Ch' 14 Electrochemistry

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Ch. 14 Electrochemistry
2
Electricity from the Ocean Floor
Oceanographic instruments can be powered by
making a battery between the water and sediment
layers.
3
Electrochemistry

• Electrochemistry is an entire sub-field within
  analytical chemistry focused on making electrical
  measurements of analytes.
• Ch. 14 discusses the basics of electrochemistry,
  while Ch. 15-16 discuss analytical methods
  potentiometry and redox titrations

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The Basics

• Redox reactions involve transfer of electrons
  from one species to another.
• Oxidized species loses electrons
• Reduced species gains electrons
• Oxidizing agent (oxidant) takes electrons
• Reducing agent gives up electrons

Reduced
Oxidized
Oxidizing Agent
Reducing Agent
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Chemistry and Electricity

• In a redox reaction, electric current is
  proportional to the rate of reaction, and the
  voltage is proportional to the free energy change
  for the reaction
• Electric charge (q) is measured in coulombs (C)
• A single electron has a charge of 1.602x10-19 C
• A mole of electrons has a charge of 9.649x104 C,
  which we call the Faraday constant (FC/mol)

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Example (Charge)

• If 5.585 g Fe3 were reduced in the reaction with
  V2, how much charge must have transferred?

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Current

• The quantity of charge flowing through a circuit
  per second is the current
• Current is given in units of ampere, AC/s
• As you probably know, current is the dangerous
  part of electricity, not voltage
• Current, because it has units of time, is related
  to the rate of a chemical reaction

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Example (Current)

• If e- are forced into a wire which acts as
  cathode to reduce tin at a rate of 4.24 mmol/hr,
  how much current passes through?

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Voltage

• The difference in electric potential (E) is the
  work needed when moving charge from one point to
  another.
• Potential difference is measured in volts (V)
• Work has dimensions of energy, joules (J)
• When a charge, q, moves through a potential
  difference, E
• One joule is the energy used when one coulomb
  moves between potential differing by one volt

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Example (Work)

• How much work can be done if 2.4 mmol of e- go
  through a potential difference of 0.70 V?
• The greater the difference in potential (V), the
  stronger the e- will be pushed around the circuit
• 12V battery 8x strong push than a 1.5V battery

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Free Energy

• The free energy change, DG, is the maximum
  possible electrical work that can be done on the
  surroundings
• This equation relates free energy to the
  electrochemical reaction, similar to how in
  precipitation chemistry the equilibrium constant
  can be related to free energy

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Resistance and Power

• Ohms law gives the relationship between current
  (I) and potential (V) or resistance (W)
• 1 Amp will flow through a circuit with potential
  difference of 1 V if the resistance is 1 W
• Power is the work per unit time, given in watts
  (W)
• A cell delivering 1 amp at a potential of 1 V
  gives an output of 1 W

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Example (Ohms Law)

• In the following circuit, a battery generates a
  potential difference of 3 V and the resistor has
  a 100 W resistance. How much current and power
  are delvered by the battery?

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Galvanic Cells

• Galvanic cells have a spontaneous chemical
  reaction that generates electricity. One
  solution must be oxidized while the other is
  reduced.
• The net reaction is composed of 2 half-
  reactions, an oxidation reaction and a reduction
  reaction.
• Free energy change for the reaction is -150 kJ
  per mol Cd

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Example (Chemical Reaction)

• Calculate the voltage that would be measured in
  the Ag/Cd reaction
• A spontaneous reaction (-DG) gives a positive
  voltage

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The Cell and the Salt Bridge
Will Work!
Wont Work!
A salt bridge can be used to keep the
half-reactions separated. The bridge has a high
concentration of an electrolyte (one not used in
the reaction).
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Notation

• We write out a cell by using two symbols
• to represent a phase boundary
• to represent a salt bridge
• So, for the cell on the previous slide

A positive voltage is obtained for spontaneous
reactions, which also means that voltage is
flowing to the negative (black) terminal of the
voltmeter.
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Standard Potential

• If we want to know voltage that would be observed
  when different half-reactions are attached, we
  need to define one as a standard
• We use hydrogen (S.H.E.)
• We can measure Eº for other half-reactions,
  relative to the hydrogen reaction, e.g. for
  silver
• Standard reduction potentials are listed in Ap. H
  in your book. Eº for the H2 reaction is for the
  reaction at 25º C

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S.H.E Half-reaction with Silver
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Short Form of Appendix H
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Nernst Equation

• The Nernst Equation relates the potential of the
  half reaction to equilibrium conditions
• For the reaction
• We usually calculate half-reactions _at_ 25º C,
  substituting that in with the gas constant and to
  base 10 log gives

The book again refers to activities, but you can
use molarities for solutes (all the calculations
do) and pressure (bar) for gases
e- in the half-reaction
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Example (Nernst)

• Write the Nernst equation for the reduction of
  phosphoric acid to solid white phosphorous
• Note that multiplying the reaction by any factor
  does not affect Eº or the calculated E

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Nernst Equation for a Cell (Complete Reaction)

• The voltage is the difference between the
  potentials of the two electrodes
• E is attached to the positive terminal of the
  voltmeter and E- is attached to the negative.
• Steps to solve problem
• 1) Write reduction half-reactions and Eº from
  App. H multiply to equal e-, but dont
  multiply Eº
• 2)Write Nernst for right half cell ( side of
  voltmeter)
• 3) Write Nernst for left half cell (- side of
  voltmeter)
• 4)Find net cell voltage (E-E-)
• 5)Write balanced net reaction (reverse left
  half-reaction)

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Example (Net Reaction)

• Find the voltage for the Ag-Cd cell and state if
  the reaction is spontaneous if the right cell
  contained 0.50 M AgNO3(aq) and if the left
  contained 0.010 M Cd(NO3)2(aq)
• 1)
• 2)
• 3)
• 4)
• 5)