The reversible electrodes

2) Components of an electrode: 1. Current collector (first-class conductor) 2. Active materials: involves in electrochemical reaction 3. Electrolytic solution.

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

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.

The reversible electrodes The reversible cells consist at least of two reversible electrodes .`There are three types of these electrodes :

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.

2) Main kinds of reversible electrodes (1) The first-type electrode: a) 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.

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

Three-phase electrode: H2 gas H+ solution (liquid) Pt foil (solid) d)-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

a)-metal – insoluble salt-anion electrode (2) The second-type electrode: a)-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

Next- 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: a) 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)

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

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:

Zn(s)| ZnSO4(c1) ||CuSO4(c2) |Cu(s) 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.