Electrochemistry: Elektrolytic and galvanic cell

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Electrochemistry: Elektrolytic and galvanic cell 1/26 Galvanic series (Beketov, cca 1860): Ca, Al, Mn, Cr, Zn, Cd, Fe, Pb, [H 2 ], Cu, Ag, Au Cell = system composed of two electrodes and an

1 Electrochemistry: Elektrolytic and galvanic cell 1/26 Galvanic series (Beketov, cca 1860): Ca, Al, Mn, Cr, Zn, Cd, Fe, Pb, [H 2 ], Cu, Ag, Au Cell = system composed of two electrodes and an electrolyte. electrolytic cell: electric energy chemical reaction galvanic cell: chemical reaction electric energy reversible galvanic cell (zero current) Electrodes anode = electrode where oxidation occurs Cu Cu e 2 Cl Cl e cathode = electrode where reduction occurs Cu e Cu Cl e 2 Cl credit: Wikipedia (free) Oxidation and reduction are separated in a cell. The charge flows through the circuit.

2 Anode and cathode 2/26 electrolytic cell galvanic cell I I Cl 2 Cl Cu2+ Cu Cu 2+ Cu Pt Cl Cl2 CuCl 2 (aq.) CuCl 2 (aq.) anode cathode anode cathode anions go to the anode

3 Galvanic cells: electrodes, convention 3/26 Electrodes(= half-cells) may be separated by a porous separator, polymeric membrane, salt bridge. Cathode is right (reduction) Anode is left (oxidation) negative electrode (anode) positive electrode (cathode) liquid junction phase boundary. (porous separator) salt bridge.. semipermeable membrane Examples: Cu(s) CuCl 2 (c = 0.1 mol dm 3 ) Cl 2 (p = 95 kpa) Pt Ag(s) AgCl(s) NaCl(m = 4 mol kg 1 ) Na(Hg) NaCl(m = 0.1 mol kg 1 ) AgCl(s) Ag(s) Pt Sn 2+ (0.1 mol dm 3 ) + Sn 4+ (0.01 mol dm 3 ) Fe 3+ (0.2 mol dm 3 ) Fe

4 Equilibrium cell potential 4/26 Also: electromotive potential/voltage, electromotive force (EMF). Should be measured at zero-current condition (balanced bridge, sensitive voltmeter) Cannot measure a potential of one electrode zero defined by the standard hydrogen electrode: 2 H + (aq.) + 2 e H 2 (g) where a H + = 1 (ph=0) and a H2 = 1 (p H2 = p st ). Construction of a hydrogen electrode: Pt sheet covered by platinum black, saturated by hydrogen credit: wikipedie symbols: E, E, φ; in physics: U

5 Cell potential II 5/26 Electrode potential of electrode X = voltage of cell NB: always the reduction potential H 2 (a = 1) H + (a = 1) X Standard (reduction) potential of an electrode: all reacting compounds have unit activities Examples: E Cu 2+ Cu = V, E Cl 2 2Cl = V (at 25 C) If the reaction are written in the way the cell produces energy: reaction = (reduction at cathode) + (oxidation at anode) E = E red cathode + Eox anode If all reactions are written as a reduction: reaction = (reduction at cathode) (reduction at anode) E = E red cathode Ered anode

6 Termodynamics of a reversible cell 6/26 Reversibility = reactions can be reversed by a small voltage change. No irreversible processes (metal dissolution, diffusion, liquid junction... ) r G m = W el = qe = zfe [p, T] Nernst equation: E = E RT zf ln i a ν i i where r G m = zfe, K = exp[ r G m/rt] = exp[zfe /RT] E = E,red cathode + E,ox anode = E,red cathode E,red anode r G < 0 i.e. E > 0 the cell produces current E = 0 i.e. r G = 0 = uncharged cell (equilibrium) distiguish from: equilibrium potential (at zero current) E Cu 2+ Cu = E Cu Cu 2+ (oxidation) but E Cl 2 2Cl = E 1 2 Cl 2 Cl hydrogen electrode right at 25 C: E = E 0 ph V

7 Termodynamics of a reversible cell II + 7/26 only electric work W el reversible [p, T] ( ) r G m r S m = T r H m = T 2 ( ( r G m /T) T ) p p = zf ( ) E T p = zft 2 ( (E/T) T ) p Q m = T r S m (II. Law for reversible processes) Oops! W p-v r U = Q + W = Q p r V + W el r H = r U + r (pv) [p] = r U + p r V = Q + W el Eq. Q = r H holds true only if the only work is pressure-volume And similarly for standard state (p = p st, a = 1), e.g.: ( ) ( r S r G ) m E m = = zf T T p p

8 Reduction potentials and different valences + 8/26 Example. E (Cr 2+ Cr) = V, E (Cr 3+ Cr) = V. Calculate E (Cr 3+ Cr 2+ ). Gibbs energies are additive (not potentials) Cr e Cr r G m = 2F ( V) Cr e Cr r G m = 3F ( V) Cr e Cr 2+ r G m = 1F E (Cr 3+ Cr 2+ ) 1F E (Cr 3+ Cr 2+ ) = 3F ( V) + 2F ( V) E (Cr 3+ Cr 2+ ) = 3 ( V) 2 ( V) = V

9 Electrodes 9/26 of the first kind (one reaction electrode ion) cationic, anionic metal, amalgam (metal in Hg), nonmetallic, gas of the second kind (nonsoluble salt two reactions) of the third kind redox (two exidation states) ion selective (membrane)

Electrodes of the first kind 10/26 cationic, metal ox: Zn Zn 2+, red: Zn 2+ Zn not for Fe, Al + ions, which are covered by oxides amalgam ox: Na(Hg) Na +, red: Na + Na(Hg) E Na + Na = E Na + Na RT F

10 Electrodes of the first kind 10/26 cationic, metal ox: Zn Zn 2+, red: Zn 2+ Zn not for Fe, Al + ions, which are covered by oxides amalgam ox: Na(Hg) Na +, red: Na + Na(Hg) E Na + Na = E Na + Na RT F ln a Na(Hg) a Na + saturated amalgam: a M(Hg)=1 cationic gas: hydrogen anionic gas: chlorine, oxygen Cl Cl 2 Pt : Cl e 2 Cl OH O 2 Pt : 1 2 O e + H 2 O 2 OH : 1 2 O e + 2H + H 2 O similarly: Br Br 2 Pt

mercury chloride (calomel) Cl Hg 2 Cl 2 Hg F ln a Cl Hg 2 Cl 2 (s) + 2 e 2 Hg + 2 Cl Usage:

11 Electrodes of the second kind 11/26 silver chloride Cl AgCl Ag AgCl(s) Ag + + Cl Ag + + e Ag(s) AgCl(s) + e Ag + Cl E AgCl Ag Cl = E AgCl Ag Cl RT F ln a Ag a Cl a AgCl = E AgCl Ag Cl RT mercury chloride (calomel) Cl Hg 2 Cl 2 Hg F ln a Cl Hg 2 Cl 2 (s) + 2 e 2 Hg + 2 Cl Usage: reference electrodes Pictures by: Analytical chemistry an introduction, 7th edition, Harcourt College, 2000

12 Curiosity: Electrodes of the third kind + 12/26 Three reactions: Ca 2+ (aq.) Ca(COO) 2 (s) Zn(COO) 2 (s) Zn(s) Ca 2+ + (COO) 2 2 Ca(COO) 2 Zn(COO) 2 (COO) Zn 2+ Zn e Zn Ca 2+ + Zn(COO) e Ca(COO) 2 + Zn E = E RT 2F ln 1 a Ca 2+ Can measure c Ca 2+ there is no Ca Ca 2+ (aq.) electrode, because Ca reacts with water (Ion-selective electrodes are more advantageous)

13 Redox electrodes 13/26 metal: quinhydrone electrode (ph 1 8): Sn 4+ Sn 2+ Pt Sn e Sn 2+ quinone (p-benzoquinone) + hydroquinone 1:1, sat. in a buffer O O (s) + 2 H e HO OH (s) quinone (Q) Nernst equation: E Q QH = E Q QH RT 2F ln Usage: measuring ph. = ( ph) V hydroquinone (QH) 1 a QH a Q a 2 = E Q QH + RT H + F ln a H +

Ion-selective electrodes Semipermeable membrane (for some ions only) 14/26 Glass electrode Membrane of special thin glass permeable for H + (and other ions). Difference of chem. pot.

14 Ion-selective electrodes Semipermeable membrane (for some ions only) 14/26 Glass electrode Membrane of special thin glass permeable for H + (and other ions). Difference of chem. pot. of both solutions µ(h +, ) µ(h +, electrode) = 1 RT ln a(h +, ) a(h +, electrode) is compensated by the electric work FE. Nernst equation credit: ref. silver chloride glass E = konst RT F ln a(h+, ) = konst RT F Usage: measuring ph (2 12), other ions ln 10 ph credit: wikipedie

Galvanic cells By the source of G: 15/26 chemical concentration electrolyte electrode By ion transfer: one electrolyte with salt bridge with membrane credit: payitoweb.

15 Galvanic cells By the source of G: 15/26 chemical concentration electrolyte electrode By ion transfer: one electrolyte with salt bridge with membrane credit: payitoweb.blogspot.com anode: Fe(s) Fe 3+ (aq.) + 3 e ( 0.04 V) Fe(s) Fe 2+ (aq.) + 2 e ( 0.44 V) cathode: 1 2 O e + 2H + H 2 O (1.23 V) or reduction of organic compounds (vitamin C)

16 Simple chemical cell [xcat ev/clanekagcl.ev] 16/26 Single electrolyte + electrodes Example. Consider a Pt electrode saturated by hydrogen and Ag wire covered by AgCl submerged in a solution of HCl (c = 0.01 mol dm 3 ) on the top of the highest Czech mountain, Sněžka (1602 m above sea level). The standard reduction potential of the Ag/AgCl/Cl electrode is V (p st = Pa). Calculate the cell voltage. A sea-level-reduced pressure is 999 mbar, temperature 25 C. p = Pa V (γ = 1) V (lim. DH) V (DH)

17 Chemical cell separated electrolytes 17/26 Porous barrier (liquid junction) (.). Irreversible liquid junction (diffusion) potential. Reduced by the salt bridge ( ). Example: Zn(s) ZnSO 4 CuSO 4 Cu(s) Electrode concentration cell Examples: Pt H 2 (p 1 ) HCl(aq.) H 2 (p 2 ) Pt (Given polarity for p 1 > p 2 ) (Given polarity for x 1 > x 2 ) Li(Hg)(x 1 ) LiCl(aq) Li(Hg)(x 2 )

18 Battery 18/26 (One or) more connected cells. Common disposable batteries: Alkaline battery (Zn, MnO 2 + C) Zn KOH (gel) MnO 2 Zn + 2 OH ZnO + H 2 O + 2 e 2 MnO 2 + H 2 O + 2e Mn 2 O OH Lithium (Li metal is light, has high potential) Electrolyte = salt (e.g., LiBF 4 ) in organic polar solvent Several possibilities, e.g.: Li Li + + e Mn IV O 2 + Li + + e Mn III LiO 2

19 Rechargeable batteries 19/26 Li-ion, Li-polymer: Li is in C (max. 1 Li in 6 C) Li (in C) LiBF 4 or polymer LiCoO 2.CoO 2 Positive electrode (e.g., in discharged state) LiCoO 2 = layers of CoO 2 intercalated by layers Li +. Charging: Li + to the solution, Co III Co IV Ni-MH: hydrogen in metal hydride (M = LaNi 5, CeAl 5, TiNi 2... ) H MH KOH(aq.) Ni(OH) 2 β-niooh Ni lead acid battery (high current) Pb PbSO 4 (s) H 2 SO 4 (20 30 wt. %) PbO 2 (s) PbSO 4 (s) Pb Summary reaction: Pb + PbO H 2 SO 4 or for anode and cathode: discharge recharge 2 PbSO H 2 O Pb + SO 4 2 PbSO 4 (s) + 2 e PbO 2 + SO H + + 2e PbSO 4 (s) + 2 H 2 O

20 Fuel cells 20/26 e.g., oxygen and hydrogen : H 2 2 H e protons permeate through a membrane : 1 2 O 2 +2 H + +2 e H 2 O expensive catalysts (Pt) purity of gases (CO) isopropanol fuel cell

21 Solubility product 21/26 Example. Determine the solubility product of AgCl using the standard potentials at 25 C. Data: E (Ag Ag + ) = V, E (Ag AgCl Cl ) = V. Ag AgCl(aq.) AgCl(s) Ag, red : Ag + + e Ag r G m = FE Ag + Ag ( 1), red : AgCl + e Ag + Cl r G m = FE Ag AgCl Cl (+1) K s = exp ( ) rg = exp RT AgCl Ag + + Cl r G m = F(E Ag AgCl Cl E Ag + Ag ) [ ] F RT (E Ag AgCl Cl E Ag + Ag ) = Short-circuit cell(virtual Ag in AgCl): the Nernst equation is E = 0 = (E Ag AgCl Cl E Ag + Ag ) RT F ln(a Cl a Ag +) = equilibrium condition a Cl a Ag + = K s

22 Kinetics of electrode phenomena 22/26 Electrode reaction: 1. diffusion of reactants to the electrode, ( 2. reaction in the adjacent layer), 3. adsorption of reactants to the electrode, 4. electron transfer of adsorbed molecules/ions and the electrode, 5. desorption of the products, ( 6. reaction in the adjacent layer), 7. diffusion of products out of the electrode. In case of slowdown: polarization of electrodes: concentration polarization (1., 7.) chemical polarization

23 Overpotential 23/26 is the voltage needed above the equilibrium potential (at one electrode) for the reaction to be actually observed a sort of the activation energy. Depeds on the electrode material (hydrogen on Pt: η 0, on Cu: 0.5 V, on Zn: 0.7 V), decreases slightly with increasing temperature, depends on the current density (η a + b ln J), increases power consumption during electrolysis + high hydrogen overpotential on metals allows electrochemical deposition of metals with (slightly) negative potentials (Cr, Co), lead-acid battery some compounds catalyze H 2 production, can be used in analysis

Corrosion 24/26 anodic phase: metal is dissolved cathodic phase: metal is deposited Cathodic protection: passive by anode of a more reactive metal (Zn, Al), which dissolves and produces a

24 Corrosion 24/26 anodic phase: metal is dissolved cathodic phase: metal is deposited Cathodic protection: passive by anode of a more reactive metal (Zn, Al), which dissolves and produces a negative protective voltage on an object (ship hull) sacrificial anode credit: RŠmi Kaupp (Wikimedia Commons) active additional voltage on the object, anode is (large pipes upto 50 V, 50 A)

25 Electroanalytical methods 25/26 coulometry charge or current needed for a chemical reaction (Faraday s laws of electrolysis; Cu, Ag, O 2 +H 2 ) calibration of ammeters (ampere meters) coulometric titration (const. current, time to equivalence) potentiometry voltage of a cell, (almost) zero current activity (concentration) of a substance is determined, then: ph (glass electrode, quinhydrone,... ) other ions acidity constants solubility products activity coefficients potentiometric titrations (ph etc.) voltammetry current vs. applied voltage: cyclic voltammetry (right) polarography credit:

mol dm 3 Jaroslav Heyrovský credits: http://canov.jergym.cz/objevite/objev2/hey.

26 Polarography 26/26 Voltammetric technique with a dropping mercury electrode Linear E: sensitivity up to mol dm 3 Problem: capacitive current Differential pulse polarography (DPP) to mol dm 3 Jaroslav Heyrovský credits: picture of polarograph by Lukáš Mižoch, CC BY-SA 3.0,

Electron Transfer Reactions

ELECTROCHEMISTRY 1 Electron Transfer Reactions 2 Electron transfer reactions are oxidation- reduction or redox reactions. Results in the generation of an electric current (electricity) or be caused by