Nils Walter: Chem 260Ion mobilityQualitatively: Large ions in viscous liquids can be expected to be drifting slowly and have low conductivitiesQuantitatively: drift velocityuEs =electric field strengthion mobilityzeEFfield=Two forces are acting on the ion:elementary chargenumber of ion chargesrsFnretardatioπη6=Stokes’ lawdrift velocityion radiusviscosityWhen the ion has reached its drift velocity, both forces are equal!⇒⇒⇒⇒rsezEπη6=⇒⇒⇒⇒rezEsπη6=][6112−−==VsmrezEsuπηu−−−+++==FuzFuzλλFaraday constantionic conductivities:Nils Walter: Chem 260Measured ion mobilitiesrezuπη6=⇒⇒⇒⇒u is high for an ion that is:•highly charged•in a solution of low viscosity•of small radius rBUTBUT: r = hydrodynamic radius(including water ligands)Special case H+: Grotthus conduction mechanismNils Walter: Chem 260Electrochemical cells= electronic conductor + surrounding electrolyteelectrode compartmentIf two different electrolytes are used:Electrolytic cell: electrochemical cell in which a non-spontaneous reaction is driven by an external source of currentGalvanic cell: electrochemical cell in which electricity is produced as a result of a spontaneous reaction (e.g., fuel cells, electric fish!)Nils Walter: Chem 260Reactions at electrodes: Half-reactionsRedox reactions: Reactions in which electrons are transferredfrom one species to another+II -II 0 -II0+IVreducedoxidizedAny redox reactions can be expressed as the difference between two reduction half-reactions in which e-are taken upCuS(s) + O2(g) →→→→Cu(s) + SO2(g)E.g.,Reduction of Cu2+: Cu2+(aq) + 2e-→→→→Cu(s)Reduction of Zn2+: Zn2+(aq) + 2e-→→→→Zn(s)Cu2+(aq) + Zn(s) →→→→Cu(s) + Zn2+(aq)More complex: MnO4-(aq) + 8H++ 5e-→→→→Mn2+(aq) + 4H2O(l)Half-reactions are only a formal way of writing a redox reactionDifference:Nils Walter: Chem 260Carrying the concept furtherReduction of Cu2+: Cu2+(aq) + 2e-→→→→Cu(s)In general: redox couple Ox/Red, half-reaction Ox + ννννe-→→→→RedAny reaction can be expressed in redox half-reactions:Expansion of gas: H2(g, pi) →→→→H2(g, pf)2 H+(aq) + 2e-→→→→H2(g, pi)2 H+(aq) + 2e-→→→→H2(g, pf)Dissolution of a sparingly soluble salt: AgCl(s) →→→→Ag+(aq) + Cl-(aq)Ag+(aq) + e-→→→→Ag(s)AgCl(s) + e-→→→→Ag(s) + Cl-(aq)Reaction