1. Chapter 7 Electrochemistry
§7.6 Reversible cell

2. Levine: pp. 417
14.4 Galvanic cells:
cell diagrams and IUPAC conventions
Levine: pp. 423
14.5 types of reversible electrodes
metal-metal ion electrode
amalgam electrode
redox electrode
metal-insoluble-salt electrode
gas-electrode
nonmetal electrode
membrane electrode

3. 1) Electrochemical apparatus
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.

4. 2) Components of an electrode:
1. Current collector (first-class conductor)
2. Active materials: involves in electrochemical reaction
3. Electrolytic solution.
Exercises: Indicate the current collector, active materials and electrolyte solution of the following electrode.
1) Zn(s)|Zn2+(sln.)
2) (Pt), H2(g, p)|H+ (sln.)

5. 3) Differences between chemical and electrochemical reactions
Sn2+ e
cathode
anode
Fe2+
Sn4+
2Fe3+ + Sn2+  2Fe2+ + Sn4+
Fe3+
Sn2+ e
at electrode / solution interface
in bulk solution
half-reactions:
Sn2+  Sn4+ + 2e
2Fe3+ + 2e-  2Fe2+
Interfacial reaction

6. 4) Basic principle for cell design
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. 5) 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

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

9. 7.6.3. Reversible electrodes
1) basic characteristics:
1) single electrode; Zn / Zn2+; Zn / H+;
2) reversible reaction; Zn Zn2+ + 2e
3) the equilibrium can be easily attained and resumed.
To have reversibility at an electrode, all reactants and products of the electrode half-reaction must be present at the electrode.

10. 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) Cu2+ (m)
Cu2+ Cu
metal electrode; amalgam electrode; complex electrode; gas electrode.

11. Basic characteristics: Two phases / One interface
amalgam electrode
Zn(Hg)xZn2+(m1):
complex electrode
Ag(s)Ag(CN)2(m1):
Cu2+ Cu
Basic characteristics:
Two phases / One interface
Mass transport: metal cations only

12. Three-phase electrode: H2 gas H+ solution (liquid) Pt foil (solid)
Gas electrode:
Hydrogen electrode
Pt(s) H2(g, p)H+(c)
Three-phase electrode:
H2 gas
H+ solution (liquid)
Pt foil (solid)
1.0 mol·dm-3 H+ solution

13. Acidic hydrogen electrode Basic acidic Basic
Pt(s), H2(g, p)H+(c)
Pt(s), H2(g, p) OH(c)
2H+(c) + 2e  H2(g, p)
2H2O(l) + 2e  H2(g, p)+2OH(c)
acidic
oxygen electrode
Basic
Pt(s), O2(g, p)H+(c)
Pt(s), O2(g, p)OH(c)
O2(g, p) + 4H+(c) + 4e  2 H2O(l)
O2 (g, p)+ 2H2O + 4e  4OH(c)

14. metal – insoluble salt-anion electrode
(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
AgCl
Cl Ag
Ag(s)AgCl(s)Cl

15. Important metal – insoluble salt-anion electrode
Hg(l)Hg2Cl2(s)Cl (c):
Hg2Cl2(s) + 2e  2Hg(l) + 2Cl(c)
There are three phases contacting with each other in the electrode.
Pb(s)PbSO4(s)SO42 (c): in lead-acid battery
PbSO4(s) + 2e  Pb(s) + SO42 (c)

16. (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
Sn4+
Sn2+ Pt
Pt(s)Sn4+(c1), Sn2+(c2)
Sn4+(c1) + 2e  Sn2+(c2)

17. Important reduction-oxidation electrode
Pt(s)Fe(CN)63(c1), Fe(CN)64(c2) :
Fe(CN)63(c1) + e  Fe(CN)64(c2)
Pt(s)Q, H2Q: quinhydrone electrode
Q = quinone
H2Q = hydroquinone
Q + 2H + + 2e  H2Q

18. 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:

19. Zn(s)| ZnSO4(c1) ||CuSO4(c2) |Cu(s)
Cell notations
1) conventional symbolism
Zn(s)| ZnSO4(c1) ||CuSO4(c2) |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.

20. (2) Design of the reversible cell
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 + CuSO4 = ZnSO4 +Cu
e.g. 2 Ag+(m) + Cl(m) = AgCl(s)
e.g. 3 H2O = H+ + OH-