Chapter 7 Electrochemistry § 7.6 Reversible cell

Electrolytic cell; Galvanic/voltaic cell 1) Electrochemical apparatus Reaction: oxidation reaction: anode, anodic reaction reductive reaction: cathode, cathodic reactions. Components: Electrodes; electrolytic solution Basic concepts of electrochemical apparatus

2) Components of an electrode: 1. Current collector (first-type conductor) 2. Active materials: involves in electrochemical reaction 3. Electrolytic solution (second-type conductor). Question: Point out the current collector, active materials and electrolytic solution of the following electrode. 1) Zn(s)|Zn 2+ (sln.) 2) (Pt), H 2 (g, p  )|H + (sln.)

3) Differences between chemical and electrochemical reactions 2Fe 3+ + Sn 2+  2Fe 2+ + Sn 4+ Fe 3+ Sn 2+ ee ee ee Fe 3+ Sn 2+ ee Fe 3+ Sn 2+ ee ee ee ee cathodeanode Fe 2+ Sn 4+ half- reactions: Sn 2+  Sn e  2Fe e -  2Fe 2+ at electrode / solution interface in bulk solution Interfacial reaction

To harvest useful energy, the oxidizing and reducing agent has to be separated physically in two different compartments so as to make the electron passing through an external circuit. 4) Basic principle for cell design

dG = -SdT + VdP +  W’ Maximum useful work 5) Relationship between chemical energy and electric energy At constant temperature and pressure  G = -  W’ Reversible process: conversion of chemical energy to electric energy in a thermodynamic reversible manner or vice versa.  G = -  W’ = QV = -nFE The relation bridges thermodynamics and electrochemistry

Reversibility of electrochemical cell 1.Reversible reaction: The electrode reaction reverts when shift from charge to discharge. reversible electrode 2. Reversible process: I  0, no current flows. Thermodynamic reversibility

Reversible electrodes 1) basic characteristics: 1) single electrode; Zn|Zn 2+ ; Zn|H + ; 2) reversible reaction; Zn Zn e  3) the equilibrium can be easily attained and resumed. In order to acquire reversibility, all reactants and products of the electrode reaction must be present at the electrode. The stability of the electrode materials: According to the active series of metals, which kind of metal can form reversible electrode? K, Ca, Na, Mg, Al, Zn, Fe, Sn, Pb, (H), Cu, Hg, Ag, Pt, Au

2) Main kinds of reversible electrodes (1) The first-type electrode: metal – metal ion electrode A metal plate immersed in a solution containing the corresponding metal ions. Cu (s)  Cu 2+ (m) Cu 2+ Cu metal electrode; amalgam electrode; complex electrode; gas electrode.

Zn(Hg) x  Zn 2+ (m 1 ): Ag(s)  Ag(CN) 2 (m 1 ): amalgam electrode complex electrode Basic characteristics: 1)Two phases / One interface 2)Mass transport: metal cations only Cu 2+ Cu

Gas electrode: Three-phase electrode: H 2 gas H + solution (liquid) Pt foil (solid) 1.0 mol·dm -3 H + solution Pt(s) H 2 (g, p  )  H + (c) Hydrogen electrode

Acidic hydrogen electrode Basic hydrogen electrode Pt(s), H 2 (g, p)  H + (c) Pt(s), H 2 (g, p)  OH  (c) 2H + (c) + 2e   H 2 (g, p) 2H 2 O(l) + 2e   H 2 (g, p)+2OH  (c) acidic oxygen electrode Basic oxygen electrode Pt(s), O 2 (g, p)  H + (c) Pt(s), O 2 (g, p)  OH  (c) O 2 (g, p) + 4H + (c) + 4e   2 H 2 O(l) O 2 (g, p)+ 2H 2 O + 4e   4OH  (c)

(2) The second-type electrode : metal – insoluble salt-anion electrode A metal plate coated with insoluble salt containing the metal, and immersed in a solution containing the anions of the salt. Type II: metal  insoluble salt  anion electrode Ag(s)  AgCl(s)  Cl  Ag AgCl Cl 

Hg(l)  Hg 2 Cl 2 (s)  Cl  (c): Pb(s)  PbSO 4 (s)  SO 4 2  (c): in lead-acid battery Hg 2 Cl 2 (s) + 2e   2Hg(l) + 2Cl  (c) PbSO 4 (s) + 2e   Pb(s) + SO 4 2  (c) Important metal – insoluble salt-anion electrode There are three phases contacting with each other in the electrode.

(3) The third-type electrode: oxidation-reduction (redox) electrodes: immersion of an inert metal current collector (usually Pt) in a solution which contains two ions or molecules with the same composition but different states of oxidation. Type III: oxidation-reduction electrodes Pt(s)  Sn 4+ (c 1 ), Sn 2+ (c 2 ) Sn 4+ (c 1 ) + 2e   Sn 2+ (c 2 ) Sn 4+ Sn 2+ Sn 4+ Sn 2+ Sn 4+ Sn 2+ Pt

Pt(s)  Fe(CN) 6 3  (c 1 ), Fe(CN) 6 4  (c 2 ) : Pt(s)  Q, H 2 Q: quinhydrone electrode Fe(CN) 6 3  (c 1 ) + e   Fe(CN) 6 4  (c 2 ) Q + 2H + + 2e   H 2 Q Q = quinoneH 2 Q = hydroquinone Important reduction-oxidation electrode

4) Membrane electrode: glass electrode The membrane potential can be developed by exchange of ions between glass membrane (thickness < 0.1 mm) and solution. Reference:

Cell notations 1) conventional symbolism 1. The electrode on the left hand is negative, while that on the right hand positive; 2. Indicate the phase boundary using single vertical bar “│”; 3. Indicate salt bridge using double vertical bar “||”; 4. Indicate state and concentration; 5. Indicate current collector if necessary. Zn(s)| ZnSO 4 (c 1 ) ||CuSO 4 (c 2 ) |Cu(s) cell notation / cell diagram

(2) Steps for Reversible Cell Design 1. Separate the two half-reactions 2. Determine electrodes and electrolytes 3. Write out cell diagram 4. Check the cell reaction EXAMPLES: e.g.1 Zn + CuSO 4 = ZnSO 4 +Cu e.g. 2 Ag + (m) + Cl  (m) = AgCl(s) e.g. 3 H 2 O = H + + OH -

Levine: pp Galvanic cells: cell diagrams and IUPAC conventions Levine: pp types of reversible electrodes metal-metal ion electrode amalgam electrode redox electrode metal-insoluble-salt electrode gas-electrode nonmetal electrode membrane electrode Outside class reading