Chemistry Week 14

Chemistry Week 14 - (e) Electrochemistry TEXTBOOK READING:...

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(e) Electrochemistry TEXTBOOK READING : BLB-10 , Chapter 20.3-9 Practice PROBLEMS: (Ch 20) 24, 32, 34, 39, 40, 47, 52, 58, 60, 62, 85, 86, 91, 94 . (i) Spontaneous vs. Nonspontaneous Redox Reactions : In spontaneous redox reactions, electrons flow from a position of high potential energy to low potential energy. Consider the following four chemical situations: 1. Placing a Zn strip in HCl(aq): Bubbles observed 2. Placing a Cu strip in HCl(aq): Nothing happens 3. Placing a Cu strip in Zn 2+ (aq): Nothing happens 4. Placing a Zn strip in Cu 2+ (aq): Black solid forms on Zn strip, blue color of the solution gradually fades. What can we conclude from these examples? 1. Zn spontaneously reacts with H + (aq) to give H 2 (g). 2. Cu does not spontaneously react with H + (aq) to give H 2 (g). 3. Zn spontaneously reacts with Cu 2+ (aq) to give Cu(s) (the black solid is CuO, which forms since the finely divided Cu metal is exposed to O 2 dissolved in water). 4. Cu does not spontaneously react with Zn 2+ (aq). Therefore, we can write the following chemical reactions: 1. Zn(s) + 2 H + (aq) ⎯→ Zn 2+ (aq) + H 2 (g); G < 0 2. Cu(s) + 2 H + (aq) ⎯×→ Cu 2+ (aq) + H 2 (g); G > 0 3. Zn(s) + Cu 2+ (aq) ⎯→ Zn 2+ (aq) + Cu(s); G < 0 4. Cu(s) + Zn 2+ (aq) ⎯×→ Cu 2+ (aq) + Zn(s); G > 0 Therefore, Zn(s) has a higher potential energy for electrons than H 2 (g), and H 2 (g) has a higher potential energy for electrons than Cu(s). OR, the strength as a reducing agent increases from Cu(s) to H 2 (g) to Zn(s) (Zn is most easily oxidized; Cu is least easily oxidized). The potential energy of an electron is proportional to its charge (in units of coulombs). The charge of one electron is 1.61 × 10 19 coulombs. The charge of one mole of electrons is ( 1.61 × 10 19 coulomb/electron) × (6.02 × 10 23 electrons/mole) = 96,500 coulombs/mole. The Faraday is defined as the number of coulombs in one mole of electrons 1 F = 96,500 coulombs/mole During a spontaneous electrochemical reaction, electrons flow from high potential energy to low potential energy. This change in potential energy is just the free energy change for the reaction, G . Therefore, the electrochemical potential E is: G = nF E
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The electrochemical potential is the free energy (potential energy) difference PER coulomb of charge transferred and has units J/coulomb = Volt (V). Since G < 0 means spontaneous from left to right, then E > 0 means spontaneous. The following table identifies the possibilities: Sign of G Sign of E Spontaneous? + Yes (left to right) 0 0 Equilibrium (quasistatic) + No (left to right) For solutions at standard conditions (all ions have 1.0 M concentrations; all gases are at 1.0 atm pressure), we use standard electrochemical potentials ( E ° ) and standard free energy changes ( G ° ): G ° = nF E ° = RT ln K Standard Reduction Potentials : Since ( G ° ) corresponds to the difference in the potential energy of the electrons between products and reactants under standard conditions, we can associate ( E ° ) with the potential difference (in V) between the substance that gains electrons and the substance that loses electrons.
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This note was uploaded on 03/27/2008 for the course CHEM 201 taught by Professor Miller during the Fall '07 term at Iowa State.

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Chemistry Week 14 - (e) Electrochemistry TEXTBOOK READING:...

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