Electrochemistry

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Chapter 14

• Electrochemistry

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Basic Concepts

• Chemical Reaction that involves the transfer of
  electrons. A Redox reaction.
• Loss of electrons oxidation
• Gain of electrons reduction
• Oxidizing agent. A species that takes electrons.
• Reducing agent. A species that gives electrons.

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Basics

• Na(s) H -gt Na H2(g)
• Sodium is a reducing agent
• Hydrogen ion is the oxidizing agent.

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Basics

• We are donating and gaining electrons. If we
  could use these electrons perhaps we could do
  some useful work.
• If we can make the electron travel in an
  electrical circuit then the amount of current can
  be measured.
• Current is related to reaction rate or amount of
  reaction
• Potential is related to free energy change of the
  reaction.

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Electron Charge

• q used to denote. Unit is Coulombs (C)
• Charge on a single electron is
• 1.602x10-19 C which will allow us to determine
  the charge on a mole of electrons.
• 1.602x10-19 C 6.022x1023 mol-1) 96490 C mol-1
• This is called the Faraday Constant
• q nF
• n is the number of moles

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Current

• Charge flowing through a circuit
• One ampere, the charge of one coulomb per second
  flowing past a given point.

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Electrodes

• The interface between a solution and an
  electrical circuit. Can be actively involved or
  just serve as a source or sink for electons.

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Electrical Potential

• Work required when moving and electric charge
  from one point to another.
• Electrical potential (E) is measured in Volts
  (V).
• Work is a measure of energy, measured in joules
  (J).
• Work E q
• Joules volts coulombs

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Free Energy

• Maximum amount of work that can be done on the
  surroundings is equal to the Gibbs free energy
  change.
• then DG -work -Eq
• Or DG -nFE

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Ohms Law

• Current is proportional to the potential and and
  inversely proportional to the resistance.
• I E/R

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Electric Circuit
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Power

• Work done per unit time. Unit is the J/s which
  is know as the watt (W).
• P work/sec Eq/sec E(q/sec) EI
• P EI I2R E2/R

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Galvanic Cells

• Spontaneous chemical reaction used to generate
  electricity.
• An example might be

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Voltmeter

• A device to measure electrical potential. When
  electrons tend to flow into the negative terminal
  then a positive voltage is measured.
• In this cell
• 2 AgCl (s) 2 e- 2 Ag 2 Cl- (aq) Red
• Cd (s) Cd2 2 e-
  Oxidation
• Cd (s) 2 AgCl (s) Cd2 2 Cl- Net
• For this reaction we have a DG of -150 kJ/mole
  per mole of Cd oxidized.

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Potential of this System

• DG -150 kJ/mole then we have
• E - DG/nF -150 x 103 J / (2
  mol)(9.649x104 C/mol)
• E 0.777 J/C 0.777 V

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Cathode/anode

• Cathode electrode where reduction occurs
• Anode electrode where oxidation occur
• Put both terms in alphabetical order to remember

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Salt Bridge

• Any bridge in upstate New York in the winter.
• Used to isolate the half cells so the work can be
  forced out into an external circuit.
• The following cell has a problem.

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What is it?

• The silver ions in solution can go directly to
  the cadmium electrode surface and be reduced
  there.
• We need to put in a barrier to rapid ionic
  transfer.

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What about this cell
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Isnt this cute

• Chemistry paper dolls?

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Line Notation - Instead of Having to Draw the
Cells

• phase boundary salt bridge
• For First Cell
• Cd(s) CdCl2(aq) AgCl(s) Ag(s)
• For Second Cell
• Cd(s) Cd(NO3)2(aq) AgNO3(aq) Ag(s)

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A Word of Connectors (Two common in USA)
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Standard Potential Eo The energy to a half
cell at standard conditions (1 M and 25 C)

• Let us look at the reduction of silver ion.
• Ag e- Ag(s)
• We will compare this to a fixed reference.
• That is the SHE or NHE Standard or Normal
  Hydrogen Electrode.
• H (aq, A1) e- ½ H2 (g, A 1)

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SHE - All other redox couples are compared to
this half cell. It is assigned a value of 0.000
V

• In our cell the left side electrode (Pt) is
  attached to the negative terminal. (Reference)
• Value of E are collected into Tables (Appendix H)

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Nernst Equation

• For the half reaction
• aA ne- bB

Eo is the standard Potential R gas constant
(8.314472 (VC)/(kmol) T Temp (K) N of
electrons in the half reaction F Faraday A
Activity
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We will often lump the constants and assume 25 C

• Nernst equation (25 C and converting to log10

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Complete Reaction

• E E - E- for full cell
• Steps
• Write both half cells as reductions, make
  electrons equal
• Half cell connected to positive terminal is E
• Other half cell is E-
• Net voltage is from the above equation
• Balance equation (reversing the left half
  reaction and adding to other half cell)
• E gt 0 spontaneous as written
• E lt 0 spontaneous in reverse

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Eo and K
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Cells as Chemical Probes

• Equilibria between the half cells
• Equilibria within each half cell

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A Probe Cell
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Probe Cell

• Right side
• We have our Ksp equilibrium
• The electrochemical reaction under this is
• AgCl(s) e- Ag(s) Cl- (aq, 0.10 M)
• Eo 0.222 v
• Left side
• We have our Ka for the weak acid.
• The electrochemical reaction
• 2 H(aq) e- H2 (g, 1.00 bar)
• E 0.00, but H is not fixed at 1 M so E varies
  with H

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Eo

• Formal Potential
• Since so many redox couples exist in the body and
  many have H we modify the potential that we use
  to pH 7. (A little more reasonable than 1 M
  acid.

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Homework

• 14- 4
• 13, 14, 15 and 27