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Chapter 7 Electrochemistry
§7.6 Reversible cell
Levine: pp. 417 14.4 Galvanic cells:
pp. 423 14.5 types of reversible electrodes
Outside class reading
Electrolytic cell;
Galvanic/voltaic cell
(1) Electrochemical apparatus
Reaction:
oxidation reaction: anode, anodic reaction
reductive reaction: cathode, cathodic reactions.
Components:
Electrodes;
electrolytic solution
7.6.1 Basic concepts of electrochemical apparatus
§7.6 Reversible cell
(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)|Zn2+(sln.)
2) (Pt), H2(g, p)|H+ (sln.)
7.6.1 Basic concepts of electrochemical apparatus
§7.6 Reversible cell
(3) Differences between chemical and electrochemical reactions
2Fe3+ + Sn2+ 2Fe2+ + Sn4+
half-reactions:
Sn2+ Sn4+ + 2e
2Fe3+ + 2e- 2Fe2+
at electrode / solution
interface
in bulk solution
Interfacial reaction
7.6.1 Basic concepts of electrochemical apparatus
§7.6 Reversible cell
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.
dG = -SdT + VdP + W’
Maximum useful work
(1) 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 thermo-
dynamic reversible manner or vice versa.
G = -W’ = QV = -nFE
The relation bridges thermodynamics and
electrochemistry
§7.6 Reversible cell
7.6.2. 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
7.6.3. Reversible electrodes
1) single electrode; Zn|Zn2+; Zn|H+;
2) reversible reaction; Zn Zn2+ + 2e
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
§7.6 Reversible cell
(1) basic characteristics:
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)
metal electrode; amalgam electrode;
complex electrode; gas electrode.
7.6.3. Reversible electrodes
§7.6 Reversible cell
Zn(Hg)xZn2+(m1):
Ag(s)Ag(CN)2(m1):
amalgam electrode
complex electrode
Basic characteristics:
1) Two phases / One interface
2) Mass transport: metal cations only
1) The first-type electrode:
7.6.3. Reversible electrodes
§7.6 Reversible cell
Gas electrode:
Three-phase electrode:
H2 gas
H+ solution (liquid)
Pt foil (solid)
Pt(s) H2(g, p)H+(c)
Hydrogen electrode
1.0 mol·dm-3
H+ solution
Acidic
hydrogen electrode
Basic
hydrogen electrode
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
oxygen electrode
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)
7.6.3. Reversible electrodes
§7.6 Reversible cell
(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.
metalinsoluble saltanion electrode
Ag(s)AgCl(s)Cl
7.6.3. Reversible electrodes
§7.6 Reversible cell
Important metal – insoluble salt-anion
electrode
Hg(l)Hg2Cl2(s)Cl (c):
Pb(s)PbSO4(s)SO42 (c): in lead-acid
battery
Hg2Cl2(s) + 2e 2Hg(l) + 2Cl(c)
PbSO4(s) + 2e Pb(s) + SO42 (c)
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.
Pt(s)Sn4+(c1), Sn2+(c2)
Sn4+(c1) + 2e Sn2+(c2)
7.6.3. Reversible electrodes
§7.6 Reversible cell
Pt(s)Fe(CN)63(c1), Fe(CN)6
4(c2) :
Pt(s)Q, H2Q: quinhydrone electrode
Fe(CN)63(c1) + e Fe(CN)6
4(c2)
Q + 2H + + 2e H2Q
Q = quinone H2Q = hydroquinone
O
O
2e-2H+
OH
OH
+ +
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:
7.6.3. Reversible electrodes
§7.6 Reversible cell
7.6.4. 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)| ZnSO4(c1) ||CuSO4(c2) |Cu(s)
cell notation / cell diagram
§7.6 Reversible cell
(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 + CuSO4 = ZnSO4 +Cu
2 Ag+(m) + Cl(m) = AgCl(s)
3 H2O = H+ + OH-
7.6.4. Cell notations
§7.6 Reversible cell