Glossary of chemical terms

ebullioscopic constant
	Ebullioscopic constant (Eb) is the constant that expresses the amount by which the boiling point Tb of a solvent is raised by a nondissociating solute, through the relation ΔTb = Eb b where b is the molality of the solute.
electrical resistivity
	Electrical resistivity (ρ) is electric field strength divided by current density when there is no electromotive force in the conductor. Resistivity is an intrinsic property of a material. For a conductor of uniform cross section with area A and length L, and whose resistance is R, the resistivity is given by The SI unit is Ω m.
electrochemical series
	Electrochemical series is a series of chemical elements arranged in order of their standard electrode potentials. The hydrogen electrode H+(aq) + e- 1/2H2(g) is taken as having zero electrode potential. An electrode potential is, by definition, a reduction potential. Elements that have a greater tendency than hydrogen to lose electrons to their solution are taken as electropositive; those that gain electrons from their solution are below hydrogen in the series and are called electronegative. The series shows the order in which metals replace one another from their salts; electropositive metals will replace hydrogen from acids.
electrode potential
	Electrode potential is defined as the potential of a cell consisting of the electrode in question acting as a cathode and the standard hydrogen electrode acting as an anode. Reduction always takes place at the cathode, and oxidation at the anode. According to the IUPAC convention, the term electrode potential is reserved exclusively to describe half-reactions written as reductions. The sign of the half-cell in question determines the sign of an electrode potential when it is coupled to a standard hydrogen electrode. Electrode potential is defined by measuring the potential relative to a standard hydrogen half cell H2(g) 2H+(aq) + 2e- The convention is to designate the cell so that the oxidized form is written first. For example Pt(s)|H2(g)|H+(aq)||Zn2+(aq)|Zn(s) The e.m.f. of this cell is e.m.f. = Eright - Eleft By convention, at p(H2) = 101325 Pa and a(H+) = 1.00, the potential of the standard hydrogen electrode is 0.000 V at all temperatures. As a consequence of this definition, any potential developed in a galvanic cell consisting of a standard hydrogen electrode and some other electrode is attributed entirely to the other electrode e.m.f. = E(Zn2+/Zn)
electrolysis
	Electrolysis is the decomposition of a substance as a result of passing an electric current between two electrodes immersed in the sample.
electromotive force
	Electromotive force (e.m.f. or EMF) is the difference in electric potential that exists between two dissimilar electrodes immersed in the same electrolyte or otherwise connected by ionic conductors.
electron
	Electron is discovered by J.J. Thompson (1856-1940) in 1896. The electron is an elementary particle with a negative electric charge of (1.602 189 2 ± 0.000 004 6) × 10-19 C and a mass of 1/1837 that of a proton, equivalent to (9.109 534 ± 0.000 047) × 10-31 kg. Electrons are arranged in from one to seven shells around the nucleus; the maximum number of electrons in each shell is strictly limited by the laws of physics (2n2). The outer shells are not always filled: sodium has two electrons in the first shell (2×12 = 2), eight in the second (2×22 = 8), and only one in the third (2×32 = 18). A single electron in the outer shell may be attracted into an incomplete shell of another element, leaving the original atom with a net positive charge. Valence electrons are those that can be captured by or shared with another atom. Electrons can be removed from the atoms by heat, light, electric energy, or bombardment with high-energy particles. Decaying radioactive nuclei spontaneously emits free electrons, called β particles.
electron affinity
	Electron affinity (EA) is the energy change occurring when an atom or molecule gains an electron to form a negative ion. For an atom or molecule X, it is the energy released for the electron-attachment reaction X(g) + e- X- (g) Often this is measured in electronvolts. Alternatively, the molar enthalpy change, ΔH, can be used.
electron spin
	Electron spin (s) is the quantum number, equal to 1/2, that specifies the intrinsic angular momentum of the electron.
electron volt
	Electron volt (eV) is a non-SI unit of energy used in atomic and nuclear physics, equal to approximately 1.602177 × 10-19 J. The electron volt is defined as the kinetic energy acquired by an electron upon acceleration through a potential difference of 1 V.
electronegativity
	Electronegativity is a parameter originally introduced by L. Pauling which describes, on a relative basis, the power of an atom to attract electrons. For example, in hydrogen chloride, the chlorine atom is more electronegative than the hydrogen and the molecule is polar, with negative charge on the chlorine atom. There are various ways of assigning values for the electronegativity of an element. Pauling electronegativities are based on bond dissociation energies using a scale in which fluorine, the most electronegative element, has value 4 and francium, the lowest electronegative element, has value 0.7.
electrophoresis
	Electrophoresis is a technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in an electric field. The migration is toward electrodes of charge opposite to that of the particles. The rate of migration of the particles depends on the field, the charge on the particles, and on other factors, such as the size and shape of the particles. Electrophoresis is important in the study of proteins. The acidity of the solution can be used to control the direction in which a protein moves upon electrophoresis.
emulsion
	Emulsion is colloidal system in which the dispersed and continuous phases are both liquids (e.g. oil in water or water in oil). Such systems require an emulsifying agent to stabilize the dispersed particles.
enantiomers
	Enantiomers are a chiral molecule and its non-superposable mirror image. The two forms rotate the plane of polarized light by equal amounts in opposite directions. Also called optical isomers.
energy
	Energy (E, U) is the characteristic of a system that enables it to do work. Like work itself, it is measured in joules (J). The internal energy of a body is the sum of the potential energy and the kinetic energy of its component atoms and molecules. Potential energy is the energy stored in a body or system as a consequence of its position, shape, or state (this includes gravitational energy, electrical energy, nuclear energy, and chemical energy). Kinetic energy is energy of motion and is usually defined as the work that will be done by body possessing the energy when it is brought to rest. For a body of mass m having a speed v, the kinetic energy is mv2/2. Kinetic energy is most clearly exhibited in gases, in which molecules have much greater freedom of motion than in liquids and solids.
enthalpy
	Enthalpy (H) is a thermodynamic property of a system defined by H = U + pV where U is the internal energy of the system, p its pressure, and V its volume. J.W. Gibbs put the concept of an ensemble forward in 1902. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction (ΔH), is to equal ΔH = ΔU + pΔV For an exothermic reaction ΔH is taken to be negative.
entropy
	Entropy (S) is a measure of the unavailability of a system's energy to do work; in a closed system, an increase in entropy is accompanied by a decrease in energy availability. When a system undergoes a reversible change the entropy (S) changes by an amount equal to the energy (Q) transferred to the system by heat divided by the thermodynamic temperature (T) at which this occurs. All real processes are to a certain extent irreversible changes and in any closed system an irreversible change is always accompanied by an increase in entropy.
enzyme
	Enzyme is a protein that acts as a catalyst in biochemical reactions. Each enzyme is specific to a particular reaction or group of similar reactions. Many require the association of certain nonprotein cofactors in order to function. The molecule undergoing reaction (the substrate) binds to a specific active site on the enzyme molecule to form a short-lived intermediate: this greatly increases (by a factor of up to 1020) the rate at which the reaction proceeds to form the product.
epoxy resin
	Epoxy resins are thermosetting resins produced by copolymerising epoxide compounds with phenols (e.g. epichlorohydrin and bisphenol A). They contain ether linkages (-O-) and form a tight, cross-linked polymer network. Toughness, good adhesion, corrosive-chemical resistance, and good dielectric properties characterize epoxy resins. Most epoxy resins are two-part type which harden when blended.
equation of state
	Equation of state is an equation relating the pressure, volume, and temperature of a substance or system. Equation of state for ideal gas pV = nRT where p is pressure, V molar volume, T temperature, and R the molar gas constant (8.314 JK-1mol-1).
equilibrium constant
	The equilibrium constant (K) was originally introduced in 1863 by Norse chemists C.M. Guldberg and P. Waage using the law of mass action. For a reversible chemical reaction represented by the equation aA + bB cC + dD chemical equilibrium occurs when the rate of the forward reaction equals the rate of the back reaction, so that the concentrations of products and reactants reach steady-state values. The equilibrium constant is the ratio of chemical activities of the species A, B, C, and D at equilibrium. To a certain approximation, the activities can be replaced by concentrations. For gas reactions, partial pressures are used rather than concentrations The units of Kp and Kc depend on the numbers of molecules appearing in the stoichiometric equation (a, b, c, and d). The value equilibrium constant depends on the temperature. If the forward reaction is exothermic, the equilibrium constant decreases as the temperature rises. The equilibrium constant shows the position of equilibrium. A low value of K indicates that [C] and [D] are small compared to [A] and [B]; i.e. that the back reaction predominates. The equilibrium constant is related to DrG°, the standard Gibbs free energy change in the reaction, by RT lnK = -ΔrG°
erg
	Erg (erg) is a non-SI (cgs) unit of energy, equal to 10-7 J.
esters
	Esters are organic compounds formed by reaction between alcohol and acids. Esters formed from carboxylic acids have the general formula RCOOR'. Triesters, molecules containing three ester groups, occur in nature as oils and fats.
ethers
	Ethers are organic compounds with formula R-O-R, where R is not equal to H. They may be derived from alcohols by elimination of water, but the major method is catalytic hydration of olefins. They are volatile highly flammable compounds; when containing peroxides they can detonate on heating. The term ether is often used synonymously with ethyl ether.
Euler number
	Euler number (Eu) is a dimensionless quantity used in fluid mechanics, defined by where p is pressure, ρ is density, and v is velocity.
eutectic
	Eutectic is a solid solution consisting of two or more substances and having the lowest freezing point of any possible mixture of these components. Eutectic point is the lowest temperature at which the eutectic mixture can exist in liquid phase. A liquid having the eutectic composition will freeze at a single temperature without change of composition.

 

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