Lecture #4 Of 26 - University Of California, Irvine

90Lecture #4 of 26

91Looking forward our review of Chapter “0” Cool applications Redox half-reactions Balancing electrochemical equations History of electrochemistry and Batteries IUPAC terminology and Ecell Ered – Eox Thermodynamics and the Nernst equation Common reference electrodes Standard and Absolute potentials Latimer and Pourbaix diagrams Calculating Ecell under non-standard state conditions Conventions

92Half reactions must be referenced to something RECALL: 𝐸cell 𝐸red 𝐸oxSince the method of half-reactions ultimately results in us taking theirdifference, we can add an arbitrary constant to all half reactions Byone (somewhat arbitrary) convention, it is often assumed that Eo for thestandard hydrogen electrode (SHE) is equal to zero.Rigorously, these twoterms in Q need to bedivided by thestandard-statecondition, and includeactivity coefficientsHalf-Reaction for Hydrogen gas (H2): H e Potential𝐸H2 1𝐻2 2𝑔Standard Potential𝐸H0 2 𝑝H2𝑅𝑇ln H 𝐹𝐸H0 2 0 vs. SHEThus, the potentials for half-cell reactions are actuallyfull-cell potential (difference(s)) versus SHE, or other!Given this value,what is thisexperimental redoxpotential versus?

93Half reactions must be referenced to something 𝐸cell 𝐸red 𝐸oxSince the method of half-reactions ultimately results in us taking theirdifference, we can add an arbitrary constant to all half reactions Byone (somewhat arbitrary) convention, it is often assumed that Eo for thestandard hydrogen electrode (SHE) is equal to zero.* Normal hydrogen electrode (NHE) is an empirical SHE ([H ] 1; not standard state)* Standard hydrogen electrode (SHE) is a hypothetical, perfect NHE (a 1; not empirical)* Reversible hydrogen electrode (RHE) is the SHE but the same regardless of pH* And generally, formal potentials (E0’) take into consideration non-idealities and changesin ionic strengths so that the reaction quotient only has concentrations, and not activities𝐸H2 𝐸H0 2 𝑝H2𝑅𝑇ln H 𝐹𝐸H0 2 0Ramette, J. Chem. Educ., 1987, 64, 885

EXAMPLE: Write a balanced chemical equation andcalculate the standard cell potential for the galvanic cell:Zn(s) Zn2 (1 M) MnO4– (1 M), Mn2 (1 M), H (1 M) Pt(s)94Look up half-reactions and standard reduction potentials in anElectrochemical Series table (CRC, B&F Appendix C, WWW):Anode:Zn2 2e–ZnE0anode –0.76 VNote: Although strictly correct, do not use “–E0” as the “E0 for oxidation”Cathode: MnO4– 8H 5e–Mn2 4H2O E0cathode 1.51 VNote: Be careful to choose the correct half-reaction with MnO4–

CRC Handbook of Chemistry and Physics, 92nd Edition95* All values versus SHEMnO4– 8H 5e–Mn2 4H2OZnZn2 pdf

EXAMPLE: Write a balanced chemical equation andcalculate the standard cell potential for the galvanic cell:Zn(s) Zn2 (1 M) MnO4– (1 M), Mn2 (1 M), H (1 M) Pt(s)96Look up half-reactions and standard reduction potentials in anElectrochemical Series table (CRC, B&F Appendix C, WWW):Anode:Zn2 2e–ZnE0anode –0.76 VNote: Although strictly correct, do not use “–E0” as the “E0 for oxidation”Cathode: MnO4– 8H 5e–Mn2 4H2O E0cathode 1.51 VNote: Be careful to choose the correct half-reaction with MnO4–To get the balanced overall reaction? multiply the anodereaction by 5 and subtract it from 2 times the cathode reaction:2MnO4– (aq) 16H (aq) 5Zn (s)5Zn2 (aq) 2Mn2 (aq) 8H2O (l)Ecell E0cathode – E0anode 1.51 V 0.76 V 2.27 V

97Q: What processes occur in electrochemistry? one more (remember this?)CRC Handbook of Chemistry and Physics, 92nd Edition* All values versus SHE* How negative can Eo f

Q: What processes occur in electrochemistry? one more A: Winter, 2017: Those involving the motion/transport of charge –carried by entities other than unsolvated electrons and holes –through phase(s), or the transfer of charge across interface(s).98Example: solvated electronsProf. Robert Hamers (Univ. of dZhu, , Hamers, Nature Materials, 2013, 12, 836

First description of conductivity using solvated electrons99During the first part of the twentieth century, E. C. Franklin and C. A. Kraus probably did more to elucidate the chemistry ofliquid ammonia solutions than everybody else combined It is perhaps little known that their work was prompted by theresearch and insight of H. P. Cady, carried out while he was an undergraduate! Whilst working on cobalt amminecomplexes, Cady proposed that ammonia in these (and other “double salts”) must function in a manner akin to water in saltswith water of crystallization. He suggested further that liquid ammonia would probably be found to resemble water in itsphysical and chemical properties—thus adding a second to our list of ionizing solvents. Cady’s undergraduate work, carriedout without supervision, published in 1897, was perhaps the first physical chemistry study of liquid ammonia solutions.Zurek, Edwards, & Hoffman, Angew. Chem. Int. Ed., 2009, 48, 8198Cady, J. Phys. Chem., 1897, 1, 707

Q: What is electrochemistry? one more A: Winter, 2017: Those involving the motion/transport of charge –carried by entities other than unsolvated electrons and holes –through phase(s), or the transfer of charge across interface(s).100Example: solvated electronsProf. Robert Hamers (Univ. of Wisconsin)http://hamers.chem.wisc.edu/peopleWhat is this?diamondZhu, , Hamers, Nature Materials, 2013, 12, 836

Absolute potentials can be measured / approximated very carefully 101Prof. Sergio Trasatti(Università de Milano, Italy)Trasatti, Pure & Appl. Chem., 1986, 58, 955

102Absolute potentials can be measured / approximated very carefully Born–Haber cycle:𝐻𝑔𝜇𝑒e(Hg)Trasatti, Electroanal. Chem. Interfac. Electrochem., 1974, 52, 313Trasatti, Pure & Appl. Chem., 1986, 58, 955

103Absolute potentials can be measured / approximated very carefully Born–Haber cycle:(two ways to solve this)𝐻𝑔𝜇𝑒e(Hg) but we need an electrode!Trasatti, Electroanal. Chem. Interfac. Electrochem., 1974, 52, 313Trasatti, Pure & Appl. Chem., 1986, 58, 955

104Absolute potentials can be measured / approximated very carefully HgPtBorn–Haber cycle:(two ways to solve this)e(Hg) but we need anFarrell and McTigue, Electroanal. Chem. Interfac. Electrochem., 1982, 139, 37Trasatti, Electroanal. Chem. Interfac. Electrochem., 1974, 52, 313electrode!Trasatti, Pure & Appl. Chem., 1986, 58, 955

105Looking forward our review of Chapter “0” Cool applications Redox half-reactions Balancing electrochemical equations History of electrochemistry and Batteries IUPAC terminology and Ecell Ered – Eox Thermodynamics and the Nernst equation Common reference electrodes Standard and Absolute potentials Latimer and Pourbaix diagrams Calculating Ecell under non-standard state conditions Conventions

106Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V7 6 4 3 2 0Chemistfrom WikiWendell Mitchell try/peopleinfo.php?pid 24644Latimer, The oxidation states of the elements and their potentials in aqueous solution, 1938

107Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V7 6 4 3 2 0from WikiDisproportionation – spontaneous and simultaneous reduction and oxidationof a molecule (the opposite is comproportionation (AKA: symproportionation))NO. Eo Ered – Eox 1.18 – 1.51 –0.33 V(1) Does Mn2 disproportionate?(2) What is the standard reduction potential of MnO4– to MnO2?Total Reaction: 3Mn2 Reduction: Mn2 Oxidation: Mn2 Mno 2Mn3 MnoMn3 Eototal ?Eo 1.18 VEo 1.51 V

108Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V7 6 4 3 2 0from WikiDisproportionation – spontaneous and simultaneous reduction and oxidationof a molecule (the opposite is comproportionation (AKA: symproportionation))NO. Eo Ered – Eox 1.18 – 1.51 –0.33 V(1) Does Mn2 disproportionate?(2) What is the standard reduction potential of MnO4– to MnO2?ΔGo -nFEo -3FEoΔGo -nFEo1 -nFEo2 -F((1 x 0.56 V) (2 x 2.26 V)) -F(5.08 V)Set them equal to each other, and thus, 3Eo 5.08 and Eo 1.69 V for #1, you can work with Eo only (not ΔGo), because the reaction is always balanced/equal in the number of electrons

109Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V?7 6 4 3 2 0from WikiRecall from before anyway, why arethese bottom E0values not on theLatimer diagram? because they areat basic/alkalinestandard state with 1 M OH–!

110Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V?7 6 4 3 2 0from WikiRecall from before What would this E0value be when atacidic standard state?

111Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V?7 6 4 3 2 0from Wiki?𝐸 0𝐸𝑎𝑐𝑖𝑑0.05916 V log𝑛𝑀𝑛𝑂2𝑀𝑛𝑂41𝐻2 𝑂2 1𝐻 24 0𝐸𝑎𝑐𝑖𝑑0.05916 V log2111110 1440 𝐸𝑎𝑐𝑖𝑑 0.02958 V 560𝐸 𝐸𝑎𝑐𝑖𝑑 1.65648 V 0.60 VWhat would this E0value be when atacidic standard state?

112Two diagrams of empirical standard potentials A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing thecomplete redox series for an element and for determining when disproportionation will occur.ReductionOxidation1,69 V?7 6 4 3 2 0from Wiki?𝐸 0𝐸𝑎𝑐𝑖𝑑0.05916 V log𝑛𝑀𝑛𝑂2𝑀𝑛𝑂41𝐻2 𝑂2 1𝐻 24 0𝐸𝑎𝑐𝑖𝑑0.05916 V log2111110 1440 𝐸𝑎𝑐𝑖𝑑 0.02958 V 560𝐸 𝐸𝑎𝑐𝑖𝑑 1.65648 V 0.60 VWhat would this E0value be when atacidic standard state?0𝐸𝑆𝐻𝐸 2,25648 VSWEET! but then why did theCRC not list this?

113 Second one (not truly standard potentials) A Pourbaix diagram is a map of the predominant equilibrium species of an aqueouselectrochemical system; it is useful for identifying which materials/species are present/stable mostly based on thermochemical dataChemistMarcel Biographies/PourbaixBio.htmOxidationfrom Wiki1,69 V? because in acid, the reaction does notoccur! but then why did thefrom WikiCRC not list this? Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 19747 6 4 3 2 0

114 Second one (not truly standard potentials) A Pourbaix diagram is a map of the predominant equilibrium species of an aqueouselectrochemical system; it is useful for identifying which materials/species are present/stable mostly based on thermochemical dataChemist1e–,1H reaction(slope –60 mV)1e–,1H reaction(slope –60 mV)Marcel Biographies/PourbaixBio.htmWhy don’t I like this? Even though EVERYONEplots it this wayE(O2,H /H2O)H transferRHEe– transfer1e–,1H reaction(slope –60 mV)2e–,3H reaction(slope –90 mV) you get the ideafrom WikiAnyway, standard state is here, at 1 M H (pH 0) SHE but if written under alkaline conditions, 1 M OH– is standard state (pH 14)Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 1974