- ? ? Au E0 1.50 V
If Au3 Ag 1 M at 298K
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Example Au plating on Ag
Spontaneous reaction 3Ag Au3 ? 3Ag Au
If E0 0.70 V Au3 Ag 0.1
M at 298K
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Example Au plating on Ag
Spontaneous reaction 3Ag Au3 ? 3Ag Au
If ½ Au3 is consumed Au3 ½ (0.10M)
0.05M Ag 0.10M3(0.05M) 0.25 M
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Experimental Parts and Key Ecell Equation
Part 1
Parts 2 and 3
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Experimental Overview

• 1. Dependence of potential on metal type
• Metal1 ? Metal2
• c1 c2
• Use 0.1 M solutions and electrodes of different
  metals
• Measure Ecell for each ( E0cell)
• Compare experimental vs. literature values

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Overview

• 2. Dependence of potential on concentration
• Metal1 Metal2
• c1 ? c2
• Use 0.1 M solution with 1?10-5 to 1?10-1 M
  solutions
• Measure Ecell for each
• Plot Ecell vs. log(cdil/cconc)
• Compare slope to Nernst equation

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Overview

• 3. Ksp Determination
• AgX(s) Ag(aq) X-(aq) Ksp AgX-
• Met1 Met2
• c1 ? c2
• Use 0.1 M Ag with satd AgX (0.1M Ag 0.2M KX)
• Measure Ecell
• Part 2 plot gives
• So

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Part 1 Notes Ecell Dependence on type of metal

• Measure Ecell for metal pairs
• 0.1 M Solutions (eliminates conc. dependence)
• TA will demonstrate cell set-up
• Each cell vial 2/3 full of solution
• liquids MUST be at equal levels
• Salt bridge filter paper soaked in 1.0 M KNO3
• dont let tweezers touch solutions in vials
• Voltmeter Clip leads to metal strips
  (electrodes)
• Insert into solutions

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Part 1 Notes Ecell Dependence on type of metal

• Measure cell voltage, Ecell
• Measure 2 or 3 metal relative to Cu
• Measure 2 or 3 metals relative to each other
• Calculate E0cell values
• Compare to literature

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Electrochemical Cell Set-up
0.47
Cu(s)
Pb(s)
Salt Bridge (KNO3)
Cu2
Pb2



Cathode
Anode
Spontaneous when Pb is oxidized and Cu2 is
reduced
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Part 1 Examples

• Copper and Lead
• Pb ? Pb2 ?? Cu2 ? Cu
• oxidation Pb metal/solution
• reduction Cu metal/solution
• Reduction potentials (table)
• Pb2(aq) 2 e- ? Pb(s) E0 0.13 V
• Cu2(aq) 2 e- ? Cu(s) E0 0.34 V
• For spontaneous reaction, E0 gt 0 so calculated E0
  is
• Pb (s) Cu2(aq)? Cu(s) Pb2(aq) E0 0.47 V

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Example Part 1 Data

• Measured vs.
• Calculated Ecell
• Ecell
• Ecathode (? Eanode)

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Part 2 Notes Ecell dependence on concentration

• Cells same metal/metal ion solution
• TA will demonstrate cell set-up
• Measure Ecell (E0 0)
• Concentrations 0.1 to 1?10-5 vs. 0.1 M
• Plot Ecell vs. log(cdil/cconc)
• Compare slope to Nernst equation (-2.303RT/nF)
• Ag(s) Ag(aq, conc) ? Ag(s) Ag(aq, dil)

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Example Part 2 Data

• Plot Ecell msd vs. log(A/B)

Theoretical Slope ?2.303RT/nF
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Part 3 Notes Silver Halides Ksp

• cell 1 0.1 M Ag?Ag electrode
• cell 2 saturated AgX solution (KCl/AgNO3)
• Measure Ecell
• Determine Agdilute
• Find Ksp AgX-
• X- unchanged
• Ag Ecell /Part 2
• Find DG0 -RTlnKsp

0.2M
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Electrochemical Ksp and DG0 Determination

• Experimental voltages good lt5 error
• Experimental Ksp good high 20 error

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Report

• Abstract
• Sample calculations including
• Reduction potentials for metals
• E0cell for cells without Cu
• log (Agdilute/Agconc)
• Agdilute, Ksp, DG0
• Results
• Ecell, msrd for all cells
• Reduction potentials for metals
• E0cell for metals
• concentrations, Emeasured, slopes, graph
• Ecell, msrd, Agdilute, Ksp, DG0
• Discussion/review questions