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Lecture 10

CHMB16Fall2012 Lecture 10 Notes.docx

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Department
Chemistry
Course
CHMB16H3
Professor
Kagan Kerman
Semester
Fall

Description
CHMB16Fall2012 Lecture 10: Electrodes and Potentiometry Concentration Effects on Cell Potential (The Nernst Equation) o For the generalized electrochemical equation reaction: o The reaction quotient, Q, may be written as: o o Q is the activity of the products/ activity of reactants the standard reduction potential does not change even though the o whole top reduction equation was multiplied the standard reduction o by 2 potential does not o when we combine all the constants then we get change even though the whole top reduction equation was multiplied 2 2 2 2 byE = + 0.222 o 2 2 2 2 E = + 0.222 o Example: AgCl Half Reaction: AgCl Half Reaction: [Cl-] E= +0.222–2.25916log1 *Approximate activity E= +0.222–0.05916 log [Cl-] *Approximate activityntration 2 1 o with concentration o NOTE: approximate activity with concentration 41 o The Nernst equation for the complete cell is E = E+ - E- o E+ is the potential of the electrode attached to the positive terminal of the potentiometer o E- is the potential of the electrode attached to the negative terminal of the potentiometer 35 o If the net cell voltage E is > 0 then the net cell reaction ill be spontaneous in the forward direction o If the net cell voltage E is <0 then the net cell reaction will be spontaneous is the reverse direction Example Problem Step-by-Step Problem: [AgNO (aq)] = 0.50 3 [Cd(N3 2 (aq)] = 0.010 M o 1 1. Write the half-reaction and E for both half cells 1. Write the half-reaction and E for both half cells Cathode Anode o 2. by convention, write both as reduction reactions (even CATHODE ANODE though one is not) --> this allows for faster look up in 38 the table of standard reduction potentials CATHODE ANODE squared 3o because there 3. Find the net cell voltage using E = E -E + - G alvanic C ells at E quilibrium this means that this o reaction is spontaneous ActiveGalvaniccells are not at equilibriumquilibriumtaneous in the forward direction 40o Cd(s) is oxidize and Ag+ is reduced At equilioriElectrons flow from the left hand electrode to the right hand electrode ActiveGalvaniccells are not at equilibrium Galvanic Cells at EQ Ao eTwo types of cell equilibrium o EQ between half cells o EQ within each half cell 0oV Non zero voltage means that the cell is not at EQ o Active galvanic cells are not at EQ Also at equilibrium Q= K 0 V o At EQ: o At EQ, Q also = K Also at equilibrium Q= K o 5 5 Example Problem: o E and K for Non-RedoxReactions E and K for Non-RedoxReactions Is it possible to find K usingthe E for a non-redox o o Ereaction? oon-R Is it possibledoxRfind K eactions E for a non-redox reaction? NOT A Is itE° and K for Non-redox Reactionsor a non-redox NOT A o we can find K using the E° for non redox reaction by writing the reaction as the difference of 2 half reactions with reaction? +2 +2 REACTION But we can write this same reaction as the difference of two But we can write this same reaction as the difference of two half reactions of knowpotentials REACTION o +2 half r+2ctions of know potentials But we can write this same reaction as the difference of two half reactions of knowpotentials o this way we can find EQ without this way we can knowing the find EQ without 7 concentrations knowing the o 7allows you to find K without known concentrations  useful for EQ constant that are too small or too large to be measured in other ways this way we can C ells as chemical probes find EQ without 7 Cells as Chemical Probes knowing the concentrations o LEFT CELL RIGHT CELL but they are relevant because there are species in these reactions that LEFT CELL RIGHT CELL but they are relevant because there are species in these reactions that also appear in the half reactions which are important o o that also appear in the half reactionst cell reaction, but they are relevant because there are species in these reactions Not part of net cell reaction p.296 8th edition C ells as chemical probes 10 o Bymeasuringthe net cell voltage (+0.503), the [H+] can be found C ells as chemical probes should be squared should be divided by 2 o o NOTE**: the Cl- concentrations above should be squared and then divided by 2 (wrong in the book) LEFT CLEFT CELL RIGHT CELL 11 Not part of net cell reaction Acid Dissociation Constant o the galvanic cell can be used to determine the EQ constant for any acid-base reaction in the left hand cell Biochemistry and Formal Reduction potential (E°’) o reduction potential values measured at pH = 7 instead of pH =0ium constant for anyacid-base reaction in the left-hand cell 12 o o potentials do not change from the normal ones in reactions that do not involve hydronium ions Potentiometry o measuring activity of ions, not concentration so it gives a better idea of the chemical properties of the ions o the use of electrodes to measure voltage in order to provide chemically relevant information about a system 4 o advantages of potentiometry:ance that can accept or donate electrons at an electrode o measures activities of ions rather than conc
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