Electrochemical potential
Encyclopedia
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
. Electrochemical potential is expressed in the unit of J
/mol
.
(for example, "water molecules", "sodium ions", "electrons", etc.) has an electrochemical potential (a number with units of energy) at any given location, which represents how easy or difficult it is to add more of that species to that location. If possible, a species will try to move from areas with higher electrochemical potential to areas with lower electrochemical potential; in equilibrium, the electrochemical potential will be constant everywhere for each species (it may have a different value for different species). For example, if a glass of water has sodium ions (Na+) dissolved uniformly in it, and an electric field
is applied across the water, then the sodium ions will tend to get pulled by the electric field towards one side. We say the ions have electric potential energy
, and are moving to lower their potential energy. Likewise, if a glass of water has a lot of dissolved
sugar on one side and none on the other side, each sugar molecule will randomly diffuse around the water, until there is equal concentration of sugar everywhere. We say that the sugar molecules have a "chemical potential
," which is higher in the high-concentration areas, and the molecules move to lower their chemical potential. These two examples show that an electrical potential and a chemical potential can both give the same result: A redistribution of the chemical species. Therefore it makes sense to combine them into a single "potential", the electrochemical potential, which can directly give the net redistribution taking both into account.
It is (in principle) easy to measure whether or not two regions (for example, two glasses of water) have the same electrochemical potential for a certain chemical species (for example, a solute molecule): Allow the species to freely move back and forth between the two regions (for example, connect them with a semi-permeable membrane that lets only that species through). If the chemical potential is the same in the two regions, the species will occasionally move back and forth between the two regions, but on average there is just as much movement in one direction as the other, and there is zero net migration (this is called "diffusive equilibrium"). If the chemical potentials of the two regions are different, more molecules will move to the lower chemical potential than the other direction.
Moreover, when there is not diffusive equilibrium, i.e., when there is a tendency for molecules to diffuse from one region to another, then there is a certain free energy
released by each net-diffusing molecule. This energy, which can sometimes be harnessed (a simple example is a concentration cell
), and the free-energy per molecule is exactly equal to the electrochemical potential difference between the two regions.
and electrochemical potential of an electron
. However, in the two fields, the definitions of these two terms are sometimes swapped. In electrochemistry, the electrochemical potential of an electron (or any other species) is by definition constant across a device in equilibrium, while the chemical potential is equal to the electrochemical potential minus the local electric potential energy
of the electron. In solid-state physics, the opposite definitions are occasionally (but not always) used, where the chemical potential of an electron is by definition constant across a device in equilibrium; while the electrochemical potential is equal to the chemical potential minus the local electric potential energy
of an electron.
This article uses the electrochemistry definitions.
done in bringing 1 mole of an ion from a standard state
to a specified concentration
and electrical potential. According to the IUPAC definition, it is the partial molar Gibbs energy of the substance at the specified electric potential, where the substance is in a specified phase. Electrochemical potential can be expressed as
,
where:
In the special case of an uncharged atom,
= 0 and so 
.
Electrochemical potential is important in biological processes that involve molecular
diffusion
across membranes, in electroanalytical chemistry, and industrial applications such as batteries and fuel cells. It represents one of the many interchangeable forms of potential energy
through which energy may be conserved
.
In cell membranes, the electrochemical potential is the sum of the chemical potential
and the membrane potential
.
(either of a corroding electrode, an electrode with a non-zero net reaction or current, or an electrode at equilibrium). This particular usage can lead to confusion. A measured electrode potential does not equal the change of the electrochemical potential (see Galvani potential
). Therefore, the recent literature usually explains the abbreviation ECP as "electrochemical corrosion potential". For an electrode at equilibrium, the phrase equilibrium or reversible potential of the electrode is used.
Electrochemistry
Electrochemistry 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 electrochemical potential,
, sometimes abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potentialChemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
and electrostatics
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
. Electrochemical potential is expressed in the unit of J
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
/mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
.
Introduction
Each chemical speciesChemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...
(for example, "water molecules", "sodium ions", "electrons", etc.) has an electrochemical potential (a number with units of energy) at any given location, which represents how easy or difficult it is to add more of that species to that location. If possible, a species will try to move from areas with higher electrochemical potential to areas with lower electrochemical potential; in equilibrium, the electrochemical potential will be constant everywhere for each species (it may have a different value for different species). For example, if a glass of water has sodium ions (Na+) dissolved uniformly in it, and an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
is applied across the water, then the sodium ions will tend to get pulled by the electric field towards one side. We say the ions have electric potential energy
Electric potential energy
Electric potential energy, or electrostatic potential energy, is a potential energy that results from conservative Coulomb forces and is associated with the configuration of a particular set of point charges within a defined system...
, and are moving to lower their potential energy. Likewise, if a glass of water has a lot of dissolved
Dissolution (chemistry)
Dissolution is the process by which a solid, liquid or gas forms a solution in a solvent. In solids this can be explained as the breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent. For liquids and gases, the molecules must be compatible...
sugar on one side and none on the other side, each sugar molecule will randomly diffuse around the water, until there is equal concentration of sugar everywhere. We say that the sugar molecules have a "chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
," which is higher in the high-concentration areas, and the molecules move to lower their chemical potential. These two examples show that an electrical potential and a chemical potential can both give the same result: A redistribution of the chemical species. Therefore it makes sense to combine them into a single "potential", the electrochemical potential, which can directly give the net redistribution taking both into account.
It is (in principle) easy to measure whether or not two regions (for example, two glasses of water) have the same electrochemical potential for a certain chemical species (for example, a solute molecule): Allow the species to freely move back and forth between the two regions (for example, connect them with a semi-permeable membrane that lets only that species through). If the chemical potential is the same in the two regions, the species will occasionally move back and forth between the two regions, but on average there is just as much movement in one direction as the other, and there is zero net migration (this is called "diffusive equilibrium"). If the chemical potentials of the two regions are different, more molecules will move to the lower chemical potential than the other direction.
Moreover, when there is not diffusive equilibrium, i.e., when there is a tendency for molecules to diffuse from one region to another, then there is a certain free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
released by each net-diffusing molecule. This energy, which can sometimes be harnessed (a simple example is a concentration cell
Concentration cell
A concentration cell is a limited form of a galvanic 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. A concentration cell produces a voltage as it attempts to reach...
), and the free-energy per molecule is exactly equal to the electrochemical potential difference between the two regions.
Conflicting terminologies
It is common in both solid-state physics and electrochemistry to discuss the chemical potentialChemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
and electrochemical potential of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
. However, in the two fields, the definitions of these two terms are sometimes swapped. In electrochemistry, the electrochemical potential of an electron (or any other species) is by definition constant across a device in equilibrium, while the chemical potential is equal to the electrochemical potential minus the local electric potential energy
Electric potential energy
Electric potential energy, or electrostatic potential energy, is a potential energy that results from conservative Coulomb forces and is associated with the configuration of a particular set of point charges within a defined system...
of the electron. In solid-state physics, the opposite definitions are occasionally (but not always) used, where the chemical potential of an electron is by definition constant across a device in equilibrium; while the electrochemical potential is equal to the chemical potential minus the local electric potential energy
Electric potential energy
Electric potential energy, or electrostatic potential energy, is a potential energy that results from conservative Coulomb forces and is associated with the configuration of a particular set of point charges within a defined system...
of an electron.
This article uses the electrochemistry definitions.
Definition and usage
In generic terms, electrochemical potential is the mechanical workMechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...
done in bringing 1 mole of an ion from a standard state
Standard state
In 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...
to a specified concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
and electrical potential. According to the IUPAC definition, it is the partial molar Gibbs energy of the substance at the specified electric potential, where the substance is in a specified phase. Electrochemical potential can be expressed as
,where:
-
is the electrochemical potential of species i, J/mol -
is the chemical potentialChemical potentialChemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
of the species i, J/mol -
is the valency (charge) of the ion i, dimensionless -
is Faraday's Constant, C/mol -
is the local electrostatic potential, V.
In the special case of an uncharged atom,
= 0 and so .Electrochemical potential is important in biological processes that involve molecular
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
across membranes, in electroanalytical chemistry, and industrial applications such as batteries and fuel cells. It represents one of the many interchangeable forms of potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
through which energy may be conserved
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...
.
In cell membranes, the electrochemical potential is the sum of the chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
and the membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
.
Incorrect usage
The term electrochemical potential is sometimes used to mean an electrode potentialElectrode potential
Electrode 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...
(either of a corroding electrode, an electrode with a non-zero net reaction or current, or an electrode at equilibrium). This particular usage can lead to confusion. A measured electrode potential does not equal the change of the electrochemical potential (see Galvani potential
Galvani potential
Galvani potential in electrochemistry, is the electric potential difference between two points in the bulk of two phases...
). Therefore, the recent literature usually explains the abbreviation ECP as "electrochemical corrosion potential". For an electrode at equilibrium, the phrase equilibrium or reversible potential of the electrode is used.
External links
- Electrochemical potential - lecture notes from University of Illinois at Urbana-Champaign