Chemistry Vocabulary (E-H)

It has occurred to me over the years that students sometimes have absolutely no idea what we teachers are talking about when it comes to chemistry terms.  To make life easier for you, I present to you the following list of vocabulary terms to make your life sunny and sweet.

Incidentally, some terms are a little more complicated than a quick synopsis can give. When this is the case, I’ll put a little asterisk at the end of the definition and you can scroll down if you’re interested in learning more.


effusion:  When gas moves through a hole into a vacuum (as would be the case if a spaceship sprung a leak).  The rate of effusion is related to the mass of the gas particles via Graham’s law of effusion.  Effusion is related to, but not the same thing, as diffusion.

electrolysis:  When electricity is used to break apart a chemical compound.  The most familiar example of this is the electrolysis of water, in which electricity is used to break water molecules into hydrogen and oxygen gases.

electrolyte:  An ionic compound that dissolves in water to conduct electricity, or the solution formed when this compound dissolves.  Strong electrolytes almost completely dissolve, while weak ones dissolve only to a very small extent.*

electron affinity:  The energy change that accompanies the addition of an electron to an atom in the gas phase.  In other words, high electron affinity means that an element really really loves to gain electrons (such as the halogens) and a low electron affinity means that it really really doesn’t (such as the alkali metals).

electron configuration:  A list of where the electrons in an atom are located.

electronegativity:  A unitless measure of how much an atom wants to steal electrons from atoms it has bonded to.  High electronegativity = loves to gain electrons (as in halogens) and low electronegativity = doesn’t love to gain electrons (alkali metals).*

empirical formula:  A ratio of the number of atoms of each element in a compound, obtained by reducing all of the subscripts by some constant number.  For example, hydrogen peroxide, H2O2, has the empirical formula of HO.*

emulsion:  When small drops of liquid are suspended in another.  An example is salad dressing in which small drops of oil are suspended in vinegar.*

enantiomers:  Molecules that are nonsuperimposable mirror images of each other.  A good picture of enantiomers can be seen here: link.*

endothermic:  An endothermic process absorbs energy.  In many cases, an endothermic reaction will cause the container it’s performed in to get cold because the energy in the surroundings is absorbed, allowing the reaction to occur.*

endpoint:  The point at which you stop a titration, usually because an indicator has told you that the equivalence point was reached.  Note:  The endpoint of a titration is not the same as the equivalence point, as indicators don’t change color at a pH of exactly 7.*

energy:  Energy is something in a system that allows the system to either to do work or give off heat.  When materials are heated, they have more energy than cold ones.

enthalpy (H):  The enthalpy of a system is equal to its internal energy plus the product of its pressure and volume:  H = U + pV.  In a less-precise (but for us, more handy) definition, the enthalpy of a system is a measure of how much heat it can give off.  Because it’s impossible to measure absolute enthalpy values, we instead find the change in enthalpy for a process (ΔH), which is much easier to figure out.  Depending on the process, we refer to the enthalpy change as the “heat of [name of change]”, such as “heat of vaporization” to indicate the amount of energy change associated with boiling one mole of a compound.*

entropy (S):  The measure of the randomness/disorder in a system.  This is unbelievably difficult to explain, but let’s just say that the randomness of the universe has to be positive for any process that occurs (2nd law of thermodynamics).  Entropy, like enthalpy, is hard to determine exactly, the change in entropy (ΔS) for some change is used instead.*

enzyme:  A biological molecule that catalyzes reactions in living things.*

equilibrium:  When the forward rate of a reversible reaction is the same as the reverse rate.  When a system reaches equilibrium, it appears as if nothing is happening because the concentrations of each chemical remain constant, but the reaction is still occurring in either direction.*

equivalence point:  The point in a titration at which the solution has a pH of exactly 7 (i.e. the amount of acid and base are exactly the same).  This is different than the endpoint (see above), and impossible to achieve in practice.

ester:  An organic molecule with R-CO-OR’ functionality.  Esters are used in many plastics and the ones you’ll run into frequently smell nice.

excess reagent:  If you do a reaction in which one reagent runs out more quickly than the other, the one that doesn’t run out is the excess reagent.  Frequently abbreviated as “XS”.

excited state:  An orbital to which electrons in lower energy ground state orbitals can jump into when energy is added.*

exothermic:  When a process gives off energy.  These processes generally cause the surroundings to get hot, as in a combustion reaction.

family:  A column in the periodic table.  Also called a “group.”

first law of thermodynamics:  The energy of the universe is constant / energy can neither be created nor destroyed.  The same thing as the law of conservation of energy.

fission:  A nuclear reaction in which a large atom breaks into smaller ones.  This process occurs in nuclear power plants and atomic bombs.*

free energy:  The capacity of a system to do work.  This is most often described by a value called the Gibbs free energy (G), and the change in free energy for a process is described at ΔG =ΔH – TΔS.

free radical:  An atom or molecule with an unpaired electron.  Because electrons prefer to be paired up, these things are unbelievably reactive.*

functional group:  A generic term for a group of atoms that cause a molecule to behave i a certain way.  Common functional groups include amino groups, hydroxyl groups, and alkyl groups.

gamma ray:  Very high energy electromagnetic radiation given off during radioactive processes as well as in stellar processes.

geometrical isomer:  Isomerism where atoms or groups of atoms take up different positions around a double bond or ring.  Another term for this is cis-trans isomerism.

ground state:  The lowest energy state in which an electron can be located.  When energy is added to ground state electrons, they become excited into a higher energy excited state.

group:  A column in the periodic table.  Also called a “family.”  Alternate definition:  Another term for “functional group.”

half-life (t1⁄2):  The amount of time required for half of something to undergo a process.  This is commonly used in radioactive decay (in which it indicates the amount of time it takes for half of the atoms to undergo decay) and in chemical kinetics (in which it indicates the amount of time it takes for half of the reagent molecules to be converted into products.)

half-reaction:  Either the oxidation or the reduction part of a redox reaction.  When these two half-reactions are combined, you get the overall equation for the entire process.

halogen:  The elements in group 17 of the periodic table.  Due to the fact that they need only one additional electron to gain a noble gas electron configuration, they are extremely reactive, behaving as oxidizing agents.

heat of reaction (ΔHr):  The heat of reaction is the enthalpy change that occurs when some chemical process takes place.*

heat:  The movement of energy from one place to another via molecular motion.  In a more casual sense, this is when energy goes from someplace hot to someplace cold.*

Hess’s law:  The enthalpy change for some process is the same whether it takes place in one big step or many smaller ones.*

heterogeneous mixture:  A mixture where the substances aren’t equally distributed (i.e. some parts have different compositions than others).*

homeogenous mixture:  Also called a “solution”, it’s a mixture in which the composition is exactly the same throughout the mix.  Salt water, for example, is a homogeneous mixture.

Hund’s rule:  Electrons are most stable if they remain unpaired in the ground state.*

hybrid orbital:  An orbital caused by the mixing of s, p, d, and f-orbitals in which lone pairs of electrons and electrons in single chemical bonds (sigma bonds) occur.

hydrate:  A compound to which water molecules have loosely bound themselves.  Hydrates don’t feel wet because the water molecules are actually bound to the compound.  One example of a hydrate is Epsom salts (magnesium sulfate heptahydrate).

hydration:  When a compound without water molecules present on it (anhydrate) has water molecules added to it to form a hydrate.

hydrocarbon:  A molecule that contains only hydrogen and carbon.

hydrogen bond:  A very strong intermolecular force that occurs when a hydrogen atom is bonded to either N, F, or O.  How this works:  If you have two molcules with O-H functionality, oxygen’s lone pair will stick tightly to the hydrogen atom, causing an unusually strong intermolecular force.*

hydrogenation:  When hydrogen is added to a carbon-carbon multiple bond, usually with the assistance of a catalyst such as platinum, nickel, palladium, or iron.

hydronium ion:  The H+ ion.  Because we see it in water a lot, it’s referred to as H3O+ in aqueous solution.  Hydronium ions are frequently referred to as “protons” in organic chemistry, as H+ consists of a hydrogen atom without an electron – in other words, a bare proton.  As a result, the gain of H+ is generally referred to as “protonation” and the loss of H+ is called “deprotonation.”

hydroxide ion:  The OH- ion.

The amazing footnotes:

electrolyte:  Though many sports drinks make a big deal about their electrolyte content, electrolytes are not inherently good for you.  At least, wouldn’t want to drink a solution of sodium cyanide.

electronegativity:  The usual values you’ll see for electronegativity are Pauling electronegativity values, which don’t have units associated with them.  However, there are other electronegativity units in use, and they generally follow the same trends that you see from Pauling electronegativity values.

empirical formula:  Though it’s common to have problems involving empirical formulas in chemistry classes, empirical formulas are nearly useless in the real world.  Though it’s possible to determine molecular formulas from empirical formulas through a series of calculations, modern equipment figures that out for you so you don’t need to bother.  That is, when you even need to find the formula at all – generally, people know what they’ve got in a bottle and are just trying to verify it, rather than trying to figure it out from nothing.

emulsion:  Emulsions may or may not be stable.  In the example of oil and vinegar, the oil drops will very quickly separate back into a separate layer, unless a surfactant is used to stabilize it.

enantiomers:  Though you’d think that the very close relationship between them would cause enantiomers to behave identically, that’s not the case.  For example, the anti-gastric reflux medication Nexium is twice as effective as the former drug Prilosec, because it contains only one of the two enantiomers of the compound – the other enantiomer present in Prilosec doesn’t do anything at all.  As a result, we would refer to Prilosec as an enantiopure drug.

endothermic:  When you break the vial inside of an instant cold pack, the contents of the vial (ammonium nitrate) dissolve in water – a highly endothermic process.

endpoint:  Because not all indicators change colors at the same time, you can put several indicators on the same strip of paper and use the differing values of color change to indicate what the specific pH of the solution is.  There is, of course, some imprecision in this method, but it works pretty well for many purposes.

enthalpy:  If you’re deep into thermodynamics, please don’t just use the heat definition above.  Because this site is intended mainly for first-year chemistry students (particularly high school students), the definition I gave is probably good enough.  So please don’t email me about the derivations for enthalpy, or the specific examples of enthalpy, because I’ll probably ignore them.

entropy:  Actually, the entropy must be greater or equal to zero according to the 2nd law, but given that the real world has friction and other forces that keep things from happening, it’s pretty safe to say that my definition, while not mathematically-correct, is functionally correct.

enzyme:  If you were to look at the plain thermodynamics of biological processes, you’d have to conclude that life is impossible.  However, the addition of enzymes allows these reactions, which would normally occur very slowly, to occur quickly enough for things to live.

equilibrium:  I’ve seen people increasingly use the term “dynamic equilibrium” in place of “equilibrium.”  This seems pretty dumb, because an equilibrium is, by definition, dynamic.  Incidentally, the equilibrium process of sublimation and deposition of ice in your freezer is what causes old ice cream to form ice crystals – essentially they’re leaving one location and redepositing on another.

excited state:  The terminology used in this definition suggests the old Bohr model of the atom in which orbitals are located on some sort of ladder with higher and lower locations. However, keep in mind that it’s not the location of the orbitals that are higher and lower, but rather the energies of the orbitals.

fission:  This process gives off so much energy that fusion reactions can be initiated, a process that takes place in thermonuclear weapons.  Interestingly, there have been cases of fission reactors being formed spontaneously in the earth, which is pretty awesome.

free radical:  The original meaning of “radical” was the same as “functional group” in organic chemistry.  That’s why generic functionality in organic formulas are shown as the letter “R”, as in R-OH to indicate an alcohol.

heat of reaction:  When a reaction is performed at standard conditions (1 atm, 0°C), this value is called the “standard heat of reaction” and is denoted by ΔHr.

heat:  Heat, it should be remembered, is not energy, but rather energy that’s moving.  This energy may start as molecular motion or chemical bonds, and may end up in the same way, but it’s only while it’s moving in the manner above that we refer to it as heat.

Hess’s law:  Or, put another way, if you’re going to climb a mountain, the altitude difference from your current point to the top will be the same, regardless of how many ups and downs you have as you get there.  More fancy descriptions of this can be found here: link.

heterogeneous mixture:  You may not be able to tell if a mixture is heterogeneous by looking at it, because the differences in composition may be on a microscopic scale.  Examples of this include milk, paint, and smoke.

Hund’s rule:  Though the usual explanation for this is that atoms are more stable because the electrons (which have the same charge) are separated from each other more if in different orbitals, this is probably not the case.

hydrogen bond:  Hydrogen bonding causes molecules to have much higher melting and boiling points than those without hydrogen bonding.  For example, methane and water have roughly the same molar mass, but methane has a boiling point about 260 degrees lower than water (-164 degrees Celsius vs. 100 degrees Celsius).  Though the word “bond” is used in the term “hydrogen bond”, hydrogen bonding is not a bonding interaction but an intermolecular force.

This webpage is licensed under the Creative Commons Attribution-NonCommerical-ShareAlike 4.0 International license (CC BY-NC 4.0).  For more information about this license and how it affects how you can use the contents of this site, click here.

The links in the body of the text are not covered under this license and the copyright holders should be contacted directly for their terms of use.




This entry was posted in Dictionary. Bookmark the permalink.