Electrochemistry Flashcards
describes any cell in which oxidation-reduction reactions take place; three types: galvanic, electrolytic, and concentration cells; all types share certain characteristics
electrochemical cells
strips of metal or other conductive material placed in an electrolytic solution
electrodes
electrodes:
always the site of oxidation; it attracts anions
anode
electrodes:
always the site of reduction; it attracts cations
cathode
electrode mnemonic
AN OX and a RED CAT
electrodes:
flows from the anode to the cathode
electrons
electrodes:
flows from the cathode to the anode
current
shorthand notation that represents the reactions taking place in an electrochemical cell; written from anode to cathode with electrolytes (the solution) in between; a vertical line represents the phase boundary, and a double vertical line represents a salt bridge or other physical boundary
e.g. Zn (s) | Zn2+ (1 M) || CU2+ (1 M) | Cu (s)
cell diagrams
cell diagram form:
anode | anode solution (concentration) || cathode solution (concentration) | cathode
house spontaneous reactions (∆G < 0) with a positive electromotive force; anode is negatively charged and cathode is positively charged
galvanic (voltaic) cells
house nonspontaneous reactions (∆G > 0) with a negative electromotive force; these negative nonspontaneous cells can be used to create useful products through electrolysis; anode is positively charged and cathode is negatively charged
electrolytic cells
a specialized form of a galvanic cell in which both electrodes are made of the same material; rather than a potential difference causing the movement of charge, it is the concentration gradient between the two solutions
concentration cells
electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states; often ranked by energy density
rechargeable batteries
rechargeable batteries:
the amount of energy a cell can produce relative to the mass of the battery material
energy density
rechargeable batteries:
when discharging, consist of 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
lead-acid batteries
rechargeable 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; theses cells have a higher energy density than lead-acid batteries
nickel-cadmium batteries (Ni-Cd)
rechargeable batteries:
have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective, and are significantly less toxic
nickel-metal hydride batteries (NiMH)
an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved
surge current
quantifies the tendency for a species to gain electrons and be reduced; the higher the ____, the more a given species wants to be ____
reduction potential (reduced)
are calculated by comparison to the standard hydrogen electrode (SHE) under the standard conditions of 298 K, 1 atm, and 1 M
standard reduction potentials (Eº(red))
has a standard reduction potential of 0 V
standard hydrogen electrode (SHE)
the difference in standard reduction potential between two half-cells
standard electromotive force (Eº(cell))
positive for galvanic cells, and negative for electrolytic cells
standard electromotive force (Eº(cell))
always have opposite signs
electromotive force and change in free energy
describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force
Nernst equation
Nernst equation
E(cell) = Eº(cell) - (.0592 / n) log Q
where:
E(cell) = emf of cell under nonstandard conditions
Eº(cell) = emf of cell under standard conditions
n = number of moles of electrons
Q = reaction quotient
relationship between K(eq) and Eº(cell): if K(eq) > 1
Eº(cell) is positive
relationship between K(eq) and Eº(cell): if K(eq) < 1
Eº(cell) is negative
relationship between K(eq) and Eº(cell): if K(eq) = 1
Eº(cell) is 0
