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Home :Glossary

Glossary: E


ebulliometry. ebulliometric.
Determination of average molecular weight of a dissolved substance from the boiling point elevation* of the solution.

EDTA. ethylenediaminetetracetic acid; versine.
A polydentate* ligand* that tightly complexes certain metal ions. EDTA is used as a blood preservative by complexing free calcium ion (which promotes blood clotting). EDTA's ability to bind to lead ions makes it useful as an antidote for lead poisoning.

effective nuclear charge. (Zeff) Compare with atomic number*.
The nuclear charge experienced by an electron when other electrons are shielding the nucleus.

efflorescent. efflorescence; efflorescing. Compare with deliquescent* and hygroscopic*.
Efflorescent substances lose water of crystallization* to the air. The loss of water changes the crystal structure, often producing a powdery crust.

effusion. effuse. Compare with diffusion* and diffraction*.
Gas molecules in a container escape from tiny pinholes into a vacuum with the same average velocity they have inside the container. They also move in straight-line trajectories through the pinhole.

electric charge. charge.
A property used to explain attractions and repulsions between certain objects. Two types of charge are possible: negative and positive. Objects with different charge attract; objects with the same charge repel each other.

electric current. current; electrical current.
A flow of electric charges*. The SI* unit of electric current is the ampere*.

electric dipole. dipole.
An object whose centers of positive and negative charge do not coincide. For example, a hydrogen chloride (HCl) molecule is an electric dipole because bonding electrons are on average closer to the chlorine atom than the hydrogen, producing a partial positive charge on the H end and a partial negative charge on the Cl end.

electric dipole moment. (µ) dipole moment.
A measure of the degree of polarity of a polar molecule*. Dipole moment is a vector with magnitude equal to charge separation times the distance between the centers of positive and negative charges. Chemists point the vector from the positive to the negative pole; physicists point it the opposite way. Dipole moments are often expressed in units called Debyes*.

electric field.
A field of forces that act on any electric charge* placed within it. The stronger the field, the stronger the force that acts on the charge. For example, the positive charge on an atomic nucleus* creates an electric field that traps electrons*.

electrical conductivity. conductivity; electric conductivity; electrical conductance; conductance. Compare with resistance*.
A measure of how easily an electric current* can pass through a material. The conductivity is the reciprocal of the resistance*. The SI* unit of conductance is the siemens*.

electrical resistance. resistance. Compare with conductivity*.
The ability of a material to oppose the flow of an electric current*, converting electrical energy into heat. The SI* unit of resistance is the ohm*.

electrochemical cell. electric cell.
A device that uses a redox reaction* to produce electricity, or a device that uses electricity to drive a redox reaction in the desired direction.

An electrically conducting surface that allows electrons to be transferred between reactants in an electrochemical cell*.

electrolytic cell.
A device that uses electricity from an external source to drive a redox reaction*.

The process of driving a redox reaction* in the reverse direction by passage of an electric current through the reaction mixture.

A substance that dissociates fully or partially into ions* when dissolved in a solvent, producing a solution that conducts electricity. See strong electrolyte*, weak electrolyte*.

electromagnetic radiation. electromagnetic wave.
A wave* that involves perpendicular oscillations in the electric and magnetic fields, moving at a speed of 2.99792458×108 m/s in a vacuum away from the source. gamma rays*, x-rays*, ultraviolet light*, visible light*, infrared radiation*, and radio waves* are all electromagnetic waves.

electron. (e-) Compare with proton* and neutron*.
A fundamental consituent of matter, having a negative charge of 1.602 176 462 × 10-19 coulombs* ± 0.000 000 063 × 10-19 coulombs and a mass of 9.109 381 88 × 10-31 kg ± 0.000 000 72 × 10-31 kg [1998 CODATA values].

electron affinity.
The enthalpy change* for the addition of one electron to an atom or ion in the gaseous state. For example, the electron affinity of hydrogen is DeltaH in the reaction
H(g) + e- rightarrow H-(g)DeltaH = -73 kJ/mol.

electron configuration. electronic configuration.
A list showing how many electrons are in each orbital or subshell*. There are several notations. The subshell notation lists subshells in order of increasing energy, with the number of electrons in each subshell indicated as a superscript. For example, 1s2 2s2 2p3 means "2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 3 electrons in the 2p subshell.

electronegativity Compare with ionization energy* and electron affinity*.
Electronegativity is a measure of the attraction an atom has for bonding electrons. Bonds between atoms with different electronegativities are polar, with the bonding electrons spending more time on average around the atom with higher electronegativity.

electron volt.
Energy required to move an electron through a potential difference of 1 volt*. An electron volt is equivalent to 1.6×10-19 J.

Electrorefining is a method for purifying a metal using electrolysis*. An electric current is passed between a sample of the impure metal and a cathode* when both are immersed in a solution that contains cations* of the metal. Metal is stripped off the impure sample and deposited in pure form on the cathode.

element Compare with compound* and mixture*.
An element is a substance composed of atoms with identical atomic number*. The older definition of element (an element is a pure substance that can't be decomposed chemically) was made obsolete by the discovery of isotopes*.

element symbol.
An international abbreviation for element names, usually consisting of the first one or two distinctive letters in element name. Some symbols are abbreviations for ancient names.

elementary reaction. Compare with net chemical reaction*.
A reaction that occurs in a single step. Equations for elementary reactions show the actual molecules, atoms, and ions that react on a molecular level.

emission spectrum. emission spectra. Compare with absorption spectrum*.
A plot of relative intensity of emitted radiation* as a function of wavelength* or frequency*.

A substance added to a formulation that gives it softening ability. For example, oils that can soften skin are added as emollients in some skin creams.

empirical formula. simplest formula. Compare with molecular formula*.
Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts. For example, the empirical formula of glucose is CH2O, which means that for every mole of carbon in the compound, there are 2 moles of hydrogen and one mole of oxygen.

empirical temperature.
A property that is the same for any two systems that are in thermodynamic equilibrium* with each other.

emulsion. Compare with colloid*.
A colloid* formed from tiny liquid droplets suspended in another, immiscible* liquid. Milk is an example of an emulsion.

enantiomer. enantiomeric. Compare with diasteromer*.
Two molecules that are nonsuperimposable mirror images of each other. One enantiomer rotates plane-polarized light to the left; the other rotates it to the right.

endothermic. endothermic reaction; endothermic process. Compare with exothermic*.
A process that absorbs heat. The enthalpy change* for an endothermic process has a positive sign.

endpoint. end point. Compare with equivalence point*.
The experimental estimate of the equivalence point* in a titration*.

energy. Compare with heat* and work*.
Energy is an abstract property associated with the capacity to do work*.

Enkephalins are molecules produced naturally by the central nervous system to numb pain. Enkephalins lock into receptors on the surface of a nerve cell and open ion channels. Ions flow into the cell and the distribution of charge on either side of the cell membrane becomes such that the nerve cell cannot fire.

enthalpy. (H) enthalpy change. Compare with heat*.
Enthalpy (H) is defined so that changes in enthalpy (DeltaH) are equal to the heat absorbed or released by a process running at constant pressure. While changes in enthalpy can be measured using calorimetry, absolute values of enthalpy usually cannot be determined. Enthalpy is formally defined as H = U + PV, where U is the internal energy, P is the pressure, and V is the volume.

enthalpy of atomization. (DeltaHat) atomization enthalpy; heat of atomization.
The change in enthalpy* that occurs when one mole of a compound is converted into gaseous atoms. All bonds in the compound are broken in atomization and none are formed, so enthalpies of atomization are always positive.

enthalpy of combustion. (DeltaHc) heat of combustion.
The change in enthalpy* when one mole of compound is completely combusted. All carbon in the compound is converted to CO2(g), all hydrogen to H2O(ell), all sulfur to SO2(g), and all nitrogen to N2(g).

enthalpy of fusion. (DeltaHfus) heat of fusion; molar heat of fusion; molar enthalpy of fusion.
The change in enthalpy* when one mole of solid melts to form one mole of liquid. Enthalpies of fusion are always positive because melting involves overcoming some of the intermolecular attractions in the solid.

enthalpy of hydration. (DeltaHhyd) hydration enthalpy; heat of hydration.
The change in enthalpy* for the process


where the concentration of A in the aqueous solution approaches zero. Enthalpies of hydration for ions are always negative because strong ion-water attractions are formed when the gas-phase ion is surrounded by water.

enthalpy of neutralization. heat of neutralization.
The heat released by an acid-base neutralization reaction* running at constant pressure.

enthalpy of reaction. (DeltaHrxn) heat of reaction.
The heat absorbed or released by a chemical reaction running at constant pressure.

enthalpy of solution. (DeltaHsoln) heat of solution. Compare with integral enthalpy of solution*.
The heat absorbed or released when a solute* is dissolved in a solvent*. The heat of solution depends on the nature of the solute and on its concentration in the final solution*.

enthalpy of sublimation. (DeltaHsub) heat of sublimation.
The change in enthalpy* when one mole of solid vaporizes to form one mole of gas. Enthalpies of sublimation are always positive because vaporization involves overcoming most of the intermolecular attractions in the sublimation.

enthalpy of vaporization. (DeltaHvap) heat of vaporization.
The change in enthalpy* when one mole of liquid evaporates to form one mole of gas. Enthalpies of vaporization are always positive because vaporization involves overcoming most of the intermolecular attractions in the liquid.

entropy. (S)
Entropy is a measure of energy dispersal. Any spontaneous change disperses energy and increases entropy overall. For example, when water evaporates, the internal energy of the water is dispersed with the water vapor produced, corresponding to an increase in entropy.

environmental chemistry. chemical ecology.
The study of natural and man-made substances in the environment, including the detection, monitoring, transport, and chemical transformation of chemical substances in air, water, and soil.

Protein* or protein-based molecules that speed up chemical reactions occurring in living things. Enzymes act as catalysts* for a single reaction, converting a specific set of reactants (called substrates*) into specific products. Without enzymes life as we know it would be impossible.

equilibrium constant. (K, Keq) equilibrium constant expression; law of mass action. Compare with reaction quotient*.
The product of the concentrations of the products*, divided by the product of the concentrations of the reactants*, for a chemical reaction at equilibrium*. For example, the equilibrium constant for A + B = C + D is equal to [C][D] / ([A][B]), where the square brackets indicate equilibrium concentrations. Each concentration is raised to a power equal to its stoichiometric coefficient* in the expression. The equilibrium constant for A + 2B = 3C is equal to [C]3/([A][B]2). For gas phase reactions, partial pressures* can be used in the equilibrium constant expression in place of concentrations.

equivalence point. Compare with end point*.
The equivalence point is the point in a titration* when enough titrant* has been added to react completely with the analyte*.

equivalent. Compare with normality*.
1. The amount of substance that gains or loses one mole of electrons in a redox reaction. 2. The amount of substances that releases or accepts one mole of hydrogen ions in a neutralization reaction. 3. The amount of electrolyte that carries one mole of positive or negative charge, for example, 1 mole of Ba2+(aq) is 2 equivalents of Ba2+(aq).

An ester is a compound formed from an acid and an alcohol*. In esters of carboxylic acids*, the -COOH group and the -OH group lose a water and become a -COO- linkage:

R-COOH + R'-OH = R-COO-R' + H2O

where R and R' represent organic groups.

ethanol. (CH3CH2OH) ethyl alcohol; grain alcohol.
A colorless, flammable liquid produced by fermentation of sugars. Ethanol is the alcohol* found in alcoholic beverages.

ethyl. (-Et, -CH2CH3) ethyl group.
A molecular fragment produced by removing a hydrogen atom from ethane (CH3-CH3). For example, ethyl chloride is CH3-CH2-Cl.

ethyl acetate (CH3COOCH2CH3)
A flammable liquid with a fruity odor, used in flavorings and as a solvent.

eutectic point. eutectic temperature; eutectic composition.
The composition and the melting point of a eutectic mixture*. For example, the eutectic point of a mixture of NaCl and water occurs at 23.3% NaCl (by mass) and -21.1°C. That means that the lowest possible temperature at which a liquid NaCl solution can exist is -21.1°C; below the eutectic point the solution will freeze into a mixture of ice and salt crystals.

eutectic mixture.
A mixture of two or more substances with melting point* lower than that for any other mixture of the same substances.

evaporation. vaporization.
Conversion of a liquid into a gas.

To convert a liquid into a gas.

excited state. Compare with ground state*.
An atom or molecule which has absorbed energy is said to be in an excited state. Excited states tend to have short lifetimes; they lose energy either through collisions or by emitting photons* to "relax" back down to their ground states*.

An excitotoxin is a toxic molecule that stimulates nerve cells so much that they are damaged or killed. Domoic acid* and glutamate* are examples of excitotoxins.

exothermic. exothermic reaction; exothermic process. Compare with endothermic*.
A process that releases heat. The enthalpy change* for an exothermic process is negative. Examples of exothermic processes are combustion reactions* and neutralization reactions*.

An experiment is direct observation under controlled conditions. Most experiments involve carefully changing one variable and observing the effect on another variable (for example, changing temperature of a water sample and recording the change volume that results).

experimental yield. actual yield. Compare with theoretical yield* and percent yield*.
The measured amount of product produced in a chemical reaction.

extensive property. extensive; extensive properties. Compare with intensive property*.
A property that changes when the amount of matter in a sample changes. Examples are mass, volume, length, and charge.

A technique for separating components in a mixture that have different solubilities*. For example, caffeine can be separated from coffee beans by washing the beans with supercritical fluid carbon dioxide; the caffeine dissolves in the carbon dioxide but flavor compounds do not. Vanillin can be extracted from vanilla beans by shaking the beans with an organic solvent, like ethanol.


General Chemistry Online! Glossary: E

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/glossary/e.shtml