Chapter 7 Electrochemistry

1. Chapter 7 Electrochemistry §7.12 Basic principal and application of electrolysis

2. Electrolysis: The chemical reactions which accompany the passage of a current through an electrolytic solution. Electrode and reaction:

3. 1. Cathode reaction 1) Order of liberation Suppose a solution in an electrolytic cell containing Ag+, Cu2+, H+, and Pb2+ of 1 molarity. If the potential is initially very high and is gradually turned down, in which order will the metals be plated out onto the cathode?

4. 0.799 ⊖ Ag+/Ag 0.000 ⊖ H+/H2 For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential. For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration. 0.337 ⊖ Cu2+/Cu Ag+, Cu2+, H+, and Pb2+ will liberates at 0.799 V; 0.337 V; 0.000 V; -0.126 V, respectively without consideration of overpotential; Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb is 1.56 V -0.126 ⊖ Pb2+/Pb

5. a(Pb2+) = 3.310-49 a(Cu2+) = 2.210-16 a(Ag+) = 1.510-8 0.337 V -1.56 V -0.126 V 0.799 V Potential sweep: polarization curve The liberation order and the residual concentration of the ions upon negative shift of potential of cathode

6. 2) Application 1) Separation of metal 2) Quantitative and qualitative analysis 3) Electroplating of single metal and alloy 4) Electrolytic metallurgy 5) Electrorefining of metal 6) Electrosynthesis

7. (1) Separation of metal ⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V; When Zn begins to plate out, the residual concentration of Cu2+ in the solution can be calculated according to: -0.763 = 0.337 + 0.05916lg aCu2+ CCu2+ aCu2+ = 2.5410-19 mol·dm-3 When Zn begins to deposit, Cu has deposited completely. When the difference between liberation potential of two metals is larger than 0.2 V, the two metal can be separated completely.

8.  A N2 Dropping mercury cathode Tl+ E1/2 Imax + Cu2+ Hg anode (2) Quantitative and qualitative analysis Polarograph Polarographic wave

9. Polarograph

10. Progress of the sensitivity of polarography 1935: 10-2 ~ 10-5 mol·dm-3 1957: 10-8 ~ 10-9 mol·dm-3 1957: 210-10 mol·dm-3 At present: 1010~1012 moldm-3 Jaroslav Heyrovský 1959 Noble Prize Czechoslovakia 1890/12/20 ~ 1967/03/27 Polarography

11. (3) Electroplating of single metal and alloy Anode: Ag  Ag++ e Cathode: Ag+ + e Ag Alloy electroplating: Zn-Fe, Cu-Zn Composite electroplating: Ni-PTFE, Ni-Diamond Electroplating of non-metals: Plastic, wood, flowers A silver-plated teapot

12. Principle of alloy deposition ⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V; When Zn begins to plate out, the residual concentration of Cu2+ in the solution is Brass can’t deposit from the solution containing Cu2+ and Zn2+. ⊖ Cu(CN)3/Cu = 1.03 V; ⊖ Zn(CN)42/Zn = 1.12 V;   < 0.2 V Zn co-deposits with Cu and form brass. When tin is added, a alloy coating with gold luster can be plated out.

13. (4) Electrolytic metallurgy Many most active metals, such as Li, Na, K, Mg, Ca, Al, Ti, rare earth metal, etc. can be only produced electrochemically. Charles Martin Hall (1863-1914), who first produced metal aluminum cheaply by electrolysis of molten mixture of Al2O3/Na3AlF6. Electroreduction of aluminum

14. Production of metal sodium The man who discovered the largest number of elements Davy, on knowing the electrolysis of water by Nicholson and Carlisle, set out his element finding trip using electrolysis as his powerful tools, he discovered 8 elements including: K, Na, Ma, Ca, Sr, Ba, B, and Si.

15. Titanium

16. Cu Cu2+ Cu2+ Zn Zn2+ Cu2+ Ag Cu2+ Au Cu2+ (5) electrorefining of metal From 95% to 99.99%, which is suitable for electric usage. Industrial electrorefining of copper

17. (6) Electrosynthesis Advantages of Electrochemical Synthesis 1) The oxidative or reductive ability can be easily adjusted. 2) The most powerful oxidation or reduction methods. 3) Without introduction of impurities.

18. 2. Anode reaction 1) Reaction over inert anode When inert material such as Platinum and graphite was used, the species in the solution discharge on the electrode in the order of liberation potential. F < Cl < Br < I Henri Moissan 1906 Noble Prize France 1852/09/28 ~ 1907/02/20 Investigation and isolation of the element fluorine

19. Fe3+  / V Fe2O3 Fe3O4 Fe2+ FeO22 Fe 0 2 4 6 8 12 14 10 pH 2) Reaction over active anode (1) Active dissolution; (2) Anodic passivation (3) Anodic oxidation We usually judge the reaction based on Porbaix diagram (1) Active dissolution: At pH=4 and low current density, active dissolution occurs. Fe  Fe2+ + 2e Pourbaix diagram of iron-water system

20. Fe3+  / V Fe2O3 Fe3O4 Fe2+ FeO22 Fe 0 2 4 6 8 12 14 10 pH (2) Anodic passivation: At pH= 12 and high potential, upon polarization, dense thin layer of Fe3O4 forms and passivation of iron takes place. 3Fe + 4H2O – 8e  Fe3O4 + 8 H+ Active dissolution Trans-passivation passivation Passivation curve of iron

21. E / V Initiation of pores Porous layer Barrier layer t / h (3) Anodic oxidation Anodic oxidation of aluminum

22. SEM photograph of the AAM Cross-section top surface

23. Application of anodic alumina membrane (AAM) • Coloring of aluminum and aluminum alloys • Corrosion protection • Template synthesis of nanomaterials.

24. Nanomaterials synthesized using AAM template Nanotubes