Zumdahl s Chapter 17 Electrochemistry Making Charges Work Chapter Contents Galvanic Cells Cell Potential Std Reduction Potential E Electrical Work Potential and Free Energy G E s concentration dependence Nernst Equation K from E Batteries Corrosion Electrolysis Electrochemistry Conversion of chemical to electrical energy discharge And its reverse electrolysis Both subject to entropic caution Convert reversibly to keep systems at equilibrium and convert all available chemical work G to and from the equivalent electrical work Q V Electrons from REDOX reactions RedOx Half Reactions The e are visible in reactions 3 H2O2 3 O2 6 H 6 e 2 Au3 6 e 2 Au 2 Au3 3 H2O2 3 O2 6 H 2 Au But while cells were a math convenience in stoichiometry they are real in electrochemistry Galvanic Cells on i One cell rxn t c u n io ed t occurs in each a R id e x Cu Zn d O o compartment h t e a d C o n 2 Zn Zn 2e in A SO42 SO42 the anode Cu2 2e Cu in Zn2 Cu2 cathode But not without a Zn Cu2 Zn2 Cu connection Ion salt Bridge But even with a connection of the electrodes no current flows We need to allow neutrality in the solutions with a salt bridge to shift counterions 2e 2e Cu Zn SO42 Zn2 SO42 Cu2 Zn Cu2 Zn2 Cu Standard Reduction Potentials E The voltage generated by the Zn Cu galvanic cell is 1 1V under standard conditions Standard conditions are T 25 C and P 1 bar for gases Solids and liquids are pure Solutions are 1 M in all species E cell is sum of cell E values cell Reduction Potentials All cells are catalogued as reduction reactions assigned reduction potentials E The lower reduction potential rxn is reversed to become the oxidation E oxidation E reduction That makes spontaneous E cell 0 But E red can t be found w o E ox Origin for Reduction Potentials We had the same problem for S ions and solved it by making H special 2H aq 2e H2 1 bar E 0 V 1 bar H2 flows over a Pt electrode and the full E cell is assigned to the other electrode E SHE 0 V E g standard calomel electrode Hg2Cl2 s 2e 2 Hg l Cl E SCE 0 27V a more physically convenient reference Active Metal Series Ag e Ag 80V Cu2 2e Cu 34 2H 2e H2 00 Fe2 2e Fe 44 Zn2 2e Zn 76 Mg2 2e Mg 2 37 Etc Remember reverse the lower potential to make it an oxidation instead of a reduction A cursory glance at the standard reduction E s at left tells us why Cu is immune to 1 M HCl while metals with lower E merrily bubble off H2 Corroding Copper Cu isn t immortal H doesn t do it We fried that penny not with HCl but with HNO3 So HNO3 isn t merely acid but oxidizing acid Cu2 2e Cu has E 0 34V NO3 4H 3e NO H2O has E 0 96V So reversing the Cu and adding HNO3 gives a cell E 0 62 V Galvanic Line Notation Shorthand for a complete redox cell is of the form Anode anodic soln cathodic soln Cathode but written all on the same line So making a cell of Cu corrosion Cu Cu2 NO3 NO g H Pt where all ions should be suffixed aq and both metals should have s Free Energy and Work Were all aq concentrations in the Cu corrosion cell at 1 M the cell potential would be 0 62V spontaneous Spontaneous reactions have negative G max non PV work Electrical work charge potential ne moles of e carry ne F Coulombs G ne F E J J CV F 96 485 C mol 1 the Faraday const Temperature Dependence of E Since E G n F and E G n F for that matter dE dT 1 n F d G dT But dG VdP SdT so dG dT S Or dE dT S n F S n F where we ve presumed that neither S nor H will change much with moderate T Since S 124 J mol K for a car battery it s harder to start in winter For 0 C the 6 cell battery puts out 0 1V less than at 25 C Nernst Eqn Potentials and Concentrations Both G and E refer to unit standard concentrations But at equilibrium G 0 and the cell potential E 0 as well see no G G RT ln Q neFE neFE RT ln Q E E 2 303 RT neF log Q E E 59 1 mV ne log Q 25 C K from E Just as G G RT ln K 0 implies G RT ln K ne F E RT ln K implies K e ne F E R T where as before ne moles of electrons involved in the overall reaction as written Very large K can be calculated Confession Time On slide 9 I touted the Hg2Cl2 Hg couple as a convenient standard and drew its E from the table But S C E stands for saturated calomel electrode and E 0 241 not E 0 268 V with saturated Cl Since Q Cl by inverting Nernst we find Cl sat d 2 86 M Cool Potential from a SINGLE Reaction Don t we need an oxidation as well as a reduction Yes but they can be the same reaction but for a reversal Concentrations must differ between the anode and cathode I e Q must less be than 1 so log Q is negative then although E 0 still E 0 The cell brings Q to 1 at equilibrium by equalizing concentrations in cells Ion selective Electrodes Ag can be obtained by E from simple Ag wire referred to SCE H is much more important pH electrodes enclosed in glass swap H for Na at silicate surface Potential difference thus induced is calibrated for H external See your Harris 15 4 for details Assaulted with Batteries Battery refers to a series of Galvanic cells whose E add Parallel hookup adds current I not E Rechargeable NiCad reactions Cd 2 OH Cd OH 2 2e NiO2 2H2O 2e Ni OH 2 2 OH Notice the cancellation of OH in final reaction Q 1 always so E fixed It doesn t run down it just stops Better Batteries NiCad though rechargeable will accept progressively smaller charges battery memory NiMH replaces anode rxn with MH OH M H2O e with a much longer recharge life M might be Mg2Ni with 4 1 and effective H density twice H2 liquid Best Battery E 2 H2 4 OH 4 H2O 4 e 0 83V O2 2 H2O 4 e 4 OH 0 40V OK I m prejudiced Is nothing more than hydrogen combustion no Greenhouse gas Best example of fuel cell so called because H2 and O2 are not built into the battery but supplied externally Notice that OH is again unchanging Corrosion A battery is electrochemistry happening where you want it Corrosion is where you don t …