Electrochemical reaction

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1 Electrochemical reaction electrochemistry electrochem. reaction mechanism electrode potential Faradays law electrode reaction kinetics 1 Electrochemistry in industry Chlor-Alkali galvano industry production of other inorganic./organic compounds recycling wastes treatment and reprocessing dezinfection / toxic wastes elemination contaminated soils treatment electric energy sources (battery, fuel cells,...) electroanalytical methods versatile selective ecological expensive mass transfer Advantages direct oxidation or reduction mediated oxidation or reduction simple construction scalable electrode potential material of electrode (overpotentials) separation of electrode chambers electron - clean reactant recycling of raw materials waste treatment Disadvantages Faradays law + price of electric energy electrode materials heterogeneous process - concentrations limited by mass transfer 3 1

proceed processes hardly realizable (or unrealizable) by other way.

2 Basic terms Electrochemical reactions - the transfer of electrons between the electrode surface and molecules in the electrolyte. reactant - electron rate of electron flow electric current oxidation/reduction power - potential (voltage) amount of electrons electrical charge Separation of electrode reactions in reactor enable to proceed processes hardly realizable (or unrealizable) by other way. 4 Basic terms I Electrode (from technological point of view) piece of electrically conductive matter (of suitable shape) where electrode reaction take place (on its surface) Anode electrode with oxidative reaction Cathode electrode with reductive reaction Electrolyte ion conductive medium. Mainly solutions with dissociated molecules (ions). 5 Electrochemical reactors: Basic terms II Electrolyser - to do electrolysis i.e. redox reaction driven by external voltage on the anode and cathode Galvanic cell batteries, accumulators } power sources Elchem. generators - fuel cells Separator permeable barrier for anodic and cathodic chamber separation. Membrane or diaphragm. 6

Process size F. C.Walsh, Pure Appl. Chem., Vol. 73, No. 1, pp. 1819 1837, 1.

another reaction occurs to enable charge flow.

3 Process size F. C.Walsh, Pure Appl. Chem., Vol. 73, No. 1, pp , 1. 7 Electrode reaction reaction with electron as a reactant or product a A + n e - = m M Simultaneously on the second electrode another reaction occurs to enable charge flow. b B = z Z + n e - total reaction in the system is the sum of both electrode reactions aa + b B = mm + zz! Electrode reactions occurs at electrodes separated by electrolyte. One electrode is called halfcell 8 Electrochemical reaction Bagdad Battery BC 9 3

.E-5 1.E-5-1.E-5 -.E-5-3.E-5-4.E-5 Pt v.5m HSO4,.E+...4.6.8 1. 1. 1.4 1.

surface reaction on electrode products desorption from he surface transport to the bulk more than one electron reaction less probable usually sequence of individual steps NO 3- + 3 H O + 5 e - = 1/ N

4 .E-5 1.E-5-1.E-5 -.E-5-3.E-5-4.E-5 Pt v.5m HSO4,.E E [V] Mechanism of electrode reaction reaction mechanism consists from several steps transport to the electrode adsorption on the el. surface reaction on electrode products desorption from he surface transport to the bulk more than one electron reaction less probable usually sequence of individual steps NO H O + 5 e - = 1/ N + 6 OH N N H 3 N H 4 N H O H N N O N O N O N O 4 N O N O alkalické prostředí 1 Polarisation curve Polarization curve - dependence of current on voltage refers to the course of events on the electrode in the system Reversible reaction I [A] Polarografický záznam Pt electrode in H SO 4 11 Galvani potential the work done in moving a unit positive charge from infinity to that point Usually difference of potentials Electrode potential Standard (reference) electrode electrode of known potential restive to the potential shift (calomel, mercurosulpahate, Ag/AgCl) For solutions in protic solvents, the universal reference electrode for which, under standard conditions, the standard electrode potential (H+ / H) is zero at all temperatures. : SHE + H ( aq) + e H ( g) Activity of each compounds in SHE must be = 1 (p(h )= 11,3 kpa, a(h + )=1) 1 4

5 Fundamental relationships I Current density j = I/A El. charge Faraday s law m is the mass of the substance produced at the electrode (in grams), Q is the total electric charge that passed through the solution (in coulombs), q is the electron charge = 1.6 x 1-19 coulombs per electron, n is the valence number of the substance as an ion in solution (electrons per ion), F is Faraday's constant, M is the molar mass of the substance (in grams per mole), and N A is Avogadro's number = 6. x 1 3 ions per mole. I is current t is time (in seconds), A is electrode area 13 Fundamental relationships II Electrode reaction potential r G= - n F E r relation between el. potential and Gibbs energy νp RT a p... Nernst equation E = Er ln νr nf ar... electrode potential calculation Tafel equation "a" and "b" are characteristic constants of the electrode system kinetic factor of electrochemical reaction 14 Faradays law m is the mass of the substance produced at the electrode (in grams), Q is the total electric charge that passed through the solution (in coulombs), q is the electron charge = 1.6 x 1-19 coulombs per electron, n is the valence number of the substance as an ion in solution (electrons per ion), F is Faraday's constant, M is the molar mass of the substance (in grams per mole), and N A is Avogadro's number = 6. x 1 3 ions per mole. I is current t is time (in seconds), A is electrode area Task: calculate volume and weight of produced Cl during 1h in the case of total current 1 ka, pressure 11,3 kpa and temp. 5 C. Current efficiency is 95 %. Mr (Cl ) = 7,9 g/mol Q = I t = 1 * 36 = 3,6 1 7 C Q(Cl ) = 3,6 1 7 *,95 = 3,4 1 7 C n(cl ) = 3,4 1 7 /(96485 * ) = 177,3 mol V = n R T / p = 177,3 * 8,314 * 98 / 11,3 1 3 = 4,3346 m 3 m = n*m = 177,3 * 7,9 = 1565,5 g = 1,5655 kg 15 5

reaction: + H ( aq) + e H ( g) RT ah E = E ln F ( a ) + H a(h + ) = 1 -ph = 1-1 =,1 a(h ) = p(h )/f std = 99,8/11,3 =,985 E = (8,314* 98)/(*965) * ln(,985/,1 ) = -,589 V 16 Cell potential Nernst

6 Electrode potential calc. Nernst equation νp... o RT a p E = Er ln νr nf a... in tables potentials in the reduction direction r Task: calculate potential of hydrogen electrode in HCl at conditions ph=1, pressure H 99,8 kpa and temp 5 C. reaction: + H ( aq) + e H ( g) RT ah E = E ln F ( a ) + H a(h + ) = 1 -ph = 1-1 =,1 a(h ) = p(h )/f std = 99,8/11,3 =,985 E = (8,314* 98)/(*965) * ln(,985/,1 ) = -,589 V 16 Cell potential Nernst equation II for cell potential the difference between electrodes (cathode and anode) is calculated. in praxis both electrodes are calculated in the reduction form and the finall cell potential ic calculated as: E celk = E kat E and U elz = - E celk Task: calculate theoretical cell potential for HCl electrolysis in the system from previous task Cl (g) + e - = Cl - E o = V E(H + /H ) = -,589 V E(Cl /Cl - ) = E o RT/nF ln(a(cl - ) /a(cl )) E (Cl /Cl - ) = 1,358 (8,314* 98)/(*965) * ln(,1 /,985) = V E celk = E kat E and = -, = V U elz = - E celk = V 17 Counter electrode reaction Electric energy demand is very high with respect to the requested charges/currents (1 ka). Each small decrease of potential causes high energy savings. Productions o Cl by HCl electrolysis. Replacemnet of cathode hydrogen production for 1 t Cl is necessary 1 kwh beside 17 kwh. E o (H + /H )=,V; E o (O /H O)= 1,9V; E o (Cl /Cl - )= 1.358V

Galvanoindustry Proper selection of galvanic bath composition it is possible to deposit simultaneously more metals (bronzes, brass) Cu + + e = Cu E o =,337 V Sn + + e = Sn E o = -,14 V Zn + + e = Zn

93 V 19 Kinetics of electrode reaction Rate of electrochemcial reaction is depenedent on various parameters electrode material, electrolyte composition, temperature etc.

7 Galvanoindustry Proper selection of galvanic bath composition it is possible to deposit simultaneously more metals (bronzes, brass) Cu + + e = Cu E o =,337 V Sn + + e = Sn E o = -,14 V Zn + + e = Zn E o = -,736 V Sn(OH) 6 + e - = HSnO + 3 OH + H O HSnO + H O + e - = Sn + 3 OH E o =.99 V E o =.93 V 19 Kinetics of electrode reaction Rate of electrochemcial reaction is depenedent on various parameters electrode material, electrolyte composition, temperature etc. Overvoltage difference between potential at OCP and potential at current density η E = ( j) E r 3 Exponential dependence of current (reaction speed) on overvoltage Anode positive sign j / A m E r E / V vs. E r Various types: Overvoltage activation rate determining step electron transfer between electrolyte and electrode concentration (difusion) mass transfer to the electrode surface other overvoltages- sorption, preceding reaction, surface reaction, 1 7

8 Kinetics of electrochemical reaction overpotential irreversibility of reaction- activation - concentration Butler-Volmer equation: j is the anodic or cathodic current density; α = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction. αvalues are typically close to.5; η act = E applied - E eq, i.e. positive for anodic polarization and negative for cathodic polarization; n = number of participating electrons; R = gas constant; T = absolute temperature; F = Faraday = C mol -1 Tafel equation η = a + b log j or for anode Task: Calculate potential of anode for chlorine production, j = 6 A/dm and temp. = 7 ºC. Activity coef. for Cl - and Cl are (a(cl - ) = 1, a(cl ) = 1). Concentration of chlorides c(cl - ) = 1mol/L. Reaction operates at pressure 76 mm Hg. 1) On graphite - Tafel plot for 7 ºC j = [A/cm ] η =,6 +,1 log j ) on ATA Tafel plot η =, E Cl 1, 3595 = Cl de = -,389 mv/k dt Kinetics of electrode reaction Cl Cl + V,6 log j j = [A/cm ] při 5 ºC ph O = 33 mm Hg 4 8

96487,693 Graphite overvoltage 6 A/dm =,6 A/cm E = r E + η Cl ATA overvoltage 1,337 V ηcl =,6 +,1 log,6 =,6,7 =, 573 V E = 1,337 +,573 = 1, 91V ηcl =, +,6 log,6 =,,8 =, 199 V E = 1,337 +,199 = 1,

9 Kinetics of electrode reaction Electrode reaction Cl + e - = Cl - Electrode potential from Nernst eq. 3 E = 1,3595,389 1 ( 7 5) = 1,3595,175 1, 34 V 1 = p ph O acl = γ a = =, 693 p Cl 76 RT a E = E ln nf a r Cl Cl de E1 = E ( T1 T ) dt 8,314 (73,15 + 7) 1 3 r E = 1,34 ln = 1,34 5,4 1 = 96487,693 Graphite overvoltage 6 A/dm =,6 A/cm E = r E + η Cl ATA overvoltage 1,337 V ηcl =,6 +,1 log,6 =,6,7 =, 573 V E = 1,337 +,573 = 1, 91V ηcl =, +,6 log,6 =,,8 =, 199 V E = 1,337 +,199 = 1, 536V 5 Concentration overvoltage transport of ion from bulk to the electrode surface J c D δ = N c s 6 3 c τ c j / A m η / V c s δ N 6 Importance of overvoltage galvanotechnics Additives for increased concentration overvolatges smooth surface Polarization curve for the potentiostatic deposition of copper. (a) Overpotential; mv, deposition time: 6 hours; (b) Overpotential: 3 mv, deposition time: 5 hours; (c) Overpotential: 7 mv, deposition time: min. 7 9

10 NaCl electrolysis Overvoltage fo Cl production High overvoltage for O enable production opf Cl E o (O /H O)= 1,9 V E o (Cl /Cl - )= V Amalgam/mercury electrolysis 8 Power sources overvoltage bateries, accumulators, fuel cells 9 1

Chapter 18 Electrochemistry. Electrochemical Cells

Chapter 18 Electrochemistry. Electrochemical Cells Chapter 18 Electrochemistry Chapter 18 1 Electrochemical Cells Electrochemical Cells are of two basic types: Galvanic Cells a spontaneous chemical reaction generates an electric current Electrolytic Cells