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In
electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...
, standard electrode potential E^\ominus, or E^\ominus_, is a measure of the reducing power of any element or compound. The IUPAC "Gold Book" defines it as: ''"the value of the standard emf (
electromotive force In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical '' tran ...
) of a cell in which molecular hydrogen under
standard pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
is oxidized to solvated protons at the left-hand electrode"''.


Background

The basis for an
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
, such as the
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
, is always a redox reaction which can be broken down into two half-reactions:
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
at anode (loss of electron) and reduction at cathode (gain of electron).
Electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describe ...
is produced due to the difference of
electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
between the individual potentials of the two metal
electrodes An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials dep ...
with respect to the
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
. Although the overall potential of a cell can be measured, there is no simple way to accurately measure the electrode/electrolyte potentials 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. At each electrode-electrolyte interface there is a tendency of metal ions from the solution to deposit on the metal electrode trying to make it positively charged. At the same time, metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged. At equilibrium, there is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode may be positively or negatively charged with respect to the solution. A potential difference develops between the electrode and the electrolyte which is called the
electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ...
. When the concentrations of all the species involved in a half-cell is unity then the electrode potential is known as standard electrode potential. According to IUPAC convention, standard reduction potentials are now called standard electrode potentials. In a galvanic cell, the half-cell in which oxidation takes place is called anode and it has a negative potential with respect to the solution. The other half-cell in which reduction takes place is called cathode and it has a positive potential with respect to the solution. Thus, there exists a potential difference between the two electrodes and as soon as the switch is in the on position the electrons flow from negative electrode to positive electrode. The direction of current flow is opposite to that of electron flow.


Calculation

The electrode potential cannot be obtained empirically. The
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
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,
standard hydrogen electrode The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis ...
(SHE), for which the potential is ''defined'' or agreed upon by convention, needed to be established. In this case the standard hydrogen electrode is set to 0.00 V and any electrode, for which the electrode potential is not yet known, can be paired with standard hydrogen electrode—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 standard hydrogen electrode 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. The electrode potentials are independent of the number of electrons transferred —they are expressed in volts, which measure energy per electron 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
electrometer An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern ...
, while the standard hydrogen electrode is connected to the negative terminal.IUPAC definition of the electrode potential
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Reversible electrode

A reversible electrode is an electrode that owes its potential to changes of a reversible nature. A first condition to be fulfilled is that the system is close to the
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the ...
. A second set of conditions is that the system is submitted to very small solicitations spread on a sufficient period of time so, that the chemical equilibrium conditions nearly always prevail. In theory, it is very difficult to experimentally achieve reversible conditions because any perturbation imposed to a system near equilibrium in a finite time forces it out of equilibrium. However, if the solicitations exerted on the system are sufficiently small and applied slowly, one can consider an electrode to be reversible. By nature, electrode reversibility depends on the experimental conditions and the way the electrode is operated. For example, electrodes used in electroplating are operated with a high over-potential to force the reduction of a given metal cation to be deposited onto a metallic surface to be protected. Such a system is far from equilibrium and continuously submitted to important and constant changes in a short period of time. Electrodes used in electroplating do not represent a reversible system and are also consumed during their use.


Standard reduction potential table

The larger the value of the standard reduction potential, the easier it is for the element to be reduced (gain
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s); in other words, they are better
oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
s. For example, F2 has a standard reduction potential of +2.87 V and Li+ has −3.05 V: : (''g'') + 2 2 = +2.87 V : + (''s'') = −3.05 V The highly positive standard reduction potential of F2 means it is reduced easily and is therefore a good oxidizing agent. In contrast, the greatly negative standard reduction potential of Li+ indicates that it is not easily reduced. Instead, Li(''s'') would rather undergo oxidation (hence it is a good
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth met ...
). Zn2+ has a standard reduction potential of −0.76 V and thus can be oxidized by any other electrode whose standard reduction potential is greater than −0.76 V (e.g., H+ (0 V), Cu2+ (0.34 V), F2 (2.87 V)) and can be reduced by any electrode with standard reduction potential less than −0.76 V (e.g. H2 (−2.23 V), Na+ (−2.71 V), Li+ (−3.05 V)). In a galvanic cell, where a
spontaneous Spontaneous may refer to: * Spontaneous abortion * Spontaneous bacterial peritonitis * Spontaneous combustion * Spontaneous declaration * Spontaneous emission * Spontaneous fission * Spontaneous generation * Spontaneous human combustion * Spontan ...
redox reaction drives the cell to produce an electric potential,
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature an ...
\Delta G^\ominus must be negative, in accordance with the following equation: : \Delta G^\ominus_ = -n F E^\ominus_      (unit: Joule = Coulomb × Volt) where is number of
moles Moles can refer to: * Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain *The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People * Abraham Moles, French engin ...
of electrons per mole of products and is the
Faraday constant In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
, . As such, the following rules apply: : If E^\ominus_ > 0, then the process is spontaneous (
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
): \Delta G^\ominus_ < 0, and energy is liberated. : If E^\ominus_ < 0, then the process is non-spontaneous (
electrolytic cell An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two electrod ...
): \Delta G^\ominus_ > 0, and energy is consumed. Thus in order to have a spontaneous reaction (\Delta G^\ominus_ < 0), E^\ominus_must be positive, where: : E^\ominus_ = E^\ominus_ - E^\ominus_ where E^\ominus_ is the standard potential at the cathode (called as standard cathodic potential or standard reduction potential and E^\ominus_ is the standard potential at the anode (called as standard anodic potential or standard oxidation potential) as given in the table of standard electrode potential.


See also

*
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...
*
Pourbaix diagram In electrochemistry, and more generally in solution chemistry, a Pourbaix diagram, also known as a potential/pH diagram, EH–pH diagram or a pE/pH diagram, is a plot of possible thermodynamically stable phases (''i.e.'', at chemical equilibrium) ...
* Solvated electron *
Standard electrode potential (data page) The data values of standard electrode potentials (''E''°) are given in the table below, in volts relative to the standard hydrogen electrode, and are for the following conditions: * A temperature of . * An effective concentration of 1  ...
*
Standard hydrogen electrode The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis ...
(SHE) * Biochemically relevant redox potentials (data page)


References


Further reading

*Zumdahl, Steven S., Zumdahl, Susan A (2000) ''Chemistry'' (5th ed.), Houghton Mifflin Company. *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


Standard Electrode Potentials tableSTEP in Non-aqueous solvent
Electrochemical concepts Electrochemical potentials