In
electrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, the
standard electrode potential, abbreviated
E° or E
o (with a superscript
plimsollPlimsoll is a surname, and may refer to:* James Plimsoll, a governor of Tasmania* John Plimsoll, South African cricketer* Oliver Plimsolls, fictional character in The League of Gentlemen * Samuel Plimsoll, British politician...
character, pronounced "standard" or "nought"), is the measure of
individual potentialElectrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...
of a reversible electrode (at equilibrium) at
standard stateIn chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...
, which is with solutes at an effective concentration of 1 mol dm
−3, and gases at a pressure of 1 bar. The values are most often tabulated at 25 °C.
In
electrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, the
standard electrode potential, abbreviated
E° or E
o (with a superscript
plimsollPlimsoll is a surname, and may refer to:* James Plimsoll, a governor of Tasmania* John Plimsoll, South African cricketer* Oliver Plimsolls, fictional character in The League of Gentlemen * Samuel Plimsoll, British politician...
character, pronounced "standard" or "nought"), is the measure of
individual potentialElectrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...
of a reversible electrode (at equilibrium) at
standard stateIn chemistry, the standard state of a material is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry recommends a conventional set of standard states...
, which is with solutes at an effective concentration of 1 mol dm
−3, and gases at a pressure of 1 bar. The values are most often tabulated at 25 °C. The basis for an
electrochemical cellAn electrochemical cell is a device used for generating an electromotive force and current from chemical reactions, or the reverse, inducing a chemical reaction by a current. The current is caused by the reactions releasing and accepting electrons at the different ends of a conductor. A common...
such as the
galvanic cellThe Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells...
is always a redox reaction which can be broken down into two
half-reactionA half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
s: oxidation at anode (loss of electron) and
reductionRedox describes all chemical reactions in which atoms have their oxidation number changed....
at cathode (gain of electron).
ElectricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
is generated due to
electric potentialAt a point in space, the electric potential is potential energy divided by charge that is associated with a static electric field. It is a scalar quantity, typically measured in volts....
difference between two electrodes. This potential difference is created as a result of the difference between individual potentials of the two metal electrodes with respect to the electrolyte.
Although the overall potential of a cell can be measured, there is no simple way to accurately measure the
electrode/electrolyte potentialsAbsolute electrode potential, in electrochemistry, according to an IUPAC definition, is the electrode potential of a metal measured with respect to a universal reference system .-Definition:...
in isolation. The electric potential also varies with temperature, concentration and pressure. Since the oxidation potential of a half-reaction is the negative of the reduction potential in a redox reaction, it is sufficient to calculate either one of the potentials. Therefore, standard electrode potential is commonly written as standard reduction potential.
Calculation of standard electrode potentials
The electrode potential may not be obtained empirically. The
galvanic cellThe Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells...
potential results from a
pair of electrodes. Thus, only one empirical value is available in a pair of electrodes and it is not possible to determine the value for each electrode in the pair using the empirically obtained galvanic cell potential. A reference electrode, the
standard hydrogen electrodeThe standard hydrogen electrode , also called normal hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
(SHE), for which the potential is
defined or agreed upon by convention, needed to be established. In this case SHE is set to 0.00 V and any electrode, for which the electrode potential is not yet known, can be paired with SHE – to form a galvanic cell – and the galvanic cell potential gives the unknown electrode's potential. Using this process, any electrode with an unknown potential can be paired with either the SHE or another electrode for which the potential has already been derived and that unknown value can be established.
Since the electrode potentials are conventionally defined as reduction potentials, the sign of the potential for the metal electrode being oxidized must be reversed when calculating the overall cell potential. Note that the electrode potentials are independent of the number of electrons transferred – that is, they are set to one
moleThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
of electrons transferred- and so the two electrode potentials can be simply combined to give the overall
cell potential even if different numbers of electrons are involved in the two electrode reactions.
For practical measurements, the electrode in question is connected to the positive terminal of the
electrometerAn electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices...
, while SHE is connected to the negative terminal.
Standard reduction potential table
Since the values are given in their ability to be reduced, the bigger the standard reduction potentials, the easier they are to be reduced, in other words, they are simply better oxidizing agents. For example, F
2 has 2.87 V and Li
+ has −3.05 V. F
2 reduces easily and is therefore a good oxidizing agent. In contrast, Li
(s) would rather undergo oxidation (hence a good
reducing agentA reducing agent is the element or compound in a redox reaction that reduces another species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox...
). Thus Zn
2+ whose standard reduction potential is −0.76 V can be oxidized by any other electrode whose standard reduction potential is greater than −0.76 V (eg. H
+(0 V), Cu
2+(0.16 V), F
2(2.87 V)) and can be reduced by any electrode with standard reduction potential less than −0.76 V (eg. H
2(−2.23 V), Na
+(−2.71 V), Li
+(−3.05 V)).
In a galvanic cell, where a
spontaneousSpontaneous acting out in complete surprise, typically requiring no outside influence or help.Spontaneity means the quality of being spontaneous and coming fromThe word spontaneous may also refer to:* Spontaneous abortion...
redox reaction drives the cell to produce an electric potential,
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
Δ
G° must be negative, in accordance with the following equation:
- ΔG°cell = −nFE°cell
where
n is number of
molesThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
of electrons per mole of products and
F is the
Faraday constantIn physics and chemistry, the Faraday constant is the magnitude of electric charge per mole of electrons. While most uses of the Faraday constant, denoted F, have been replaced by the standard SI unit, the coulomb, the Faraday is still widely used in calculations in electrochemistry...
, ~96485 C/mol. As such, the following rules apply:
- If E°cell > 0, then the process is spontaneous (galvanic cell
The Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells...
)
- If E°cell < 0, then the process is nonspontaneous (electrolytic cell
An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased...
)
Thus in order to have a spontaneous reaction (−ΔG°),
E°
cell must be positive, where:
- E°cell = E°cathode − E°anode
where
E°
anode is the standard potential at the anode (reverse the sign of the standard reduction potential value for the electrode)
and
E°
cathode is the standard potential at the cathode as given in the table of standard electrode potential.
Non-standard condition
The standard electrode potentials are given at standard conditions. However, real cells may operate under non-standard conditions. Given the standard potential of the half-cell, its potential at non-standard effective concentrations can be calculated using the
Nernst equationIn electrochemistry, the Nernst equation is an equation which can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
:
where
Q is the
reaction quotientIn chemistry, reaction quotient: Qr is a function of the extent of reaction: ξ, the relative proportion of products and reactants present in the reaction mixture at some instant of time....
.
The values of
E° depend on temperature (except for SHE, for which the potential has been, arbitrarily, declared 0 at all temperatures) and are normally referenced to the SHE at the same temperature. For condensed phases, they are also expected to depend somewhat on pressure (see the article on equilibrium constant). For example, the standard electrode potential for Ni/NiO redox couple has been well studied because such a solid has applications in high-temperature pseudo-reference electrodes (when enclosed inside an yttrium-stabilized zirconia ceramic membrane). The half-cell reaction for this redox couple is:
- Ni + H2O NiO + 2H+ + 2e−
The standard potential of Ni/NiO has been correlated for temperatures between 0 and 400 °C to be approximately:
- E°(T) = −0.0003T + 0.1414
where
E° is in volts, and
T is in degrees Celsius.
In
biochemistryBiochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
, potentials are usually defined for pH 7, with the standard potential under these conditions being
E°
′ - also referred to as the
mid-point potential or
Em,7 because it is the potential at which the concentrations of the oxidized and reduced forms of the redox pair are equal.
The actual redox potential for a pair at a given pH of x (
Eh, pH = x) is related to the midpoint potential by:
See also
- Reference electrode
A Reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each participants of the redox reaction.There are many ways reference...
- Table of standard electrode potentials
- Reduction potential
Reduction potential is the tendency of a chemical species to acquire electrons and thereby be reduced...
- Absolute electrode potential
Absolute electrode potential, in electrochemistry, according to an IUPAC definition, is the electrode potential of a metal measured with respect to a universal reference system .-Definition:...
- Electrochemical potential
In electrochemistry, the electrochemical potential, , sometimes abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potential and electrostatics.- Conflicting terminologies :...
- Redox
Redox describes all chemical reactions in which atoms have their oxidation number changed....
- Galvanic series
The galvanic series determines the nobility of metals and semi-metals. When two metals are submerged in an electrolyte, while electrically connected, the less noble will experience galvanic corrosion. The rate of corrosion is determined by the electrolyte and the difference in nobility...
- Nernst equation
In electrochemistry, the Nernst equation is an equation which can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
- Half cell
A half cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally-occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a...
- Electrochemical cell
An electrochemical cell is a device used for generating an electromotive force and current from chemical reactions, or the reverse, inducing a chemical reaction by a current. The current is caused by the reactions releasing and accepting electrons at the different ends of a conductor. A common...
- Galvanic cell
The Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells...
- Concentration cell
A Concentration cell is an electrochemical cell that has two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst Equation...
Further reading
- Zumdahl, Steven S., Zumdahl, Susan A (2000) Chemistry (5th ed.), Houghton Mifflin Company. ISBN 0-395-98583-8
- Atkins, Peter, Jones, Loretta (2005) Chemical Principles (3rd ed.), W.H. Freeman and Company. ISBN 0-7167-5701-X
- Zu, Y, Couture, MM, Kolling, DR, Crofts, AR, Eltis, LD, Fee, JA, Hirst, J (2003) Biochemistry, 42, 12400-12408
- Shuttleworth, SJ (1820) Electrochemistry (50th ed.), Harper Collins.
External links