electrochemistry

electrochemistry - E le c t r o c h e m is t r y Nernst...

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Electrochemistry Nernst Equation Ion selective electrodes Redox reactions oxidation - loss of electrons M n+ ! M n+1 + e - M is oxidized - reducing agent reduction - gain of electrons N n+ + e - ! N n-1 N is reduced - oxidizing agent Half Reactions Fe 3+ + e - ! Fe 2+ Zn 2+ + 2e ! Zn if mix these which will donate, which will accept electrons? can’t measure equilibrium concentrations - only represent this reaction
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-ve, ANODE +ve CATHODE Galvanic Cell Cu 2+ + Zn ! Cu + Zn 2+ Half reactions are: Cu 2+ + 2e - ! Cu Zn - 2e - ! Zn 2+ transfer of electrons - electric current - can be measured half-cell with greater driving force makes other cell accept electrons Spontaneous reaction copper more easily reduced electrons flow spontaneously from zinc half cell to copper half cell Zn - 2 e- ! Zn 2+ (zinc dissolves) Cu 2+ +2e - ! Cu (copper bar gains weight) Zn electrode -ve (loses electrons) ANODE Cu electrode +ve (gains electrons) CATHODE
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Salt Bridge Salt bridge maintains electrical neutrality by transport of ions Cu deposits leaves excess SO 4 2- - neutralized by K + from KCl bridge Zn dissolves to give excess Zn 2+ in solution, neutralized by Cl - from salt bridge Also SO 4 2- and Zn 2+ could migrate into bridge - does not matter which Driving Force Driving force of half-cell can’t be measured - only by comparison to other half cells All potentials quoted against hydrogen half cell - assigned zero potential 2H + + 2e- ! H 2 if half cell causes H 2 cell to accept electrons - assigned -ve potential (Zn -0.763) if H 2 call causes half cell to accept electrons - assigned +ve potential (Cu +0.86) Oxidizing/Reducing agents Strong oxidizing agents e.g. permanganate +ve potential Strong reducing agents, e.g. zinc -ve potential Potentials shown in next slide are for gases at 1 atm pressure, and 1M for solutions relative to hydrogen electrode
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Which way is spontaneous? Fe 3+ + e - ! Fe 2+ 0.771V Zn 2+ + 2e - ! Zn -0.763V 1. Spontaneous reaction has +ve potential 2. Subtract one reaction potential from other to make difference +ve i.e 0.771 - (-0.763) = +1.534 V 3. potential of Zn has to be subtracted to make final number positive - Zn goes in reverse: Zn ! Zn 2+ + 2e - Overall Reaction 2Fe 3+ + 2 e - ! 2 Fe 2+ E 0 1 = 0.771 Zn ! Zn 2+ + 2e - E 0 2 = -0.763 2 Fe 3+ + Zn ! Zn 2+ + 2 Fe 2+ E 1 - E 2 = + 1.534 V
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The Nernst Equation if species not in standard state, E 0 changes depending on concentrations E = E 0 ! RT nF ln [ red ] b [ ox ] a or [products] [reactants] " # $ % ' where a ox + n e - ( b red a, b coefficients in balanced equation at 25 0 C : E = E 0 ! 0.059
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This note was uploaded on 02/25/2012 for the course CHEM 232 taught by Professor Michel, r during the Fall '08 term at UConn.

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electrochemistry - E le c t r o c h e m is t r y Nernst...

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