Introduction to Electrochemistry (Chapter 22) Many different electroanalytical methods: ·fast ·inexpensive ·in situ ·information about oxidation states stoichiometry rates charge transfer equilibrium constants CEM 333 page 10.1
Introduction to Electrochemistry (Chapter 22) Many different electroanalytical methods: • fast • inexpensive • in situ • information about oxidation states stoichiometry rates charge transfer equilibrium constants CEM 333 page 10.1
Electrochemical Cells: Oxidation and reduction (redox)reactions Separate species to prevent direct reaction (Fig 22-1) Voltmeter 1.100V Salt bridge Saturated KCl solutior Zn electrode Cu electrode 0.0100M 0.0100M ZnSO CuS04 solution solution Zn(s)=Zn2+(ag)+2e Cu2+(aq)+2e-=Cu(s) azm2+=0.010 acu2+=0.010 Anode Cathode Most contain· external wires (electrons carry current) ion solutions (ions carry current) interfaces or junctions All contain.complete electrical circuit conducting electrodes (metal,carbon) CEM 333 page 10.2
Electrochemical Cells: Oxidation and reduction (redox) reactions Separate species to prevent direct reaction (Fig 22-1) Most contain • external wires (electrons carry current) • ion solutions (ions carry current) • interfaces or junctions All contain • complete electrical circuit • conducting electrodes (metal, carbon) CEM 333 page 10.2
Electrons transferred at electrode surface at liquid/solid interface Potential difference (voltage)is measure of tendency to move to equilibrium Galvanic cell-cell develops spontaneous potential difference Overall: Zn(s)+Cu2+(aq)>Zn2+(aq)+Cu(s) Zn(s)→Zn2++2e Oxidation Half reactions: Cu2++2e→Cu(S) Reduction Convention: Reduction at Cathode Oxidation at Anode Galvanic cell-Zn anode (negative),Cu cathode (positive) CEM 333 page 10.3
Electrons transferred at electrode surface at liquid/solid interface Potential difference (voltage) is measure of tendency to move to equilibrium Galvanic cell - cell develops spontaneous potential difference Overall: Zn(s) + Cu2+ (aq) ® Zn2+ (aq) + Cu(s) Half reactions: Zn(s) ® Zn2+ + 2e- Oxidation Cu2+ + 2e- ® Cu(s) Reduction Convention: Reduction at Cathode Oxidation at Anode Galvanic cell - Zn anode (negative), Cu cathode (positive) CEM 333 page 10.3
Electrolytic cells-require potential difference greater than galvanic potential difference(to drive away from equilibrium) Zn(s)→Zn2++2e Oxidation Galvanic cell Cu2++2e→Cu(s) Reduction Zn2++2e→Zn(s) Reduction Electrolytic cell Cu(s)>Cu2++2e-Oxidation Electrolytic cell-Zn cathode(positive),Cu anode(negative) Many chemically reversible cells Short-Hand Cell notation: Convention: Anode on Left Zn|ZnSO(0.01 M)CuSO(0.01 M)Cu liquid-liquid interface Galvanic cell as written Electrolytic cell if reversed CEM 333 page 10.4
Electrolytic cells - require potential difference greater than galvanic potential difference (to drive away from equilibrium) Zn(s) ® Zn2+ + 2e- Oxidation Cu2+ + 2e- ® Cu(s) Reduction Galvanic cell Zn2+ + 2e- ® Zn(s) Reduction Cu(s) ® Cu2+ + 2e- Oxidation Electrolytic cell Electrolytic cell - Zn cathode (positive), Cu anode (negative) Many chemically reversible cells Short-Hand Cell notation: Convention: Anode on Left Zn|ZnSO4 (0.01 M)||CuSO4 (0.01 M)|Cu liquid-liquid interface Galvanic cell as written Electrolytic cell if reversed CEM 333 page 10.4
Not all cells have liquid-liquid junctions(Fig 22-3) H2 10 P=1.00atm) 0.01MHC1 saturated with AgCl Ptelectrode(anode Hz(aq)-H+(aq)+e Solid AgCl AgCl(s)->Ag(aq)+CI(aq) H2(g)→H2(aq) Cathode:Ag(aq)+e->Ag(s) Anode: H2(aq)→2H(aq)+2e Overall:2AgCl(s)+H(g)>2Ag(s)+2H++2CI- Pt,H2(p=1atm)H(0.01 M),CI(0.01 M),AgCI(sat'd)Ag CEM 333 page 10.5
Not all cells have liquid-liquid junctions (Fig 22-3) AgCl(s) ® Ag+ (aq) + Cl- (aq) H2 (g) ® H2 (aq) Cathode: Ag + (aq) + e - ® Ag(s) Anode: H2 (aq) ® 2H+ (aq) + 2e - Overall: 2AgCl(s) + H2 (g) ® 2Ag(s) + 2H+ + 2ClPt,H 2 (p = 1atm)|H + (0.01 M),Cl - (0.01 M),AgCl (sat'd)|Ag CEM 333 page 10.5