Chemistry 163B Electrochemistry 1
activity coefficients for ions (HW9 #58) BaCl s Ba aq Cl aq a a ( ) ( ) ( ) K sp BaCl ( s) 1 a ( ) a Ba aq Cl ( aq) Ba Ba ( aq) Ba a Cl ( aq) Cl a Cl BaCl ( s) cannot determine and independently but only Ba Ba Cl Cl K sp 3 Ba 1M Cl 1M 1 (1) 3 K sp Ba Cl
work of expansion H (10 bar) H (1 bar) pfft 1 bar 1 bar 10 bar 10 bar 3
hydrogen pressure [ concentration ] cell (reaction I of III) H (10 bar) H (1bar) RT lnq reaction 0 is for reaction H ( P1 bar) H ( P1 bar) P 1 ) RT ln 5. 706 kj per mole H P( 10 ) 4
d and work-other (did before for dg) d dh TdS SdT d d q TdS SdT VdP d w d T, P 0 d w other other for a spontaneous process at constant T,P the MAXIMUM work done ON SURROUNDINGS is and this occurs when the process approaches REVERSIBILITY 5
responsible for 3 redox reactions; here s II (HW9, prob #60) e oxidation anode reduction cathode e + H (g, 1 bar) + AgCl(s) H + [1M] +Cl - [1M] +Ag(s) + e 6
for the reaction (see Appendix A, Table 4.1 for data; additional decimal places from other tables) e +H (g, 1atm) + AgCl(s) H + [1M] + Cl - [1M] + Ag(s) +e f G f (kj) 0-109.79 0-131.3 0 G = - (0) - (-109.79) + (0) + (-131.3) + (0) = -4.88 kj =for moles e transferred e + ½ H (g, 1atm) + AgCl(s) H + [1M] + Cl - [1M] + Ag(s) + e G=-1.44 kj per ½ mole H for 1 mole e transferred 7
and FINALLY w other!!! (p. 0) 8
w other (p. 60) p. 60 vs is overcomplicated BUT - d w electrical dq electric potential dq Fdn charge transfer moles of e s transferred from negative charge on e - d w w electrical electrical w E nf & R Fdn nf p60 z -n F is Faraday constant 96,458 coulomb (mole e) 1 (n electrons transferred) electromotive force= rev UNITS: [w] = [Q] [] joule= coulomb volt 9
sign of and spontaneity T, P w other nf irrev irrev T, P cell cell nf T, P cell cell for irreversible = mf for reversible T,P < 0 spontaneous > 0 spontaneous 10
vs RT lnq nf reaction nf RT nf lnq reaction RT nf T 98K ln Q reaction 0.0569 V ln Q n reaction n = moles electrons transferred n =mol n= n mol n 1 unitless 11
responsible for 3 redox reactions; here s II (HW9, prob #60) oxidation anode reduction cathode 0 0 0.33 V e + H (g, 1bar) + AgCl(s) H + [1M] +Cl - [1M] +Ag(s) + e 0.33 V 1
example incorporating activities e +H (g, 1bar ) + AgCl(s) H + [1M] + Cl - [1M] + Ag(s) +e 0.0569 V a a a ln Ag s n ah a AgCl ( s) H Cl ( ) a AgCl 1 a [ H ] a [ Cl ] can't independently meansure a Ag H H Cl Cl H Cl H and Cl 4 0.0569 V [ H ] [ Cl ] ln n H P H 13
example incorporating activities 4 0.0569 V [ H ] [ Cl ] ln n H P H 0.33 V e s 4 0.0569 V [1 M] [1 M] 0.33 V ln H (1 bar) unitless; have dropped standard state concs and pressure from denominators 14
example incorporating activities 4 0.0569 V [1 M] [1 M] 0.33 ln H (1 bar) Calculate s from observed (HW9, prob 60) If s =1 0.0569 V 0.33 V ln1 0.33 V nf 1 mol 96, 485 C mol 0.33V 4 4.90 10 CV 4.90 kj = -4.88 kj for moles e transferred [from f earlier ] 15
intensive vs extensive = - ( /nf) H (g, 1 bar) H + [1M] +e e + AgCl(s) Cl - [1M] + Ag(s) = 0 V = 0.33 V e +H (g, 1 bar) + AgCl(s) H + [1M] + Cl - [1M] + Ag(s) +e = -4.88 kj for moles e transferred cell = 0.33 V 0.0569 V a a a ln H Cl Ag ( s ) cell e' s cell ah aagcl ( s) 16
intensive vs extensive = - ( /nf) ½H (g, 1 bar) H + [1M] +e e + AgCl(s) Cl - [1M] + Ag(s) = 0 V = 0.33 V e +½H (g, 1 bar) + AgCl(s) H + [1M] + Cl - [1M] + Ag(s) +e cell= 0.33 V = -1.44 kj for 1 moles e transferred 0.0569 V a a a ln H 1 1 1 H Cl cell 1e cell 1 1 1 a aagcl ( s) Al ( s ) intensive same as for mole e s is oomph per electron 17
intensive vs extensive = - ( /nf) e 1e two times greater = -4.88 kj for moles e transferred 0.0569 V a a a ln H Cl Ag ( s ) cell e' s cell ah aagcl ( s) = -1.44 kj for 1 moles e transferred cell e cell 1e same 0.0569 V a a a ln H 1 1 1 H Cl cell 1e cell 1 1 1 a aagcl( s) Ag ( s ) extensive: depends on stoichiometry nf intensive: independent of how reaction is written ooomph PER electron 18
biological example: cytochrome C iron containing enzyme (reaction III) CytC=cytochrome C standard state ph=7, [H+]=10 7 standard REDUCTION potentials e + CytC(Fe 3+ ) CytC(Fe + ) e + ½O (g) + H + (aq) H O () red (V) ph7 0.5 0.816 oxidation reduction reaction: the oxidation of CytC(Fe + ) CytC(Fe + ) CytC(Fe 3+ ) + e e + ½O (g) + H + (aq) H O () (V) - 0.5 0.816 ½O (g) + H + (aq) +CytC(Fe + ) CytC(Fe 3+ ) + H O () standard state [H + ]=10-7?= cell 19
biological example (redox equation III) ½O (g) + H + (aq) +CytC(Fe + ) CytC(Fe 3+ ) + H O () RT RT ' ' cell cell ln Q cell ln 1 nf nf [ ] H O P O 7 10 M 1bar what s º? what s Q? what s n? standard state 0
and thermodynamic derivatives, etc. (HW9, prob #59) nf nf RT RT ln Keq ln K nf S S T T nf P P eq T H T H T T T nf T P P 1
ΔC p from Φ S S T T nf P H TS H TS nf T nf T P P H Cp nf nf nft T T P T P T P C p nf T T P P
relationships on final Electrochemistry: reaction =nf cell RT ln Q nf 0.0569 n ln Q at T 98K 3
batteries and fuel cells battery - nicely package electrochemical cell closed system runs irreversibly ( < ) may be recharged (storage battery) fuel cell- electrochemical cell open system (reactants continuously flow in) 4
efficiency of w electrical vs w P-V compare: w electrical (on surr) from lead storage battery G=-377 kj mol -1, H =-8 kj mol -1 with w P-V (on surr) of heat engine using q upper = -H storage battery heat engine: T u =600K, T L =300K =(600-300)/600= 0.5 w P-V =(0.5)*8 kj mol -1 = 114 kj mol -1 battery: G=-377 kj mol -1, T=300K w electrical =- G = 377 kj mol -1 the winner: welectrical 377 3. 31 w 114 P V 5
types of batteries Lead storage Pb(s) oxidized to Pb + PbO reduced to Pb + Alkaline storage (no liquids) Zn(s) oxidized to Zn + MnO reduced to Mn O 3 Zn (s) + OH - (aq) ZnO (s) + H O (l) + e +1.6V MnO (s) + H O (l) + e Mn O 3 (s) + OH (aq) 0.75 Zn (s) + MnO (s) ZnO (s) + Mn O 3 (s) +1.5 V 6
types of batteries NiMH- Nickel Metal Hydride M= intermetallic compound, e.g. M=AB 5, A is a rare earth mixture of lanthanum, cerium, neodymium, praseodymium B is nickel, cobalt, manganese, and/or aluminum Ni + oxidized to Ni 3+ H + reduced to H, M oxidized H O + M + e - OH - + MH Ni(OH) + OH - NiO(OH) + H O + e - High power Ni-MH battery of Toyota NHW0 Prius 7
Li-ion batteries Li-ion batteries can pack more energy into smaller and lighter weight units than other types of batteries. Those attributes have spurred enormous growth in their use for cell phones, laptop computers, and other portable electronic devices. A downside of Li-ion cells, however, is that they contain a flammable electrolyte solution consisting of lithium salts in organic solvents such as ethylene carbonate and ethyl methyl carbonate. This is not the case for other commercial battery types. 8
battery property comparison Zinc 1.5V Non-rechargeable first the forerunner and later an inexpensive alternative to Alkaline batteries. However, reductions in the price of Alkalines have made both Zinc-Carbon and Zinc-Chloride batteries all but obsolete. Alkaline 1.5V Rechargeable Alkaline rechargeable batteries are lower capacity (don t hold a charge as long) than the more popular NiMH rechargeables. The advantage of the rechargeable Alkaline over the NiMH or the NiCAD is that it loses its charge gradually, Nickel-Metal Hydride (NiMH) 1.5V Rechargeable- Lightweight and rechargeable, the NiMH has a higher capacity than the NiCAD plus you can throw it away since it doesn t contain toxic metals and it isn t classed as a hazardous waste item. Lithium ion 3.6V Rechargeable For a given voltage, a lithium ion battery is smaller in size and lighter in weight than a nickel cadmium (NiCd) or nickel metal hydride (NiMH) battery. In addition, lithium ion has virtually no self-discharge. This allows a lithium ion battery to be stored for months without losing charge. The battery chemistries can be compared as follows: http://batteryuniversity.com/learn/article/whats_the_best_battery 9
fuel cells PEM- proton exchange membrane H H + + e 0 V ½O + H + + e H O 1.3 V load H O Fuel cells come in a variety of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, The energy efficiency of a fuel cell is generally between 40 60%. 30
End of Lecture 31
i the electrochemical potential dg dw SdT VdP reaction T, P elec i i i, elec i i i, dn reaction G w = 0 dg SdT VdP VS G 0 reaction T, P i i i, i, i dn i i ( ) i i dw i elec assigns electrical work to each species including electrons transferred 3