Chapter 7 Electrochemistry §7.12 Basic principal and application of electrolysis

Electrolysis: The chemical reactions which accompany the passage of a current through an electrolytic solution. Electrode Positive electrodeNegative electrode AnodeCathode ReactionOxidationReduction Electrode and reaction:

1. Cathode reaction Suppose a solution in an electrolytic cell containing Ag +, Cu 2+, H +, and Pb 2+ 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? 1) Order of liberation Ag + + e   Ag:  ⊖ Ag + /Ag = V Cu e   Cu:  ⊖ Cu 2+ /Cu = V 2H + + 2e   H 2 :  ⊖ H + /H 2 = V Pb e   Pb:  ⊖ Pb 2+ /Pb = V

For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration. Ag +, Cu 2+, H +, and Pb 2+ will liberates at V; V; V; V, respectively without consideration of overpotential; Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb is 1.56 V  ⊖ Cu 2+ /Cu  ⊖ Pb 2+ /Pb  ⊖ Ag + /Ag  ⊖ H + /H 2 For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential.

a(Ag + ) = 1.5  V a(Cu 2+ ) = 2.2  V a(Pb 2+ ) = 3.3  V V The liberation order and the residual concentration of the ions upon negative shift of potential of cathode Potential sweep: polarization curve

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

 ⊖ Cu 2+ /Cu = V;  ⊖ Zn 2+ /Zn = V; When Zn begins to plate out, the residual concentration of Cu 2+ in the solution can be calculated according to: (1) Separation of metal = lg a Cu2+ C Cu2+  a Cu2+ = 2.54  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.

(2) Quantitative and qualitative analysis Polarograph Polarographic wave Dropping mercury cathode N2N2 A +  Hg anode Cu 2+ Tl + E 1/2 I max

Polarograph

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

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

 ⊖ Cu 2+ /Cu = V;  ⊖ Zn 2+ /Zn = V; When Zn begins to plate out, the residual concentration of Cu 2+ in the solution is Principle of alloy deposition Brass can’t deposit from the solution containing Cu 2+ and Zn 2+.  ⊖ Cu(CN) 3  /Cu =  1.03 V;  ⊖ Zn(CN) 4 2  /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.

(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. Electroreduction of aluminum Charles Martin Hall ( ), who first produced metal aluminum cheaply by electrolysis of molten mixture of Al 2 O 3 /Na 3 AlF 6.

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.

Titanium

(5) electrorefining of metal From 95% to 99.99%, which is suitable for electric usage. Cu Zn Ag Au Cu 2+ Zn 2+ Cu 2+ Industrial electrorefining of copper

(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.

2. Anode reaction 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 1) Reaction over inert anode

(1) Active dissolution; (2) Anodic passivation (3) Anodic oxidation 2) Reaction over active anode Pourbaix diagram of iron-water system (1) Active dissolution: At pH=4 and low current density, active dissolution occurs. Fe  Fe e  Fe 2+ Fe 2 O 3 Fe pH  / V Fe 3 O 4 Fe 3+ FeO 2 2  We usually judge the reaction based on Porbaix diagram

(2) Anodic passivation: At pH= 12 and high potential, upon polarization, dense thin layer of Fe 3 O 4 forms and passivation of iron takes place. 3Fe + 4H 2 O – 8e   Fe 3 O H + Passivation curve of iron Fe 2+ Fe 2 O 3 Fe pH  / V Fe 3 O 4 Fe 3+ FeO 2 2  Active dissolution passivation Trans- passivation

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

SEM photograph of the AAM top surface Cross-section

Application of anodic alumina membrane (AAM) 1)Coloring of aluminum and aluminum alloys 2)Corrosion protection 3)Template synthesis of nanomaterials.

Nanomaterials synthesized using AAM template Nanotubes