Brain Warmup : Brain Warmup Half-Reaction E° (V)
Ag+ + e- ? Ag 0.80
Cu2+ + 2e- ? Cu 0.34
Zn2+ + 2e- ? Zn -0.76
Al3+ + 3e- ? Al -1.66 What is E° for each of the following reactions?
Which reaction(s) are spontaneous? 3 Ag+(aq) + Al(s) ? 3 Ag(s) + Al3+(aq)
Cu2+(aq) + Zn(s) ? Cu(s) + Zn2+(aq)
2 Al3+(aq) + 3 Zn(s) ? 2 Al(s) + 3 Zn2+(aq) E°
2.46 V
1.10 V
-0.90 V Spontaneous?
Y
Y
N Zn can reduce Cu2+, but not Al3+
Slide 2 : 3 Ag+(aq) + Al(s) ? 3 Ag(s) + Al3+(aq) Polishing Silver with Aluminum Foil E° = 2.46 V ?G° = -nFE° = -712 kJ 3 Ag2S + 2 Al + 3 H2O ? 6 Ag + Al2O3 + 3 H2S ?
Stinky!! (Add NaHCO3 to neutralize H2S)
Redox Reaction Equilibrium Constants : Redox Reaction Equilibrium Constants ?G° = -nFE° ?G° = -RT lnK nFE° = RT lnK lnK = nFE°/RT What is K at 25°C for the aluminum/silver polishing reaction? 3 mol (96485 C/mol)(2.46 J/C)
(8.314 J/mol K)(298 K) 3 Ag+(aq) + Al(s) ? 3 Ag(s) + Al3+(aq) E° = 2.46 V lnK = = 287 K = e287 = 4.39 ? 10124
Concentration Cells : Concentration Cells A voltage is generated just by a difference in concentration between the two half-cells Cu(s)?Cu2+(aq) (0.5 M)??Cu2+(aq) (2.5 M)?Cu(s) Cu2+(aq) + 2e- ? Cu(s) Cu(s) ? Cu2+(aq) + 2e- Oxidation Reduction E° = 0 = 0.02 V
Concentration Cells in Nature : Concentration Cells in Nature Living cells contain low-pH vesicles surrounded by neutral-pH cytoplasm
The [H+] concentration gradient creates a voltage pH = 7 4 H+ + O2 + 4e- ? 2 H2O E° = 1.23 V Ecell = -(0.059/4) log(10-16) = 0.24 V
Batteries : Batteries Car Battery (Lead storage battery)
Anode: Pb + HSO4- ? PbSO4 + H+ + 2e-
Cathode: PbO2 + HSO4- + 3H+ + 2e- ? PbSO4 + 2H2O
Cell: Pb + PbO2 + 2H+ + 2 HSO4- ? 2 PbSO4 + 2H2O
2 volts per cell, 6 cells to a battery ? 12 volt battery
Alkaline Battery
Anode: Zn ? Zn2+ + 2e-
Cathode: 2 MnO2 + H2O + 2e- ? Mn2O3 + 2OH-
1.5 volts
Lemon Battery
Anode: Zn ? Zn2+ + 2e-
Cathode: Cu2+ + 2e- ? Cu
Cell reaction: Zn + Cu2+ ? Zn2+ + Cu
1.1 volts (if all goes well)
Want more volts? Link cells in series...
Electrolytic Cells : Electrolytic Cells Galvanic cell (battery): spontaneous reaction, E° > 0
Electrolytic cell: non-spontaneous reaction, E° < 0
An external power source is used to force the reaction to occur
Used for:
Charging (rechargeable) batteries
Producing or purifying metals (aluminum, copper)
Electroplating Charles Hall
Discovered how to produce aluminum by electrolysis
Electrolysis of Water : Electrolysis of Water Spontaneous reaction: formation of water
2 H2(g) + O2(g) ? 2 H2O(l) ?G° = -474 kJ
0 0 +1 -2
As a galvanic cell:
Cathode: O2 ? H2O (O2(g) + 4 H+(aq) + 4 e- ? 2 H2O(l))
Anode: H2 ? H2O (2 H2(g) + 4 OH-(aq) ? 4 H2O(l) + 4 e-)
E° = 2.06 V (pure water, E = 1.23 V)
Non-spontaneous reaction: electrolysis of water
2 H2O(l) ? 2 H2(g) + O2(g) ?G° = +474 kJ
As an electrolytic cell:
Anode (oxidation): 2 H2O(l) ? O2(g) + 4 H+(aq) + 4 e-
Cathode (reduction): 4 H2O(l) + 4 e- ? 2 H2(g) + 4 OH-(aq)
E° = -2.06 V (pure water, E = -1.23 V)
Electrolysis of Water : Electrolysis of Water 2 H2O(l) ? 2 H2(g) + O2(g) E° = -2.06 V Cathode (reduction):
4 H2O + 4 e- ? 2 H2 + 4 OH-
Produces:
2 mol gas
Base Anode (oxidation):
2 H2O ? O2 + 4 H+ + 4 e-
Produces:
1 mol gas
Acid Battery
> 2.06 V + - Pt electrodes e- ? ? e-
Electrolysis Calculations : Electrolysis Calculations How many grams of O2(g) will a 3.0 amp power source produce in 5 minutes?
Amperes (A) = electric current = coulombs/second (C/s) André-Marie
Ampère Oxygen half-reaction: 2 H2O ? O2 + 4 H+ + 4 e- Current
and time Charge Moles of e- Moles of
product Grams of
product 3.0 C/s ? 300 s = 900 C 900 C ? 1 mol e-/96485 C = 0.0093 mol e- 0.0093 mol e- ? 1 mol O2/4 mol e- = 0.0023 mol O2 0.0023 mol O2 ? 32 g O2/1 mol O2 = 0.075 g O2
Electroplating : Electroplating Cathode
Cu2+(aq) + 2e- ? Cu(s) Anode
Cu(s) ? Cu2+(aq) + 2e- Battery + - Cu ? Cu2+ Cu2+ ? e- ? ? e- How long will it take a 15 amp power source to deposit 5.9 g of copper? 15 C
s ? t ? ? ? 1 mol e-
96485 C 1 mol Cu
2 mol e- 63.55 g Cu
1 mol Cu = 5.9 g Cu t = 1200 s = 20 minutes