Download Chapter Nineteen Outline

A.P. Chapter 19 Outline
Electrochemistry and its Applications
Electrochemistry is the study of the relationship between electron flow and redox
reactions.
I Redox Reactions
 A reaction is a redox reaction in which species are reduced and oxidized. A
species is reduced when it gains electrons and a species is oxidized when it loses
electrons
 LEO and GER, OIL RIG, OP
 The oxidizing agent is reduced and the reducing agent is oxidized.
II. Using Half-Reactions
 Reactions can be broken down into half-reactions, one for what is reduced and
one for what is oxidized.
 Atoms and electrons can not be gained or lost
III. Electrochemical Cells
 In an electrochemical cell, an oxidizing agent and a reducing agent pair are
arranged in such a way that they can react only if electrons flow through an
outside conductor. These are known as voltaic cells or batteries.
 The anode (-) produces electrons (oxidation)
 The cathode (+) consumes electrons (reduction)
 Oxidation occurs at the anode, and reduction occurs at the cathode. Electrons
move from the anode to the cathode through an external wire. The electrical
circuit is completed in the solution by the movement of ions – anions move from
the salt bridge compartment to the anode compartment and cations move from the
slat bridge compartment to the cathode compartment.
 All voltaic cells must have these characteristics:
1. The redox reaction must favor products
2. there must be an external circuit through which electrons flow
3. there must be a salt bridge, porous barrier, or some other means of
allowing ions to flow between the electron compartment
The shorthand representation for representing this cell is
Zn(s) ‫ ׀‬Zn2+(aq) ‫ ׀׀‬Cu2+ (aq) ‫ ׀‬Cu(s)
IV.
V.
Electrochemical Cells and Voltage
 Electrical charge = charge x potential energy difference
 Electrical work = number of electrons x potential energy difference
 1 electron has a charge of 1.6-22 x 10-19 Coulombs
 One joule of work is performed when one coulomb of charge moves
through a potential difference of one volt
 1 volt = 1joule/coulomb
 The cell voltage shows how much work a cell can produce for each
coulomb of charge that the chemical reaction produces
 Standard conditions (same as for ΔºH) will produce standard voltages, E°.
 Cell voltages for product favored reactions are positive
 Eºcell = Eºcathode - Eº anode
 If Eºcell is positive, the reaction is product favored
 The reaction chosen as standard is
2 H3O+ (aq, 1M) + 2 e- → H2 (g, 1 bar) + 2 H2O (l) with V = 0
Using Standard Cell Potentials
 Cell potentials are all written as reductions. You can either use Eºcell =
Eºcathode - Eº anode and not reverse sign of cell potential if reaction is
reversed, or use Eºcell = Eºcathode + Eº anode , and then you do need to reverse
sign if reaction is reversed. This is because each half-reaction can occur in
either direction.
 The more positive the value of the reduction potential, Eº, the more easily
the substance on the left hand side of a half reaction can be reduced.
 The les positive the value of the reduction potential, the less likely the
reaction will occur as a reduction, and the more likely the reverse reaction,
an oxidation, will occur.
 Under standard conditions, any species on the left of a half-reaction will
oxidize any species that is below it on the right side of the table.
 A positive cell potential denotes a product favored reaction
 Electrode potentials depend on the nature and concentration of reactants
and products, not o the quantity of each that reacts.
VI. E° and Gibbs Free Energy
 For a product favored reaction, ΔG must be negative. The “free’ in free energy
indicates that energy is available to do work.
 Quantity of charge = moles of electrons x coulombs per mole of electrons
 Charge on 1 mol of electrons = 96, 500 C/mol
 The Faraday constant, F, = 96,500 C/mol
 The electrical work that can be done by a cell = the number of moles of
electrons transferred times the cell potential times F
 ΔGº = -nFE°
 -nFEºcell = -RT ln K°
 Eºcell = [(RT)/(nF)] x lnKº
 Assuming standard state temperature, one then gets
Eºcell = (.0592v/nF) x log K° and log Kº = nEºcell/.0592v
VI.
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Effect of Concentration on Cell Potential
As the reactions proceed in an electrochemical cell, reactants are consumed
and products are generated, so the concentrations of the species change
continuously. As the reactant concentration decreases, the voltage produces
drops.
ΔGº = ΔG + RT ln Q
The Nernst equation : Ecell = Eºcell – (2.303RT/nF) x ln Q
Assuming standard conditions, Ecell = Eºcell – (.0592v/n) x log Q
The Nernst equation can be used to calculate the voltage produced by an
electrochemical cell under standard conditions and it can also be used to
calculate the concentration of a reactant or product in an electrochemical
reaction from the measured value of the voltage produced.
A concentration cell is a cell in which the voltage is generated because of a
difference in concentrations.
pHunknown = Ecell/.0592