幻灯片 1 - Shandong University

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Chapter 7 Electrochemistry
§7.6 Reversible cell
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.
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.)
3) Differences between chemical and electrochemical reactions
e
2Fe3+ + Sn2+  2Fe2+ + Sn4+
Sn2+
Fe3+
e
Fe3+
Fe3+
Sn2+
Fe3+
e
Sn2+
e Fe3+
e
e
2+
Fe3+ Fe
Fe3+
e
Fe3+
Fe2+
Fe2+
Fe3+
Sn4+
Sn2+
Sn4+
Sn2+
e
Sn2+
Sn2+ 4+
Sn
Sn2+
in bulk solution
Interfacial reaction
cathode
anode
at electrode / solution interface
halfreactions:
Sn2+  Sn4+ + 2e
2Fe3+ + 2e-  2Fe2+
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.
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
7.6.2. Reversibility of electrochemical cell
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:
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
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)
Cu

 2+


Cu

 Cu
Cu 2+ +2e 

Cu2+
Cu2+ Cu2+
Cu2+
Cu2+
metal electrode; amalgam electrode;
complex electrode; gas electrode.
amalgam electrode
Zn(Hg)xZn2+(m1):

 Zn(Hg) x
Zn 2  2e  xHg 

complex electrode
Ag(s)Ag(CN)2(m1):



Ag(CN)2  e 
Ag

2CN

Cu
Basic characteristics:

 2+


Cu
Cu2+
1) Two phases / One interface
2) Mass transport: metal cations only
Cu2+ Cu2+
Cu2+
Cu2+
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
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)
(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:
AgCl
Ag
metalinsoluble saltanion electrode
Cl
Cl
Cl
Cl
Cl
Cl Cl
Ag(s)AgCl(s)Cl



AgCl(s)  e 
Ag(s)

Cl

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)
(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:
Pt
oxidation-reduction electrodes
Sn2+
Sn4+
Sn4+
Sn2+
Sn4+
Sn2+
Pt(s)Sn4+(c1), Sn2+(c2)
Sn4+(c1) + 2e  Sn2+(c2)
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
OH
O
+
+ 2H + 2e-
O
Q = quinone
OH
H2Q = hydroquinone
Q + 2H + + 2e  H2Q
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:
7.6.4. 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.
(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 + CuSO4 = ZnSO4 +Cu
e.g. 2 Ag+(m) + Cl(m) = AgCl(s)
e.g. 3 H2O = H+ + OH-
Outside class
reading
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

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