General Chemistry Chapter 12: Electrochemistry Flashcards
Electrochemical cell
describes any cell in which oxidation-reduction reactions 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 side of reduction. It attracts cations.
Electrons flow
from the anode to the cathode
Current flows
from the cathode to the anode
Cell diagrams
are shorthand notation that represent the reactions taking place in an electrochemical cell.
Cell diagrams are written
from anode to cathode with electrolytes (the solution) in between.
A vertical line represents
a phase boundary
A double vertical line represents
a salt bridge or other physical boundary
Galvanic (voltaic) cells
have spontaneous reactions (delta G
Electrolytic cells
house nonspontaneous reactions (delta G > 0). They can be used to create useful products through electrolysis.
Concentration cells
Are 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.
Charge on an electrode for galvanic cells
Anode is negatively charged and the cathode is positively charged
Charge on an electrode for electrolytic cells
Anode is positively charged and the cathode is negatively charged
Rechargeable batteries
electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states. Often ranked by energy density - the 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 restonre the original Pb and PbO2 electrodes and concentrate the electrolyte. 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 electrodes and concentrate the electrolyte. These cells have a higher energy density than lead-acid batteries.
Nickel-metal hydride (NiMH)
Have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective and are significantly less toxic.
Surge current
An above average current transiently released at the beginning 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 reduced. The higher the reduction potential, the more a given species wants to be reduced.
Standard reduction potentials (E’red)
calculated by comparison to the standard hydrogen electrode (SHE) under the standard conditions of 298 K, 1 atm pressure and 1 M concentrations
Standard electromotive force (Ecell)
the difference in standard reduction potential between the two half-cells.
For galvanic cells, the difference of the reduction potentials of the two half reactions = ____
For electrolytic = ___
galvanic = positive electrolytic = negative
Relationship between E cell and Delta G
always have opposite signs
Ecell and Delta G in galvanic cells
E = + and G = -
Ecell and Delta G in electrolytic cells
negative
In concentration cells, Ecell and Delta G
both = 0
Nernst equation
describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force.
When Keq is greater than 1, Ecell
is +
When Keq is less than 1, Ecell
is -
When Keq is equal to 1, Ecell
is 0
