Electrochemistry-TheoryThe Scope of Electrochemistry Electrochemistry – provide information about activities rather than concentrations of chemical species. • Battery • Corrosion • Electrolysis • Electroanalysis Electrochemical Cells • Consist of two electrical conductors immersed in an electrolyte solution. • Generally contain: – External wires (electrons carry current). – Conducting electrodes (metal, carbon). – Ion solutions (ions carry current). – Interfaces or junctions. – A complete electrical circuit.Electrochemical Cells • Galvanic cell: a cell that produces electrical energy • Electrolytic cell: a cell that consumes electrical energyElectrochemical (Galvanic) Cells Open%circuit%–%no%current%flow%(measure%poten3al)%%%%%%%%Closed%circuit%–%current%flow%(measure%current)%%Oxida3on%occurs%at%the%anode%(AnOx);%reduc3on%occurs%at%the%cathode%(RedCat)%%Net%reac3on:%%Cu(s)%+%2Ag+%%%%%%%%%%%Cu2+%+%2Ag(s)%←%ClB%%%K+%→%• Copper (Cu) wants to ionize more than Silver (Ag).Electrochemical (Electrolytic) Cells Net%reac3on:%%Cu2+%+%2Ag(s)%%%%%%%%%Cu(s)%+%2Ag+%Conduction in an Electrochemical Cell • Electrons serve as carriers (e.g. moving from Cu through the conductor to the Ag) • In the solution, electricity involves the movement of cations and anions • In the salt bridge both chloride and potassium ions move • At the electrode surface: an oxidation or a reduction occurs – Cathode: the electrode at which reduction occurs – Anode: the electrode at which oxidation occursElectrolytic cell 1. Requires potential/voltage input. 2. The potential/voltage input + the cell potential must be > 0 for the reactions to occur. 3. The cell potential is negative – consume electrical energy. Galvanic cell 1. Requires no potential/voltage input. 2. The voltage becomes smaller and it ultimately reaches 0V when the system achieves equilibrium. 3. The cell potential is positive – produce electrical energy. Galvanic cell vs. Electrolytic cellWhat happens at the electrode surface? • Electrons*are*transferred*at*electrode*surface*by*redox*reac2ons:*– Occur*at*liquid/solid*interface**(solu2on/electrode).*• An*electrical*double*layer*is*formed:*– Tightly*bound*inner*layer*–*Stern*layer.*– Loosely*bound*outer*layer*–*Diffuse*layer.*Types*of*Current*in*an*Electrochemical*Cell*• Two*types*of*processes*can*result*in*current*flow*across*the*electrodeIsolu2on*interface:*– Faradaic*currents:*• Propor2onal*to*ionic*species*concentra2on*(follows*Faraday’s*Laws).*• Due*to*redox*reac2ons*at*electrodes.*– NonIfaradaic*currents:*• Result*from*charging*of*the*electrical*double*layer*(capacitance).*• Not*due*to*redox*reac2ons.*• Redox*reac2ons*occur*close*to*electrode*surface*(<10*Å).*• Faradaic*currents*require*con2nual*mass*transport*of*ions*to*the*electrode*surface.**This*occurs*by:*– Convec2on*(s2rring,*flowing)*.*– Diffusion*(concentra2on*gradient).*– Migra2on*(electrosta2c*force).*Electrochemical Cell Notation • Convention: – Anode is listed on the left. – Electrolyte solutions and liquid-liquid interface in center. – Cathode is listed on the right. – Activities or concentrations listed in parentheses. Cu|CuSO4 (0.02 M)||AgNO3 (0.02 M)|Ag • Each vertical line indicates a phase boundary. • Galvanic cell as written; electrolytic cell if reversed.Electrode Potentials • An electrochemical cell as made up of two half-cell reactions (one at the anode and the other at the cathode), each of which has its own electrode potential (E). • When combined, the two reactions will determine the overall potential of the cell (Ecell) and the direction of current flow. • The cell potential is the difference between anode and cathode potentials: Ecell = Ecathode - Eanode • By convention, the half-reactions are always written as reductions: Ecell = EAgCl/Ag – EH+/H2 Ecell = Eright – Eleft **The Standard Hydrogen Electrode (SHE) • A universal reference, but is really a hypothetical electrode (not used in practice) – Uses a platinum electrode, which at its surface oxidizes 2H+ to H2 gas. – Very sensitive to temperature, pressure, and H+ ion activity • Because the SHE is difficult to make, the saturated calomel electrode (SCE) is used instead. – Ag/AgCl electrode – Calomel = mercury (I) chloride Pt, H2 (p atm)| H+ (aH+ = x)Use of the SHE to Measure Electrode Potentials • An electrode potential is defined as the potential of a cell with the electrode under study as the right hand electrode and the SHE as the left hand electrode. • If a = 1.00 M, p = 1 atmosphere, the electrode potential, E becomes the standard electrode potential, E0.Some Standard Electrode Potentials Oxidizing agents • E"is*temperature*dependent;*E0*determined*at*25°C.*• E0*is*calculated*rela2ve*to*the*SHE*(assigned*E0*=*0.000*V).*• E0*is*measured*at*standard*condi2ons*(298K,*1M*of*solutes*and*1*bar*of*gas*pressure*.*• E0*is*a*measure*of*the*driving*force*(ΔG)*of*the*half*reac2on.*Reducing agentsNernst Equation • Compensates*for*non*unit*ac2vity*(i.e.,"not*1*M).*• Rela2onship*between*cell*poten2al*and*electrolyte*ac2vi2es.*aA*+*bB*+*neI********cC*+*dD***• At*298°K,*2.3RT/F*=*0.0592.*• What*is*the*poten2al*of*an*electrode*of*Zn(s)*and*0.01*M*Zn2+?*Zn2+*+*2eI*******Zn;* E0*=*I0.763*V.*ac2vity*of*Zn(s)*is*1:*• Work*done*by*cell:*– ΔG*=*InFE""=""0RTlnK;"F"=*96,485*coulombs/mole*of*electrons.*V 822.001.01log20592.0763.0 −=−−=EKnFRTEBADCnFRTEEbadclog30.2][][][][log30.200−=−=e- e- reference electrode counter electrode working electrode indicator electrode detector electrode Potentiometry: Measures equilibrium E Voltammetry: Control E, measures I as function of time control measurement Electrochemical Cells and Analytical MethodsPotentiometry an indicator electrode, a reference electrode an a potential measuring deviceIndicator Electrode ReferenceElectrodeGeneral*Principles*Ecell = (Eind – Eref) + EjElectrodes and PotentiometryReference Electrodes Standard Hydrogen Electrode Requires freshly prepared Pt surface.Hg2Cl2 + 2e- 2Hg(l) + 2Cl– SCE reference electrode Hg|Hg2Cl2(sat’d), KCl (xM)||AgCl(s) + e- Ag(s) + Cl– Ag/AgCl reference electrode Vycor plug• Electrodes of