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Chapter 20

Lecture and Textbook Collaborated Notes - Chapter 20 - CHEM 1A03

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Aadil Merali Juma

Chem 1A03 Chapter 20: Electrochemistry 20.1 Electrode Potentials and Their Measurement Redox Reactions: Examples a) Spontaneous: Cu + 2 Ag+ Cu + 2 Ag b) Non-spontaneous: Cu + Zn  no reaction + Electrochemical Cells: Cu/Ag  An electrode immersed in a solution containing the same metal, M , is called a half cell  Two connected half cells  The 2 half reactions are separated, yet connected electrically (salt bridge, external wire) o Charge is carried by ions, so a wire cannot be used o Anions (NO 3 migrate from the salt bridge into the copper half cell to neutralize the excess positive charge from the Cu ions + o Cations (K ) migrate from the salt bridge into the silver half cell to neutralize the excess negative charge from the NO 3ons  Solid metals = electrodes (anode, cathode), M -  Movement of e from anode to cathode generates violated  Movement of cations and anions through salt bridge maintains electro neutrality in solution n+ -  Oxidation (anode) – surface of electrode erodes as M are produced; n e then travels to cathode o M  M + n e - 2+ - o Cu(s)Cu ( aq) 2 e  Reduction (cathode) – cathode gains mass as M ions gain n e and form M(s) o M + n e  M + - o 2 Ag (aq) 2e  2 Ag (s)  Remember – “an ox and a red cat” anode ox, reduce cathode; or vowels OA, consonants CR + 2+  Overall Cell Reaction: Cu(s)2 Ag (aq) Cu ( aq)+ 2 Ag (s)  Galvanic (voltaic) cell – results from spontaneous chemical reactions  Electrolytic cell – uses electricity to accomplish non-spontaneous chemical change  Cell voltage, or potential difference between two half cells o Measured in volts (V), the energy per unit charge  Potential difference of one volt signifies an energy of one joule for every coulomb of charge passing through an electric circuit; 1 V = 1 J/C o Driving force for electrons – the greater the voltage, the greater the dirving force o Also called electromotive force (emf) or cell potential – represented by the symbol Ecell Cell Diagrams – shows the components of an electrochemical cell in a symbolic way  Symbolic notation of cell:  Sometimes an electrode is inert (does not chemically participate, but is needed for electron transfer to occur Chem 1A03 20.2 Standard Electrode Potentials Standard Electrode Potentials  Absolute half-cell potentials cannot be measured  All potentials are measured relative to the standard hydrogen electrode (SHE); assigned a potential of 0V o Involves equilibrium established on the surface of an inert metal between H O io3s from a solution in which they are at unit activity (that is, a = 1) and H molecules from the gaseous state at a pressure H3O+ 2 of 1 bar o 2 H (a=1) + 2 e  H (2, 1 bar) E° = exactly 0 V (volt) Simplify: a = 1 becomes [H+] = 1M; 1 bar = 1 atm o Half-cell diagram: Pt(s) | H2(g, 1 atm) | H (aq, 1M)  3 phases are present – solid platinum, gaseous hydrogen and aqueous hydrogen ion  Standard electrode potentials (E°) measures the tendency for reduction to occur (E° ) atred electrode  Standard conditions: Conc. = 1M, pressures = 1 atm Standard Cell Potentials (E° cell  E°cell voltage of a cell formed from 2 standard electrodes  Can be expressed as E°(cathode right= - E°(anode left ALTERNATIVELY  For a given half-cell we can say E°ox -E° red (reverse the sign on the potential for the oxidation half cell) 2+  Eg/ Zn /Zn E° red - 0.763, therefore E° ox+ 0.763  E° cell E°red+ E°ox  Note: A standard potential is an intensive property – it does not depend on the amount of substance reacting  We do not multiply E° redor E°oxy a factor even if we multiply a half-reaction by a factor when balancing  E°cell 0 = spontaneous  Magnitude of cell potential indicated driving force for cell o Eg/under standard conditions Oxidation of metallic Zn by Cu (E° cell 1.100V) has a greater tendency to go to completion than does + oxidation of metallic Cu by Ag ions (E° cell 0.4600V) Standard Reduction Potentials (E° ) red  Table 20.1: Selected Standard Electrode (Reduction) Potentials at 25°, page 872  Species most easily reduced on left side, at most positive potential  Species most easily oxidized on right side, at least positive potential Chem 1A03 20.3 E cellΔG and K Spontaneity: E°cell and ΔG°  Electrical work is done when a reaction occurs in a voltaic cell; product of three terms o Ecell;ectromotive force or cell potential o z; the number of electrons transferred between the electrodes  electron number (charge number) of an electrochemical reaction, no units o F; Faradays constant, the electric charge per mole of electrons; 96,485 coulombs/mol of electrons (C/mol)  joule = volt x coulomb (or volt = energy/unit charge)  w elec zFEcell o Only applies if the cell operates reversibly (requires
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