Chapter 19 Electrochemistry Mr. Watson HST

Mr. Watson Redox Reactions Oxidation loss of electrons Reduction gain of electrons oxidizing agent substance that cause oxidation by being reduced reducing agent substance that cause oxidation by being oxidized

HST Mr. Watson Electrochemistry In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.

HST Mr. Watson Oxidation-Reduction Reactions

HST Mr. Watson Voltaic Cells harnessed chemical reaction which produces an electric current

HST Mr. Watson Voltaic Cells Cells and Cell Reactions Daniel's Cell Zn (s) + Cu +2 (aq) ---> Zn +2 (aq) + Cu (s) oxidation half reaction anodeZn (s) ---> Zn +2 (aq) + 2 e - reduction half reaction cathodeCu +2 (aq) + 2 e - ---> Cu (s)

HST Mr. Watson Voltaic Cells copper electrode dipped into a solution of copper(II) sulfate zinc electrode dipped into a solution of zinc sulfate

HST Mr. Watson Voltaic Cells

HST Mr. Watson Hydrogen Electrode consists of a platinum electrode covered with a fine powder of platinum around which H 2(g) is bubbled. Its potential is defined as zero volts. Hydrogen Half-Cell H 2(g) = 2 H + (aq) + 2 e - reversible reaction

HST Mr. Watson

HST Mr. Watson

HST Mr. Watson Standard Reduction Potentials the potential under standard conditions (25 o C with all ions at 1 M concentrations and all gases at 1 atm pressure) of a half- reaction in which reduction is occurring

HST Mr. Watson Some Standard Reduction Potentials Table 18-1, pg 837 Li + + e - ---> Li v Zn e - ---> Zn-0.763v Fe e - ---> Fe-0.44v 2 H + (aq) + 2 e - ---> H 2(g) 0.00v Cu e - ---> Cu+0.337v O 2(g) + 4 H + (aq) + 4 e - ---> 2 H 2 O (l) v F 2 + 2e - ---> 2 F v

HST Mr. Watson If the reduction of mercury (I) in a voltaic cell is desired, the half reaction is: Which of the following reactions could be used as the anode (oxidation)? A, B

HST Mr. Watson Cell Potential the potential difference, in volts, between the electrodes of an electrochemical cell Direction of Oxidation-Reduction Reactions positive value indicates a spontaneous reaction

HST Mr. Watson Standard Cell Potential the potential difference, in volts, between the electrodes of an electrochemical cell when the all concentrations of all solutes is 1 molar, all the partial pressures of any gases are 1 atm, and the temperature at 25 o C

HST Mr. Watson Cell Diagram the shorthand representation of an electrochemical cell showing the two half- cells connected by a salt bridge or porous barrier, such as: Zn (s) /ZnSO 4(aq) //CuSO 4(aq) /Cu (s) anode cathode

HST Mr. Watson Metal Displacement Reactions solid of more reactive metals will displace ions of a less reactive metal from solution relative reactivity based on potentials of half reactions metals with very different potentials react most vigorously

HST Mr. Watson Ag+ + e- --->Ag E°= 0.80 V Cu e- ---> Cu E°= 0.34 V Will Ag react with Cu 2+ ? yes, no Will Cu react with Ag+? yes, no

HST Mr. Watson Gibbs Free Energy and Cell Potential  G = - nFE wheren => number of electrons changed F => Faraday’s constant E => cell potential

HST Mr. Watson Applications of Electrochemical Cells Batteries – device that converts chemical energy into electricity Primary Cells – non-reversible electrochemical cell – non-rechargeable cell Secondary Cells – reversible electrochemical cell – rechargeable cell

HST Mr. Watson Applications of Electrochemical Cells Batteries Primary Cells "dry" cell & alkaline cell 1.5 v/cell mercury cell 1.34 v/cell fuel cell 1.23v/cell Secondary Cells lead-acid (automobile battery) 2 v/cell NiCad 1.25 v/cell

HST Mr. Watson “Dry” Cell

HST Mr. Watson “Dry” Cell

HST Mr. Watson “Flash Light” Batteries "Dry" Cell Zn (s) + 2 MnO 2(s) + 2 NH > Zn +2 (aq) + 2 MnO(OH) (s) + 2 NH 3 Alkaline Cell Zn (s) + 2 MnO 2(s) ---> ZnO (s) + Mn 2 O 3 (s)

HST Mr. Watson “New” Super Iron Battery Mfe(VI)O 4 + 3/2 Zn 1/2 Fe(III) 2 O 3 + 1/2 ZnO + MZnO 2 (M = K 2 or Ba) Environmentally friendlier than MnO 2 containing batteries.

HST Mr. Watson Mercury Cell

HST Mr. Watson Lead-Acid (Automobile Battery)

HST Mr. Watson Lead-Acid (Automobile Battery) Pb (s) + PbO 2(s) + 2 H 2 SO 4 = 2 PbSO 4(s) + 2 H 2 O 2 v/cell

HST Mr. Watson Nickel-Cadmium (Ni-Cad) Cd (s) + 2 Ni(OH) 3(s) = Cd(OH) 2(s) + 2 Ni(OH) 2(s) NiCad 1.25 v/cell

HST Mr. Watson Hydrogen-Oxygen Fuel Cell

HST Mr. Watson Automobile Oxygen Sensor

HST Mr. Watson Automobile Oxygen Sensor see Oxygen Sensor Movie from Solid-State Resources CD-ROM

HST Mr. Watson pH = (E glass electrode - constant)/ pH Meter

HST Mr. Watson Effect of Concentration on Cell Voltage: The Nernst Equation E cell = E o cell - (RT/nF)ln Q E cell = E o cell - (0.0592/n)log Q whereQ => reaction quotient Q = [products]/[reactants]

HST Mr. Watson EXAMPLE: What is the cell potential for the Daniel's cell when the [Zn +2 ] = 10 [Cu +2 ] ? Q = ([Zn +2 ]/[Cu +2 ] = (10 [Cu +2 ])/[Cu +2 ] = 10 E o = (0.34 V) Cu couple + (-(-0.76 V) Zn couple n = 2, 2 electron changeE cell = E o cell - (0.0257/n)ln Q thusE cell = ( (0.0257/2)ln 10) V E cell = ( (0.0257/2)2.303) V E cell = ( ) V = 1.07 V

HST Mr. Watson Nernst Equation [H + ] acid side  [H + ] base side E = E o – RT nF ln Q = – 2.3 RT F log [H + ] base side [H + ] acid side [h + ] p-type side  [h + ] n-type side E (in volts) = – 2.3 RT F log [h + ] n-type side [h + ] p-type side

HST Mr. Watson Electrolysis non-spontaneous reaction is caused by the passage of an electric current through a solution

HST Mr. Watson Electrolysis of KI (aq)

HST Mr. Watson Electrolysis Electrolysis of Sodium chloride (chlor-alkali process) molten reactants =>liquid sodium and chlorine gas aqueous reactants => caustic soda (sodium hydroxide) and chlorine gas

HST Mr. Watson Aqueous Reactants => caustic soda (sodium hydroxide) and chlorine gas

HST Mr. Watson Molten Reactants => liquid sodium and chlorine gas

HST Mr. Watson Electrolysis Preparation of Aluminum (Hall process)

HST Mr. Watson Electrolytic Refining of Copper Cu (s) + Cu +2 (aq) --> Cu +2 (aq) + Cu (s) impure pure anode cathode impurities: anode mud; Ag, Au, Pb

HST Mr. Watson Copper Purification

HST Mr. Watson Copper Purification

HST Mr. Watson Hall Process for Aluminum

HST Mr. Watson Quantitative Aspects of Electrolysis 1 coulomb = 1 amp sec 1 mole e - = 96,500 coulombs

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current?

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? (45 min) #g Cr =

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? definition of minute (45 min)(60 sec) #g Cr = (1 min)

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? (45) (60 sec) (25 amp) #g Cr = (1)

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? definition of a coulomb (45)(60 sec)(25 amp)(1 C) #g Cr = (1) (1 amp sec)

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? Faraday’s constant (45)(25)(60)(1 C)(1 mol e - ) #g Cr = (1)(1)(96,500 C)

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? atomic weight (45)(60)(25)(1)(1 mol e - )(52 g Cr) #g Cr = (1)(1)(96,500) (6 mol e - )

HST Mr. Watson Electroplating EXAMPLE: How many grams of chromium can be plated from a Cr +6 solution in 45 minutes at a 25 amp current? (45)(60)(25)(1)(1 mol e - )(52 g Cr) #g Cr = (1)(1)(96,500)(6 mol e - ) = 58 g Cr

HST Mr. Watson Rusting of Iron

HST Mr. Watson Corrosion O 2(g) + 4 H + (aq) + 4 e > 2 H 2 O (l) E o = 1.23 V Rusting Fe (s) -----> Fe +2 (aq) + 2 e - E o = 0.44 V O 2(g) + 4 H + (aq) + 4 e > 2 H 2 O (l) E o = 1.23 V Fe (s) + O 2(g) + 4 H + (aq) -----> 2 H 2 O (l) + Fe +2 (aq) E o = 1.67 V

HST Mr. Watson Preventing Corrosion painting galvanizing sacrificial anode