Galvanic Cells
Terms and Formulae
Terms
Anode
-
The electrode that is the source of the negative charge, designated by a
minus sign (-); this electrode is the site of oxidation.
Battery
-
A galvanic cell or cells connected in series with a constant amount of
reagents. A battery stores energy in the form of chemical potential energy, which is released as electrical energy.
Cathode
-
The electrode that is the source of positive charge, designated by a plus
sign (+); this
electrode is the site of reduction.
Cell Potential
-
The overall electrical potential of an electrochemical cell. It is the sum
of
the reduction potential of the cathode and the oxidation potential
of the anode.
Concentration Cell
-
A galvanic cell that has two compositionally equivalent half-cells of differing concentrations. One can calculate the
potential developed by such a cell using the Nernst Equation.
Current
-
Flow of charge per unit time. I(current)=dQ/dt.
Electrochemistry
-
The study of the exchange between electrical and chemical energy.
Electrode
-
A conducting material placed in physical contact with a half-reaction
on which the electron transfers in the redox reaction take place.
Fuel Cell
-
A galvanic cell with a constant flow of reagents in and products out used
for the prodction
of a constant supply of energy. Whereas batteries have a finite lifetime
of useful energy
production, fuel cells are only limited in their duration of energy
production by the
ammount of available fuel reagents.
Galvanic Cell
-
An electrochemical cell with a positive cell potential that allows chemical energy to be converted into electrical energy.
Half-Cell
-
A half-reaction and its electrode; it is half of a galvanic cell.
Half-Reaction
-
Either an oxidation or a reduction reaction, it represents half of the
redox reaction.
Half-Reaction Method
-
The set of rules that have been developed to aid in balancing redox
reactions. See Balancing Redox Reactions for details.
Hess's Law
-
The sum of the state functions of a series of reactions is the same as the
state function for
the overall sum of the reactions.
Line Notation
-
A shorthand way of describing an electrochemical cell without drawing a
picture. This system is further described in Line
Notation.
Oxidation
-
The loss of an electron from a species (an increase in its oxidation
number).
Oxidation Number
-
A conceptual bookkeeping numbering system that allows us to track the
number of electrons
transferred during a redox reaction. The rules for determining the
oxidation number
of a species are discussed in Galvanic Cells, Oxidation
State.
Oxidation Potential
-
The potential of a half-reaction written as an oxidation reaction, it is
the opposite sign of the same reaction written as a reduction.
Oxidizing Agent
-
A reactant in a redox reaction that accepts an electron from the
oxidized species. The oxidizing agent is reduced.
pH Meter
-
A concentration cell that places a known concentration of acid in the
meter's
electrode. When that probe is dipped into a solution of unknown acid
concentration, a
potential develops due to the differences in concentration which can be
calculated by the
Nernst Equation.
From that potential,
the meter
calculates the concentration of the unknown acid and, therefore, its pH.
Porous Disk
-
A disk placed between two half-reactions allowing ion flow between
half-reactions without
mixing the reactions. The mixing of the two cells would cause a short in
the circuit.
Potentiometer
-
A device that measures electrical potential.
Reaction Quotient
-
Similar to the form of an equilibrium constant, the reaction quotient is
the ratio of the
product of the each product in a reaction raised to its stoichiometric
power divided by the
product of each reactant raised to its stoichiometric power from the
balanced equation.
Redox
-
A reaction involving the transfer of one or more electrons from the
reducing agent to
the oxidizing agent.
Reducing Agent
-
A reactant in a redox reaction that donates an electron to the
reduced species. The
reducing agent is oxidized.
Reduction
-
The gain of an electron by a species (a decrease in oxidation number).
Reduction Potential
-
Arbitrarily setting the potential of the standard hydrogen electrode, SHE, to zero,
all other half-reactions are measured by their power to reduce hydrogen. The voltage
given by the construction of a galvanic cell between the SHE and the reduction of
interest gives the standard reduction potential of that reduction.
Salt Bridge
-
A tube plugged with pourous material at either end (usually cotton) filled
with a gel allowing
ion flow between half-reactions without mixing the reactions. The actual mixing
of the two cells
would cause a short in the circuit.
Standard State
-
An arbitrarily defined set of conditions--273K, 1atm for gasses, or 1M for
solutions.
Work
-
Force over a distance.
Formulae
| Adding Cell Potentials | Eo cell = Eo 1 + Eo 2 |
| Nernst Equation | E = Eo - (RT/nF) ln Q |
