Ch. 12: Electrochemistry Flashcards
Electrochemical Cell
Describes any cell in which oxidation reduction rxns take place. Certain characteristics are shared between all types of electrochemical cells
Electrodes
Strips of metal or other conductive materials placed in an electrolyte solution
Anode
Always the site of oxidation. It attracts anions.
Cathode
Always the site of reduction. It attracts cations
Electron flow and current flow
Electrons flow from anode to cathode; current flows from the cathode to the anode
Cell Diagrams
Shorthand notation that represent the reactions taking place in an electrochemical cell
- Written from anode to cathode w electrolytes (the solution) in between
- A vertical line represents a phase boundary, and a double vertical line represents a salt bridge or other physical boundary
Galvanic (voltaic) cells
House spontaneous rxns (🔼G<0) w a positive electromotive force
Electrolytic Cells
House nonspontaneous rxns (🔼G>0) w a negative electromotive force. Can be used to create useful products through electrolysis
Concentration cells
Specialized from of a galvanic cell in which both electrodes are made of the same material. Rather than a potential diff causing the movement of charge, it is the concentration gradient between the two solutions
Charge on an electrode
Dependent on the type of electrochemical cell
- galvanic: anode = negative charge; cathode = positive charge
- electrolytic cels: anode = positive charge; cathode = negative charge
Rechargeable batteries:
Electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states. Rechargeable batteries are often ranked by energy density
Energy Density
Amount of energy a cell can produce relative to the mass of battery material
Lead-acid batteries
When discharging, consist of a Pb anode and a PbO2 cathode in a concentrated sulfuric acid solution. When charging the PbSO4- plated electrodes are dissociated to restore the original Pb and PbO2 electrodes and concentrate the electrolyte. These cells have low energy density
Nickel-Cadmium Batteries
When discharging, consist of a Cd anode and a NiO(OH) cathode in a concentrated KOH solution. When charging the Ni(OH)2- and Cd(OH)2- plated electrodes are dissociated to restore the original Cd and NiO(OH) electrodes and concentrate the electrolyte. These cells have a higher energy density than lead-acid batteries
Nickel-metal hydride (NiMH) batteries
Have more or less replaced Ni-Cd batteries bc they have higher energy density, are more cost effective, and are significantly less toxic
Surge Current
Above average current transiently released at the begining of the discharge phase; it wanes rapidly until a stable current is achieved
Reduction potential
Quantifies the tendency for a species to gain electrons and be rudeced. The higher the reduction potential, the more a given species wants to be reduced
Standard Reduction Potentials (Ered)
Calculated to the statdard hydrogen electrode (SHE) under the standard conditions of 298 K, 1 atm pressure, and 1 M concentrations
Standard Hydrogen electrode
Has a standard reduction potential of 0 V
Standard electromotive force
E*cell; diff in standard reduction potential between the two half cells
Difference of Half rxn reduction potentials
For galvanic cells, the diff of the reduction potential of the two half reactions is positive; for electrolytic cells, the diff of the reduction potentials of the 2 half-reactions is negative
Electromotice force and change in free energy…
Always have opposite signs”
- when E*cell is positive, 🔼G is negative, this is the case in galvanic cells
- when E*cell is negative, 🔼G is positive, this is the case in electrolytic cells
- when E*cell is 0, 🔼G is 0, this is the case in concentration cells
Nernst equation
Describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force
Relationship between equilibrium constant Keq and E*cell
- when Keq (the ratio of products’ concentrations at equilibrium over reactants’, raised to their stoichiometric coefficients) is greater than 1, E*cell is positive
- when Keq is less than 1, E*cell is negative
- when Keq is equal to 1, E*cell is )
