Chemical Electrochemistry: Change and Electrical Work
21.1 Half-Reactions and Electrochemical Cells 21.2 Voltaic Cells: Using Spontaneous Reactions to Generate Electrical Energy 21.3 Cell Potential: Output of a Voltaic Cell
Chapter 21
Electrochemistry: Chemical Change and Electrical Work
21.4 Free Energy and Electrical Work 21.5 Electrochemical Processes in Batteries 21.6 Corrosion: A Case of Environmental Electrochemistry 21.7 Electrolytic Cells: Using Electrical Energy to Drive a Nonspontaneous Reaction
Basic Lessons/Concepts
Oxidation Numbers electron book keeping Identifying Redox reactions Changes in ox. numbers Electrochemistry = Redox chemistry at electrodes The ability to separate chemical reagents and control electron transfer = ability to store and control release of chemical potential energy (batteries, electrolysis) Electrons flow in direction of spontaneous rxn (release stored chemical potential energy) cell potential Reduction potentials tabulated relative to a standard reference (hydrogen electrode) Cell potentials are fundamentally related to the reaction free energy (Gspontaneity, max. work, equilibrium constants) Nonspontaneous reactions can be driven with external energy source (by doing work on system) electrolysis (fuels, separation of materials, storing energy)
Review of Previous Lecture
1. 2. Spontaneity of Electrochemical Reactions G < 0 E > 0 .reation is spontaneous as written G = -nFE (max. work mol-1) G = G0+RTlnKeq; E = E0 RTlnKeq/nF G=0 at equil AND E = 0 (dead battery) at equil E0 = RTlnKeq/nF Nernst equation: E0 RTlnKeq/nF .follows from G = G0+RTlnQ allows determination of E for nonstandard conditionseffects of conc. etc. Quantifying Electrochemical Processes Nernst Equation enables quantitative predictions of cell potentials and spontaneity for nonstandard reaction conditions (max. amount of work that can be extracted) Measurements of current enables determination of amount of product produced (and vice versa) Corrosion Nemesis of Technology Oxidation of support materials as well as microelectronics (GaAs is the material of the future....and always will be due to oxidation of surface)
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Storing Energy
Storing Energy
A lead storage battery is recharged by forcing electrons to flow in the opposite direction from galvanic operation.
PbO2(s) + Pb(s) + 2HSO4-(aq) + 2H+ 2PbSO4(s) + 2H2O(l); E0 = 2.05 V BUT at equilibrium E = 0 (battery is dead) Need to convert PbSO4 back to PbO2 and Pb Expect PbSO4(s) + 2H2O(l) PbO2(s) + Pb(s) + 2HSO4-(aq) + 2H+ would need an applied voltage of 2.05 V..but actually need moreOverpotential needed in addition to compensate for kinetic barrier (see later reaction kinetics).
Microscopic View of Electrodes Electrode Polarisation
+++ + + -++ -
OverpotentialsMicroscopic Origins
Need to rearrange polar solvent molecules to accommodate negative charge costs energy
+-+-
-
+ -+ -+ + + -+ Bulk
electrode Higher voltage pumps electrons into higher lying levels (orbitals of metal)
Double Layer; induced when current starts to flow. Will die off depending on mobility of ions in the bulk solution.
Overpotential = Need extra potential to bring electrons to up the same energy as the electron would have on acceptor in solution
Overpotential Reorganization of solvent molecules barrier Determines rate of electron transfer or current
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Electrolysis: Water and Fuel Generation
Electrolysis of Water
Reactions:
Hydrogen Fuels
H2O(l) O2(g) + H2(g) (Note: redox reaction costs energy water can be split electrochemically as it provides a means to externally put energy into reactions E0 Anode rxn: 2H2O O2(g) + 4H+ + 4e-1.229 V Cathode rxn: 4H2O + 4e2H2(g) + 4OH- -0.828 V ____________________________________________ Overall: 2H2O* O2(g) + 2H2 (g) E0 = -2.057 V Need to apply an external voltage of 2.057 V to drive the reaction Actually need additional voltage re: overpotential to get any appreciable current. *4H+ + 4OH 4H2O (nonredox) to leave overall 2H2O
A photograph and schematic diagram of a cell for the electrolysis of water. The reaction generates hydrogen and oxygen in a 2:1 ratio (text)
Electrolysis of Water Demo Electrolysis of Water Demo What to look for: Which electrode is cathode? Expect to produce 2x moles of H2(g) relative to O2(g) Expect cathode to generate OH- in reducing water Expect anode to produce H+ in oxidizing water How can we tell we have H2 and O2?
Photo courtesy Brian Or, 2004
Look for: volume differences, colour changes re: PH, soap bubbles entrain H2(g) and O2(g) as gas escapes.if stoichiometric H2 + O2 H2O
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Example Problem in Hydrogen Fuel Economy An electrolysis process is operating at .10 amps of current. Calculate the amount of hydrogen (moles) produced and the volume after 15 minutes of operation at 1 atm pressure and 250C. Strategy: Calculate the number of electrons produced by this current (1 amp = 1C/sec; 1.6x10-19 C/e-) From balanced reaction calculate moles of H2 Use ideal gas law to calculate volume
Faradays Law of Electrolysis:
Large Scale Electrolysis
The amount of product formed or reactant consumed by an electric current is stoichiometrically equivalent to the amount of electricity (electrons supplied).
Weve seen how Na(s) and Cl2(g) are produced on an industrial scale using electrolysis. Similar processes are used in Al(s) and Mg(s) production.
So, how else can electrolysis be exploited?
Electroplating
Electric deposition of a thin metal film on object.
Electroplating: Methodology
Fe based cutlery cathode
Ag+(aq), Na+(aq), CN-(aq)
Ag(s) anode M+ + e- M(s)
The first person to do this?
FARADAY, Who else?
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Hardly limited to small things!
Fuel Cells
CrO42-(aq) + 6e- Cr(s) (s) Lots of electricity needed!
http://lapin.lavrans.net:8080/photos/2002/octobre/chrome%20car.jpg
CO2 also produced by some
http://www.newmango.com/artwork/inf/fuel-cell.jpg
Yes, and?
Subtleties in the shape of the curve can be traced to electron transfer reactions. A few systems of interest come to mind:
http://mswebs.aist-nara.ac.jp/LABs/kobuke/zn2e.jpg
Biologically Relevant Electron Transfer
http://www.blc.arizona.edu/interactive/metabolism2.95/atp_synthetase.gif
PHOTOSYNTHESIS; Develop higher efficiency solar power.
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Tracking Electron Transfer
Plastics that conduct electricity could make flexible screens true electronic newspaper!
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