chapter 7 electrochemistrycourse.sdu.edu.cn/g2s/ewebeditor/uploadfile/20171112211134582.pdf ·...

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


(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

2Fe3+ + Sn2+ 2Fe2+ + Sn4+

half-reactions:

Sn2+ Sn4+ + 2e

2Fe3+ + 2e- 2Fe2+

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.

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


7.6.2. 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


(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.



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:



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)



(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



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)



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:



7.6.4. 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


(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


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