Chapter 7 Electrochemistry Outside class reading Levine: pp. 417 14.4 Galvanic cells: pp. 423 14.5 types of reversible electrodes
7.6.1 Basic concepts of electrochemical apparatus (1) Electrochemical apparatus Electrolytic cell; Galvanic/voltaic cell Components: Electrodes; electrolytic solution Reaction: oxidation reaction: anode, anodic reaction reductive reaction: cathode, cathodic reactions.
7.6.1 Basic concepts of electrochemical apparatus (2) Components of an electrode: Question: 1. Current collector (first-type conductor) 2. Active materials: involves in electrochemical reaction 3. Electrolytic solution (second-type conductor). 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.)
7.6.1 Basic concepts of electrochemical apparatus (3) Differences between chemical and electrochemical reactions 2Fe 3+ + Sn 2+ 2Fe 2+ + Sn 4+ at electrode / solution interface Interfacial reaction half-reactions: Sn 2+ Sn 4+ + 2e 2Fe 3+ + 2e - 2Fe 2+ in bulk solution 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.
7.6.2. Reversibility of electrochemical cell (1) Relationship between chemical energy and electric energy dg = -SdT + VdP + W 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 Maximum useful work The relation bridges thermodynamics and electrochemistry Thermodynamic reversibility 1. Reversible reaction: The electrode reaction reverts when shift from charge to discharge. reversible electrode 2. Reversible process: I 0, no current flows.
7.6.3. Reversible electrodes (1) basic characteristics: 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
7.6.3. 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) metal electrode; amalgam electrode; complex electrode; gas electrode.
7.6.3. Reversible electrodes 1) The first-type electrode: amalgam electrode Zn(Hg) x Zn 2+ (m 1 ): complex electrode Ag(s) Ag(CN) 2 (m 1 ): Basic characteristics: Gas electrode: Hydrogen electrode Pt(s) H 2 (g, p ) H + (c) Three-phase electrode: H 2 gas H + solution (liquid) Pt foil (solid) 1) Two phases / One interface 2) Mass transport: metal cations only 1.0 mol dm -3 H + solution
7.6.3. Reversible electrodes Acidic hydrogen electrode Pt(s), H 2 (g, p) H + (c) 2H + (c) + 2e H 2 (g, p) Basic hydrogen electrode Pt(s), H 2 (g, p) OH (c) 2H 2 O(l) + 2e H 2 (g, p)+2oh (c) acidic oxygen electrode Pt(s), O 2 (g, p) H + (c) O 2 (g, p) + 4H + (c) + 4e 2 H 2 O(l) Basic oxygen electrode Pt(s), O 2 (g, p) OH (c) O 2 (g, p)+ 2H 2 O + 4e 4OH (c)
7.6.3. Reversible electrodes Important metal insoluble salt-anion (2) The second-type electrode: electrode metal insoluble salt-anion electrode Hg(l) Hg 2 Cl 2 (s) Cl (c): A metal plate coated with insoluble salt containing the metal, and immersed in a solution containing the anions of the salt. metal insoluble salt anion electrode Hg 2 Cl 2 (s) + 2e 2Hg(l) + 2Cl (c) There are three phases contacting with each other in the electrode. Ag(s) AgCl(s) Cl Pb(s) PbSO 4 (s) SO 4 2 (c): in lead-acid battery PbSO 4 (s) + 2e Pb(s) + SO 4 2 (c)
7.6.3. Reversible electrodes 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. Important reduction-oxidation electrode Pt(s) Fe(CN) 3 6 (c 1 ), Fe(CN) 4 6 (c 2 ) : Fe(CN) 3 6 (c 1 ) + e Fe(CN) 4 6 (c 2 ) Pt(s) Q, H 2 Q: quinhydrone electrode O OH Pt(s) Sn 4+ (c 1 ), Sn 2+ (c 2 ) Sn 4+ (c 1 ) + 2e Sn 2+ (c 2 ) + 2H + + O Q = quinone 2e - OH H 2 Q = hydroquinone Q + 2H + + 2e H 2 Q
7.6.3. Reversible electrodes 4) Membrane electrode: Reference: The membrane potential can be developed by exchange of ions between glass membrane (thickness < 0.1 mm) and solution. glass electrode
7.6.4. Cell notations (1) conventional symbolism Zn(s) ZnSO 4 (c 1 ) CuSO 4 (c 2 ) Cu(s) cell notation / cell diagram 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.
7.6.4. Cell notations (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: 1 Zn + CuSO 4 = ZnSO 4 +Cu 2 Ag + (m) + Cl (m) = AgCl(s) 3 H 2 O = H + + OH -