Ley de Faraday:
Q=nFM
i= Q/t
y
EJEMPLO : calculo de E de una Celula de Ioduro de Plata
Reacción anódica la 1º
Reacción catódica
emf de la reacción =
y lo mismo si hacemos el potencial Standard de la célula electroquímica
y
Convención de SIGNOS
Terminología
Anode: electrode where the oxidation takes place.
Auxiliary (Counter) Electrode (CE): electrode that helps pass the current flowing through
the cell; the current travels between the WE and CE; typically no processes of interest
(under study) occur at the surface of CE; example: Pt wire.
Battery: one or more galvanic cells.
Cathode: electrode where the reduction takes place.
Cell Potential (E [V]): the sum of electrical potentials within an electrochemical cell that
also accounts for all redox processes occurring at the electrodes.
Chronoamperometry: electrochemical methods that utilize a potential step and have the
output current readings vs. time.
Electrochemical Cell: device that involves the presence of faradaic currents as a result of
redox chemical reactions; it can be either a galvanic cell, when the reactions are spontaneous,
or an electrolytic cell, when the reactions are non-spontaneous.
Electrochemical Mediator: the electrochemically active species with fast kinetics that will
undergo a reduction or an oxidation reaction at the electrode, thus yielding a faradaic current
that can give information about a kinetically slow process.
Electrode: it represents an electrical conductor/semiconductor in an electrochemical cell;
in electrochemistry, it is the conductive phase where the electron transfer occurs; it can be
an anode or a cathode.
Electrolytic Cell: energy-consuming device that converts electrical energy into chemical
energy; it consists of at least two electrodes and an electrolyte solution; cathode is negative
as compared to the anode; example: electrolysis cell.
Equilibrium Potential (Eeq [V]): the potential associated with an electrode when all redox
processes are in equilibrium; the net current is zero under these conditions and the electrode
potential is given by the Nernst equation.
Faradaic: the multitude of processes involving redox chemical reactions.
Fuel Cell: device similar to a galvanic cell used for the conversion of chemical energy into
electricity, sustained by a continuous supply of chemical reactants from outside the cell;
example: H2–O2 cell.
Galvanic (Voltaic) Cell: self-powered device that produces electricity by means of chemical
energy; it is limited by the supply of the chemicals contained inside the device; it consists
of two electrodes (anode and cathode) and an electrolyte solution; the cathode is
positive as compared to the anode.
Half-cell: the anode or the cathode compartment of an electrochemical cell, including all
reactions that occur at that particular electrode.
Half-reaction: the redox reaction that occurs in one half-cell, either at the anode or at the
cathode.
Ideal Non-polarizable (Depolarized) Electrode: it is a type of electrode that does not
change its potential upon passage of current; example: reference electrode.
Ideal Polarizable (Polarized) Electrode (IPE): it is a type of electrode that exhibits a large
change in potential for an infinitesimal change in current; example: WE.
Interface (Junction): in an electrochemical cell, it represents the location where two distinct
phases come in contact with each other: solid–liquid (electrode–solution), two liquids
of different concentrations and/or compositions (reference electrode–solution), etc.
Nernstian: a reversible redox process that follows equilibria equations.
Non-faradaic: processes that follow Ohm’s law; they are comprised of all processes that
occur at the electrode (excluding chemical reactions) and account for solution conductivity
and capacitive charging.
Normal/Standard Hydrogen Electrode (NHE/SHE): it is the standard reference electrode;
all standard potentials are referred to NHE; its potential is by definition 0.000 V.
Oxidation: refers to the process in which a chemical species loses one or more electrons;
it is the reverse of the reduction.
Oxidized Species/Oxidizing Agent/Oxidant (O): the chemical species that undergoes the
reduction, enabling the oxidation of a different species.
Overpotential (_ [V]): deviation of the electrode/cell potential from its equilibrium value,
_ _ E – Eeq; it can be either positive or negative.
Potentiometry: electrochemical methods that utilize near zero currents and have the output
potential readings vs. time, using the Nernst equation to find analyte concentrations.
Redox: a process that involves both a reduction and an oxidation.
Redox Couple: the chemical species that has at least two oxidation states, and thus can act
either as the reduced or the oxidized species (depending on the oxidation state); example:
Fe3+/Fe2+.
Reduced Species/Reducing Agent/Reductant (R): the chemical species that undergoes the
oxidation, enabling the reduction of a different species.
Reduction: refers to the process in which a chemical species gains one or more electrons;
it is the reverse of the oxidation.
Reference Electrode (RE): electrode that can maintain a constant potential under changing
experimental conditions; the WE potential is referenced vs. the RE potential; REs are
typically anodes in electrochemical cells; example: NHE.
Standard Reduction Potential (E0 [V vs. NHE]): is defined as the potential of the reduction
half-reaction at the electrode, with respect to the NHE; each redox couple has a fixed
standard reduction potential.
Supporting Electrolyte: an ionic substance (typically a salt) that is present in a solution
to ensure its conductivity; example: KCl. The supporting electrolyte reduces the migration
effects in the solution; it does not undergo redox chemistry, and thus its ions are
called spectator ions. Sometimes the supporting electrolyte is referred to simply as the
electrolyte.
Voltammetry: electrochemical methods that utilize a potential ramp (e.g., increase or
decrease with time) and have the output current vs. potential.
Working Electrode (WE)/Indicator Electrode: electrode where the redox processes under
study occur; WEs are typically cathodes; example: ion-selective electrode or noble metal.
Ecuaciones fundamentales
Ecuación de Nernst
Sobrepotencial
Para que ocurra una reaccion no espontanea debe aplicarse un sobrepotencial superior al de equilibrio
Eeq , que es aquel en el que no hay corriente neta, la corrienta anodica es igual a la catodica
Ecuación de Butler Volmer
Cuando una celula electroquimica está en equilibrio, no hay corriente neta la corrienta anodica es igual a la
catódica
Ecuacion De Tafel
Relaciona el sobrepotencial aplicado con la corriente que pasa por el circuito
¿ Para que sirve?
Para calcular ALFA
Ecuacion que gobierna el modo de Transferencia de Masa Solucion- Electrodo
Ecuacion de Nernst-Planck
Leyes de Ficks ( transferencia de masa de disolucion hacia el electrodo por diffusion)
1ª
2ª Ley de
El flujo puede describirse como : J=mCj
siendo m el coeficiente de transferencia de masa ( cm.seg-1) representa la velocidad con
que la especie j llega a la superficie del electrodo
Podemos escribirlo como
si espresamos la velocidad de la reaccion como la variacion de moles por Segundo y de
la ley de faraday tenemos :
sustituyendo en la ecuacion del flujo tendremos:
si tenemos en cuenta que la transferencia de masa limita la corriente producida y que
es independiente del potencial
donde δ (cm) es la capa de difusion de Nernst
y m( coeficiente de transferencia de masa) = D/δ
y si solamente hay difusion .
y la intensidad generada es
ECUACIÖN DE Cottrell ( ecuación cronoamperométrica)
Celula Galvanica
Celula Electroquímica
Doble Capa Eléctrica
Celulas de trabajo de 2 y tres electrodos
Dos electrodos
Tres electrodos
Disolventes Electrodos Electroactividad
ELECTRODOS DE REFERENCIA
CALOMELANOS
Ag/ClAg
y
Ecuaciones Curvas Intensidad-Potencial