10/9/2020 64 Lecture #3 of 26 65 Looking 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 RECALL: Daniell (galvanic) Cell (1836) 66 No more H2 from the (primary) battery! anode cathode oxidation reduction Chemist, Meteorologist Half-reactions are physically separated! 2+ – 2+ – Zn (s) → Zn (aq) + 2e Cu (aq) + 2e → Cu (s) John Frederic Daniell (1790–1845) NET REACTION: Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s) from Wiki 1 10/9/2020 Voltage Produced by Galvanic Cells 67 The difference in electric potential between the anode and the cathode is called: ✓ Cell potential ✓ Cell voltage ✓ emf (electromotive force) This should be –0.76 V! Cell Diagram (we will discuss this soon) Zn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq) [Cu2+] = 1 M and [Zn2+] = 1 M Zn (s) | Zn2+ (aq, 1 M) || Cu2+ (aq, 1 M) | Cu (s) anode salt bridge cathode EXAMPLE: What is the (standard) potential of a galvanic cell 68 consisting of a Cd electrode in a 1.0 M Cd(NO3)2 solution and a Cr electrode in a 1.0 M Cr(NO3)3 solution? Which half-reaction is reducing? 2+ – 0 Cd (aq) + 2e Cd (s) E = -0.40 V Cd2+ will get reduced to Cd Cr3+ (aq) + 3e– Cr (s) E0 = -0.74 V Cr will get oxidized to Cr3+ More negative of the two … thus, it is reducing Anode (oxidation): Cr (s) Cr3+ (1 M) + 3e– x 2 Cathode (reduction): 2e– + Cd2+ (1 M) Cd (s) x 3 2Cr (s) + 3Cd2+ (1 M) 3Cd (s) + 2Cr3+ (1 M) 0 0 0 Ecell = Ecathode – Eanode 0 Ecell = -0.40 V – (-0.74 V) 0 Ecell = +0.34 V (positive = spontaneous, since ∆퐺 = −푛퐹퐸) … if your answer is negative then you switched the anode/cathode in the galvanic cell Electrochemistry: 69 conventions… oh, conventions! Cathode – electrode where catholyte species are reduced Anode – electrode where anolyte species are oxidized 2 10/9/2020 Electrochemistry: 70 conventions… oh, conventions! Cathode – electrode where catholyte species are reduced Anode – electrode where anolyte species are oxidized Does a Negative/Positive Electrode = Cathode or Anode?… It depends! For the discharging (galvanic) battery, label the anode and the cathode. http://autoshop101.com/ Electrochemistry: 71 conventions… oh, conventions! Cathode – electrode where catholyte species are reduced Anode – electrode where anolyte species are oxidized Does a Negative/Positive Electrode = Cathode or Anode?… It depends! For the charging (electrolytic) battery, label the anode and the cathode. e– e– This is a generator (AKA power supply) cathode anode http://autoshop101.com/ Electrochemistry: 72 conventions… oh, conventions! Positive electrode – positively charged; immersed in the posolyte Negative electrode – negatively charged; immersed in the negolyte … I’m not kidding! … Sheesh!… … Take-home message: For batteries, don’t call electrodes anodes and cathodes (but naming convention used by most is for discharge) e– e– e– e– anode cathode cathode anode http://autoshop101.com/ 3 10/9/2020 Chemist, Meteorologist The Daniell Cell (1836) low impedance to measure current – – Icell John Frederic Daniell (1790–1845) from Wiki As drawn, current flows for < 1 sec and then stops due to Zn Cu lack of charge neutrality… … capacitive charging Zn(s) Zn2+(aq) Cu2+(aq) Cu(s) Zn | Zn2+(aq) || Cu2+(aq) | Cu Chemist, Meteorologist The Daniell Cell (1836) Now it works = “electrochemistry”! low impedance to measure current – – Icell John Frederic Daniell + Ai (1790–1845) from Wiki As drawn, current flows B – i for < 1 sec Salt bridge and then contains an inert, stops due to Zn redox inactive Cu lack of charge salt solution neutrality… (electrolyte) … capacitive charging Zn(s) Zn2+(aq) Cu2+(aq) Cu(s) Zn | Zn2+(aq) || Cu2+(aq) | Cu The Daniell Cell (1836) Quick Quiz * Name the cell type * Identify the anode * Identify the cathode low impedance to measure current * Name the – – electrode signs Icell + * primary galvanic cell Ai … Ultimately, this cell will have fully discharged, and at which point it will B – i be at equilibrium (ΔG = Ecell = 0)… (A/–) (C/+) Zn Cu … Then, any direction of polarization bias will result Zn(s) Zn2+(aq) Cu2+(aq) Cu(s) in electrolytic function (i.e. Zn | Zn2+(aq) || Cu2+(aq) | Cu charging)! 4 10/9/2020 76 Quick Check on Your Understanding You try! (a) What is the standard Ecell for a galvanic cell based on zinc and silver? Ecell = +0.80 V – (-0.76 V) = 1.56 V (b) If one wanted to electrolytically charge the cell from part a (before any reactions took place), what potential would one have to provide? Ebias < –1.56 V… such that Ecell + Ebias < 0 (this is the observed value) 77 International Union of Pure and Applied Chemistry (IUPAC) (Accepted) Nomenclature and Terminology that you’ve learned, but may have forgotten • Coulomb (in units of C = A·s) is the unit of charge (96,485 C are in a mole of singly charged species = Faraday constant, F ≈ 96,500 C/mol ≈ 105 C/mol) differentiate, with respect to time integrate, over time • Electricity is the flow of current (I; in units of A = C/s) and is negative (cathodic) or positive (anodic) depending on the direction and sign of the current-carrying species (e.g. e–, H+) • (Electrode) (electric) potential (V or E; in units of V = J/C) is written as a reduction This relates to Gibbs free energy as ΔG = –RT ln K = –nFEcell (electrical work per mole), and… … partial molar Gibbs free energy is the electrochemical potential (휇ҧ, in units of J/mol) ➢ Chemical potential (휇, in units of J/mol) ➢ Galvani/Inner (electric) potential (ϕ, in units of V) … and in summary, 휇ҧ = 휇 + 푧퐹ϕ Also, standard state is a solvent, a solid, and a species at unit activity (~1 M solutes, ~1 bar gases) Also, E = IR (Ohm’s law) when resistance is constant • Galvanic cells produce power (in units of W = A x V = C/s x J/C = J/s) by spontaneous redox reactions • Electrolytic cells require a power input to drive redox reactions; thus, the reactions are thermodynamically unfavorable • A battery has an anode/anolyte and a cathode/catholyte, but these descriptors change depending on whether the battery is being discharged(Sources (galvanic): B&F, M3LC or charged course (electrolytic); textbook, negative electrode/negolyte and positive electrode/posolyte are better and http://goldbook.iupac.org/) 78 International Union of Pure and Applied Chemistry (IUPAC) (Accepted) Nomenclature and Terminology that you’ve learned, but may have forgotten • Coulomb (in units of C = A·s) is the unit of charge (96,485 C are in a mole of singly charged species = Faraday constant, F ≈ 96,500 C/mol ≈ 105 C/mol) differentiate, with respect to time integrate, over time • Electricity is the flow of current (I; in units of A = C/s) and is negative (cathodic) or positive (anodic) depending on the direction and sign of the current-carrying species (e.g. e–, H+) • (Electrode) (electric) potential (V or E; in units of V = J/C) is written as a reduction This relates to Gibbs free energy as ΔG = –RT ln K = –nFEcell (electrical work per mole), and… … partial molar Gibbs free energy is the electrochemical potential (휇ҧ, in units of J/mol) Based on➢ourChemicalcurrentpotentialsign convention (휇, in units of J/, molit is) E0(Cu2+/0) = +0.34 V vs. SHE best to ➢onlyGalvani/Innerwrite reduction (electric) potentialpotentials (ϕ, in units; of V) E0(Cu0/2+) = –0.34 V vs. SHE however,… andif inwe summary,lived 휇ҧ =in휇 +an푧퐹ϕ oxidation- Also, standard state is a solvent, a solid, and a species at unit activity (~1 M… solutes, is incorrect! ~1 bar gases) potentialAlso,-centric E = IR (Ohm’sworld, law) whenwe resistancecould iswrite constant them all (i.e. everything) as oxidation You can subtract redox • Galvanic cells produce power (in units of W = A xpotentials V = C/s x J/C but = J/s)do not by spontaneous change the potentialsredox; reactionssimply put, it is best to not sign of the potential and then mix the conventions and so stick with • Electrolytic cells require a power input to drive redoxcall reactions;it an oxidation thus, the potential! reactions are reductionthermodynamicallypotentials unfavorable • A battery has an anode/anolyte and a cathode/catholyte, but these descriptors change depending on whether the battery is being discharged (galvanic) or charged (electrolytic); negative electrode/negolyte and positive electrode/posolyte are better 5 10/9/2020 79 International Union of Pure and Applied Chemistry (IUPAC) (Accepted) Nomenclature and Terminology that you’ve learned, but may have forgotten • Coulomb (in units of C = A·s) is the unit of charge (96,485 C are in a mole of singly charged species = Faraday constant, F ≈ 96,500 C/mol ≈ 105 C/mol) differentiate, with respect to time integrate, over time • Electricity is the flow of current (I; in units of A = C/s) and is negative (cathodic) or positive (anodic) depending on the direction and sign of the current-carrying species (e.g. e–, H+) • (Electrode) (electric) potential (V or E; in units of V = J/C) is written as a reduction This relates to Gibbs free energy as ΔG = –RT ln K = –nFEcell (electrical work per mole), and… … partial molar Gibbs free energy is the electrochemical potential (휇ҧ, in units of J/mol) ➢ Chemical potential (휇, in units of J/mol) ➢ Galvani/Inner (electric) potential (ϕ, in units of V) Cannot be … and in summary, 휇ҧ = 휇 + 푧퐹ϕ measured Also, standard state is a solvent, a solid, and a species at