Appendix 4 (cont.) - Glossary Definitions
This term is used to describe
(1) a process in which one substance is absorbed, or chemically soaks into, another material, eg. water is absorbed into a sponge, or a gas is absorbed by a liquid. The process may be purely physical, or may involve chemical bonding (see also adsorption);
(2) the process in which energy enters a substance and brings about change, eg. electromagnetic radiation in the visible region may bring about electronic transitions, or infra-red radiation may cause the vibration of bonds.
This term is used to describe
(1) a non-metal oxide that reacts with water to give an acidic solution, eg. sulfur trioxide is the acid anhydride of sulfuric acid;
(2) an organic acid derivative containing the anhydride functional group (-CO-O-CO-). An acid anhydride reacts with water to give the acid or acids from which it is derived, eg. ethanoic anhydride, CH3-CO-O-CO-CH3 gives ethanoic acid on reaction with water.
Acid ionisation constant, Ka
The equilibrium constant for the ionisation of a weak acid, also known as the acid dissociation constant.
An acid salt is a compound derived from a polyprotic acid where at least one of the acid hydrogen atoms in the acid molecule is replaced by a metal, while some 'acid' hydrogen still remains, eg. sodium hydrogensulfate is an acid salt derived from sulfuric acid.
Activated complex - see transition state.
This term is used to describe
(1) the capacity of a substance to undergo chemical change. It can be used to indicate that a substance undergoes numerous reactions, and it may also imply that the reactions are vigorous;
(2) the activity of a radioactive substance is the number of atoms of that substance that disintegrate per unit time;
(3) the effective concentration of a substance in a solution. Activity values are used in thermodynamic calculations to allow for the non-ideality of real systems.
Activity series - see reactivity series
An allotrope is a form of an element that exists in the same state as other forms of the element at a given temperature and pressure. Different allotropes of the same element have different physical properties, but usually have very similar chemical properties. Examples of elements that form allotropes are oxygen (with allotropes dioxygen and trioxygen in the gas state), and carbon (with allotropes diamond, graphite and fullerine in the solid state).
The word 'amount' in IB vernacular means the number of moles of a subsance. This term should be reserved for use when amount of substance is intended. In other cases the term quantity may be used, eg. a quantity of charge may be measured in coulombs, but the amount of electrons carrying the charge would be measured in moles.
Amount of substance
The term is a quantity that is proportional to the number of particles in a given sample of a substance. The particles may be atoms, ions, molecules, electrons, etc., but should be specified. The amount of substance may also refer to combined particles of different types, as indicated by a formula, eg. the amount of substance of magnesium chloride, MgCl2, would indicate a value proportional to both magnesium ions and chloride ions, with the amount of chloride ions being twice that of the magnesium ions. The amount of substance is measured in units of moles.
Ampholyte - see amphoteric (2)
This term is used
(1) to describe an oxide or hydroxide that is a water-insoluble metal oxide or hydroxide, which may exhibit properties both of an acid and a base. It may behave like an acid in that it reacts with a strong alkali to give a salt, but it may also behave like a base in that it reacts with an acid to give a (different) salt, eg. the amphoteric oxide, aluminium oxide, reacts with sodium hydroxide to give sodium aluminate, and with hydrochloric acid to give aluminium chloride;
(2) to refer to a water soluble substance, such as an amino acid, that can behave both as an acid or a base. Such a substance is an electrolyte and may be referred to as an ampholyte.
The electrode at which oxidation (ie. loss of electrons) takes place.
Atomic mass unit (symbol u)
A unit of mass equal to exactly one-twelfth of the mass of the carbon-12 atom, sometimes also called the dalton (Da).
This is an obsolete term for relative atomic mass; its use is not recommended.
This is the process whereby a substance (usually in its standard state) is converted into gaseous atoms at infinite seaparation (which just means that they are far apart enough to not influence one another). In terms of energy this proces is normally quoted per mole of substance.
e.g. the enthalpy of atomisation of iron is the energy required to transform 1 mole of iron metal into 1 mole of gaseous iron atoms.
Avogadro's constant (L)
This is defined as the number of particles in one mole of substance. The particles may be atoms, molecules, ions, electrons, etc. but should be clearly specified. Avogadro's constant may be defined in terms of a particular number of particular number of particles, eg. 6.02 x 1023. However, it should be noted that this value is continually being refined. The unit of Avogadro's constant is mol-1, and the term Avogadro's number is to be discouraged because it implies a unitless quantity. Avogadro's constant is also sometimes given the symbol, NA.
A unit of pressure equal to 100 kPa and approximately equal to 750 mm Hg. It has been proposed that one bar be used as standard pressure, but this is not common practice.
A basic salt is a salt that retains a portion of the base from which it is derived. The retained part may be oxygen (e.g. bismuth oxide chloride) or a hydroxyl group (eg. tin (II) hydroxide chloride. The formation of basic salts is an indication of weakness on the part of the base, and both the base and the basic salt are usually water-insoluble. Basic salts are often found in naturally occurring minerals, eg. malachite, basic copper(II) carbonate, Cu(OH)1.CuCO3.
A number of electrochemical cells connected in series. Often used in everyday speech to mean a single electrochemical cell.
This prefix is used to describe
(1) an organic compound which contains two identical rings, eg. biphenyl. Sometimes replaced by the prefix di-;
(2) an acid salt, eg. sodium bicarbonate for sodium hydrogencarbonate. The use of bi-in this sense is not recommended;
(3) incorrectly, a species formed by condensation of two other species, eg. bichromate for dichromate. This term is no longer used.
Bond dissociation energy - see bond dissociation enthalpy
Bond dissociation enthalpy
The enthalpy increase that accompanies the homolytic fission of one mole of bonds in a covalently bonded species, to give individual atoms and/or radicals, with both the original species and the resulting fragments being in their standard states, and at standard temperature and pressure. The bond dissociation enthalpy refers to a specific bond in a specific compound. Note that the bond dissociation enthalpy is always positive. Also called (often/still) bond dissociation energy. The symbols E and D HD are used for bond dissociation enthalpies.
Bond energy - see bond enthalpy
The enthalpy increase that accompanies the homolytic fission of one mole of bonds in a covalently bonded species, to give individual atoms and/or radicals, with both the original species and the resulting fragments being in their standard states, and at stand temperature and pressure. The bond enthalpy is the average value of the bond dissociation enthalpy (see above) for the same kinds of bonds, but in a number of different compounds. Also called bond energy term or bond energy. The symbols E, E, D HD and D HB are all used for bond enthalpy, which is sometimes confused with bond dissociation enthalpy.
A term that is used loosely to indicate bond enthalpies: the larger the bond enthalpy, the greater the strength of the bond. It is often used to compare different bonds qualitatively; thus, the intramolecular bonding in chlorine is stronger than the intermolecular bonding.
An aqueous solution the pH of which remains nearly constant despite the addition of small amounts of acid or alkali. Buffers that maintain a pH of less than 7 are often made from a weak acid and one of its salts (eg. ethanoic acid and sodium ethanoate): buffers that maintain a pH of greater than 7 are often made from a weak alkali plus one of its salts (eg. ammonia solution and ammonium chloride). Acid salts anions, which are amphiprotic, any also give buffer solutions, eg. hydrogencarbonate ions that act as a buffer in blood. The use of strong acids as buffers at very low pH values, or strong alkalis at very high pH values, is often overlooked. A strong acid gives high concentration of hydrogen ions in solution, and this does not change appreciably when a small amount of base is added. Likewise a strong alkali contains a high concentration of hydroxide ions, which does not change appreciably when a small amount of acid is added.
This is a term no longer used in the SI system to describe the quantity of heat required to raise the temperature of one gram of water by 1 oC (specifically from 14.5 to 15.5 ºC). One calorie = 4.148 J. The term finds everyday use in food, ie. the term food calorie or Calorie (1 Calorie = 1000 calories).
A species containing a trivalent carbon atom with one electron missing, it is thus associated with a positive charge eg. (CH3)3C+.
Carbonium ion - an alternative name for a carbocation
The electrode at which reduction (ie. gain of electrons) takes place.
An apparatus that consists of a container with an electrolyte solution and electrodes immersed in it. The electrodes are connected into an external electrical circuit. There are two main types of cells : (1) galvanic cells, which produce electricity from chemical reactions, and (2) electrolytic cells, in which electrical energy is used to produce chemical reactions.
A temperature scale in which the fixed points are the melting point of ice (00C) and the boiling point of water (100 0C) at standard pressure. One Celsius degree is equal to 1/100 the interval between the two fixed points, and is equal to a one degree interval on the absolute (Kelvin) temperature scale. The Celsius scale was formerly called Centigrade scale.
Centigrade - a commonly used alternative to Celsius
This term is used in Valence Shell Electron Pair Repulsion Theory. It commonly
describes a single electron pair (bonding or non-bonding), or the two or three
electron pairs found in double or triple bonds respectively, which are treated
as a single centre for the purposes of the theory. It may also be used for a
single electron in a half-filled orbital, or for bonds in delocalised systems.
A type of redox reaction is which the same element in two different oxidation states react together to form one other, different, oxidation state, eg. iodide ions and iodate(V) ions react together to form iodine in the elemental state. (opposite disproportionation)
Concentration (symbol C, or indicated by square brackets around the substance under consideration, eg. [NaOH(aq)])
(1) A term used to describe the proportion of solute in a solution. It usually refers to the chemical amount of solute in a given volume of solution, however it may also refer to the mass of solute in a give volume of solution. Sometimes concentration is expressed as a percentage - meaning the mass of solute in 100 g of aqueous solution - or 100 cm3 if the solution is dilute. Occasionally the use of molal concentration is given : this refers to the chemical amount of solution in one kilogram of solvent. The term normality, referring to the concentration in terms of equivalent of solute per litre of solution, is obsolete.
The most common measure of concentration is mol dm-3.
(2) The process of increasing the proportion of solute in a solution.
This term is used to describe
(1) the process of changing from a gas to the liquid or solid state;
(2) a type of reaction in which two molecules combine to form a larger molecule, eliminating a small molecule as they do so, eg. carbonyl compounds react with hydrazine to form hydrazones, eliminating water as they do so.
A formula often used to depict organic compounds, in which the structure is indicated, without fully displaying all the bonds, eg. the condensed formula for butan-2-ol is CH3CH2CH(OH)CH3.
This term is used to describe
(1) the property of allowing electricity, heat, or sound to pass through a material. Also called conduction;
(2) a measure of the ease with which a substance (usually an electrolyte in solution) allows electricity to pass through it. Measured in units of W-1 m-1, it is the reciprocal of the resistivity. Formerly called the specific conductance;
(3) a measure of the ability of a substance to allow heat to flow through from a high temperature to a low temperature. Measured in units of J s-1m-1K-1. Also called the thermal conductivity.
Conical flask - also called an Erlenmeyer flask. The term conical flask is recommended.
The unit process in the manufacture of sulfuric acid in which oxygen and sulfur
dioxide are reacted together over a vanadium pentoxide catalyst to form sulfur
trioxide. Often incorrectly used to refer to the entire process of manufacturing
This term is determine as follows :
(1) in a metal the number of atoms that touch (or come closest to) any one particular atom. Thus close-packed structures have a coordination number of 12, but in body-centred cubic structures the coordination number is 8;
(2) in ionic lattices, the coordination number of a particular ion is the number of ions of the opposite charge that touch (or come closest to) it. Thus the coordination number of calcium ions is 8 but the coordination number of fluoride ions is 4 in the fluorite (CaF2) structure;
(3) in complex ions the coordination number is determined by the number of coordinate bonds around the central ion. Thus the hexacyanoferrate (III) ion has a coordination number of 6. Note, however, that the coordination number is not necessary the same as the number of ligands around the central atom, as it is possible for some ligands to form more than one bond.
As alternative name for a dative covalent bond, often used in the context of
Dative covalent bond
A chemical bond formed when two atoms share a pair of electrons, both of which may be considered to originate from the same atom.
Once formed a dative covalent bond cannot be distinguished from other covalent
bonds in which the electrons in the bond are provided by different atoms. Also
called a dative bond or a coordinate bond.
d- Block element
A metallic element with two s-electrons in its outer shell, and with between
one and ten d-electrons in its penultimate shell. Sometime incorrectly used
as a synonym for a transition metal: although all transition metals are d-block
elements, the converse is not necessarily true.
This term is used to describe
(1) the process of removing water from a substance, for example, the removal of water of crystallisation from a hydrated salt;
(2) the process of removing the elements of water from a compound, ie. removing
hydrogen atoms and oxygen atoms in the ratio 2 : 1, from a compound in which
they are chemically bonded to other atoms, eg. ethanol is dehydrated to give
A phenomenon in which valency electrons provided by individual atoms are no longer held in the near vicinity of that atom, but are mobile and shared by a number of atoms. Delocalisation occurs
(1) in metals, where electrons can move throughout the entire crystal structure;
(2) in organic compounds (and graphite) that have alternate double and single carbon-carbon bonds, orientated in such a way that p-electron can overlap, providing a pathway for electron movement;
(3) in certain inorganic species, such as nitrate and carbonate ions, where
p-orbital overlap can occur. Delocalisation stabilises a structure, giving it
a lower enthalpy than it would have if double and single bonds are arranged
in such a way that orbital overlap cannot occur.
Describes a base one mole of which is capable of neutralizing two moles of protons, eg. calcium hydroxide may be described as diacidic. The term diacid is sometimes also used in the same context, but this should be avoided as it is also occasionally used to describe a diprotic acid, or (more often) a molecule containing two acid groups such as ethanedioic acid.
Diaphragm cell - see membrane cell
Describes an acid one mole of which is capable of producing two moles of protons
in a neutralisation reaction. Also called diprotic.
These forces are a type of intermolecular bonding caused by attractions between
permanent dipoles in polar molecules. Also known as dipole forces or dipole-dipole
interactions or permanent dipole-dipole interactions.
Diprotic - see dibasic
This term is used to describe
(1) the discharge seen at an electrode during electrolysis, eg. chlorine is discharged at the anode during electrolysis of molten sodium chloride;
(2) the electrical discharge in a discharge tube;
(3) in common language, the removal of charge, ie. send away.
Forces between temporary dipoles induced in atoms or molecules also known as
London forces or temporary dipole-dipole interactions. See van der Waal's forces.
A type of redox reaction in which an element in one particular oxidation state
is simultaneously oxidised and reduced to give two products in different oxidation
states, eg. chlorine molecules react with water to form chloride ions and chlorate
This term is used to describe
(1) separation of a compound into ions when it is dissolved in a polar solvent. For example molecules of sulfuric acid are dissociated into hydrogen ions and sulfate ions in water. Ionic compounds, such as salts, may also dissociate if the ion-dipole interactions are sufficiently strong. Dissociation is often reversible and equilibrium conditions prevail;
(2) a reversible process in which a compound splits up into smaller species.
Thus ammonium chloride is dissociated into ammonia and hydrogen chloride when
heated, but the ammonium chloride is formed again on cooling.
Dissociation constant - see acid ionisation
Double decomposition - see double
An exchange reaction between two ionic compounds in solution, in which the cations and anions change partners, thus :
AB + CD AD + CB
The products of the reaction may be insoluble or volatile compounds, so that
they are removed from solution, allowing the equilibrium to swing in favour
of the products. Not that the products are not necessarily ionic, thus water
is formed by the reaction of hydrogen and hydroxide ions in the neutralisation
reaction. Double displacement is also called double decomposition and metathesis.
Also see ionic precipitation.
This term is used to describe :
(1) a substance that loses water of crystallisation, becoming powdery in the process. Thus hydrated sodium carbonate is efflorescent, gradually losing water of crystallisation to the atmosphere to become anhydrous sodium carbonate;
(2) a substance that becomes encrusted with powder or crystals as a result of chemical change or the evaporation of a solution. For example, copper sulfate solution may gradually evaporate to give an encrustation that may climb up and over the sides of the container.
This is often defined as a cell in which a spontaneous chemical reaction is
used to produce electrical energy, and may also be called a galvanic cell or
a voltaic cell. The term electrochemical cell is sometimes used for electrolysis
cells as well, so it is recommended that the term galvanic cell is used for
a cell in which a chemical reaction produces electrical energy.
A list of elements written in order of their standard reduction potentials,
having those element with the most negative reduction potentials written first.
As this list refers to reductions in aqueous solutions, it is slightly different
from the reactivity series which is based on the reduction of metal oxides.
The electrochemical series may also be extended to include non-metals and other
reduction systems. The electrochemical series is sometimes written with the
most positive reduction potentials first, or (in very old-fashioned books) as
oxidation potentials. However, the use of such lists is not recommended.
(1) The first electron affinity of an element is the enthalpy change that occurs
when one electron is gained by each atom in a mole of gaseous atoms of the element
to give one mole of ions, each with a single negative charge, at standard temperature
(2) This term is also used to describe the enthalpy increases that occur when
subsequent electrons are gained. For example the second electron affinity would
refer to the gain of one electron by each ion, each with a single negative charge,
in a mole of such ions in the gas state, to give one mole of ions with a double
negative charge in the gas state. Note that according to these definitions most
common first electron affinities are negative, but second electron affinities
are often positive.
Electron dot formula - see Lewis
The ability of an atom to attract electrons towards itself in a diatomic bond. There are two different Electronegativity scales:
(1) Mulliken's scale in which the Electronegativity of an atom is the arithmetic mean between the ionisation energy and the electron affinity;
(2) the more commonly used Pauling scale, in which all values are measured relative to fluorine, which has the maximum Electronegativity of 4.0.
Using a Pauling scale, it is possible to estimate the degree of ionic character of a covalent bond, by calculating the difference in Electronegativity of the two atoms in the bond: a difference of $ 1.8 indicates that the bond is essentially ionic.
This term is used to describe
(1) on a macro scale, the simplest form of substance: it cannot be further broke down by chemical means;
(2) on an atomic scale, a substance the atoms of which all have the same nuclear charge. All atoms of the same element display the same chemical characteristics.
Empirical formula - also called simplest
End point - see equivalence point
Enthalpy change of atomisation
Note that different definitions apply according to whether the substance being atomised is an element or a compound. However, the enthalpy changes involved in either case are positive as they involve bond breaking reactions.
(1) The standard enthalpy change of atomisation of an element, symbol D Hqatm, is the enthalpy increase that takes place when one mole of gaseous atoms is made from the element in the defined physical state under standard conditions.
(2) The standard enthalpy change of atomisation of a compound, symbol D
Hqatm, is the enthalpy increase that takes
place when one mole of the compound in the defined physical state, is broken
down into gas atoms under standard conditions.
A reversible reaction that has reached a dynamic situation where the rate of
the forward reaction is equal to the rate of the reverse reaction under the
ambient conditions. As a consequence the concentrations of the reactants and
the products are constant at equilibrium. An equilibrium is represented by a
double arrow between the reactants and the products: Reactants
Equilibrium expression - see equilibrium
The equilibrium law for a reaction is the expression in which the equilibrium
constant is equal to a fraction in which the numerator is the product of the
concentrations of the substances on the right of the equation, each raised to
a power equal to its coefficient in the chemical equation. The denominator is
the product of the concentrations of the substances on the left of the equation,
each raised to a power equal to its coefficient in the chemical equation. The
equilibrium law is also called the equilibrium expression.
That point in a titration where stoichiometrically equivalent amounts of reactants have been added.
Erienmeyer flask - see conical flask
(1) The common name for ethoxyethane.
(2) Any organic compound containing a C-O-C linkage.
The term is used to describe
(1) the breaking of a chemical bond, eg. Cl2(g) ® 2Cl. (g);
(2) the breaking up of a nucleus.
The term is used to describe
(1) melting, for example, in enthalpy change of fusion
(2) the combining of two nuclei, eg. 21D + 21D
An electrochemical cell that generates electricity by means of spontaneous
redox reaction, also known as a Voltaic cell.
Giant molecular solids - see network
Heats of reaction - see enthalpy changes
This is also called the law of constant heat summation/the first law of thermodynamics
The reaction of a substance with water to form two or more products,
eg. CH3COCl + H2O ® CH3CO2H
This ion, H3O+, is also oxonium ion and hydroxonium ion.
Hydroxonium ion - see hydronium ion
Describes a group having a hydrogen and oxygen atom covalently bonded together.
The hydroxyl group is often covalently bonded via the oxygen atom into a molecule
or ion, as in an alkanol or a hydrogensulfate ion. However, hydroxyl is also
often used as a synonym for hydroxide.
A form of double decomposition where ions combine to produce a precipitate.
This term is used to describe
(1) the formation of ions, eg. Ar ® Ar+ + e;
(2) the dissolving of a solid in water to form ions eg. CH3CO2H
Ionisation constant - see acid ionisation
A temperature scale with intervals measured in Kelvin. Also known as the absolute
A study of the rates of chemical reactions. This is separate from the kinetic
The kinetic theory
The theory in which the behaviour of matter is explained in terms of the movement
of small particles. Not to be confused with the study of kinetics.
The enthalpy change that occurs when one mole of a solid ionic crystal is separated
into each of its component ions in the gaseous state, under standard temperature
and pressure conditions. Lattice enthalpy defined in this way will always have
values that are positive. Many texts define it the opposite way, ie. for the
change from separate ions to ionic crystal. Its value is then negative.
Law of conservation of energy
- see Hess' law
A diagram showing the covalent bonds in a molecule or ion by using the symbol(s) of the element(s) involved and some representation of the valency electrons. This representation can be by dots, crosses, a combination of dots and crosses or by using a line to represent a pair of electrons.
eg. H : H, Hx0 H, or H - H
Also known as a Lewis formula or electron dot formula.
Litre (symbol L, dm3)
A non-SI unit of volume equivalent to one dm3, which is the preferred
unit of volume measurement in chemistry, along with the centimetre cubed cm3
for the simple reason that the scale is convenient as regards laboratory measurements.
1 litre = 1000cm3.
London forces - see dispersion forces
Macromolecular solids - see network
covalent solids for recommended term.
Mass of one mole - see molar mass
The most modern process used for the electrolysis of sodium chloride solution
in the chlor-alkali industry. It replaces the diaphragm cell.
This means 'divided by the amount of substance in moles'. Thus the molar volume
is the volume per mole of substance. There are some exceptions to this definition.
For example, molar conductivity where molar means 'divided by the concentration
in mol dm-3'. It is recommended that the term molar is not used to
refer to solutions, where it is often incorrectly used to indicate the chemical
amount in a given volume of solution.
Molar mass, M
This is the mass per mole of substance, and has the units of g mol-1.
The molar mass should always be accompanied by a statement indicating the nature
of the particles in the substance, or the formula of the substance, eg. for
chlorine, it should be specified whether the molar mass refers to chlorine atoms,
Cl, or chlorine molecules Cl2, as the molar masses are different
for the two species.
Molar volume, Vn
This is the volume per mole of substance, and usually has the units of dm3.mol-1.
The conditions under which the molar volume is measured should always be stated,
and this is particularly important for gases, where small changes in temperature
and pressure can make a significant difference to the volume. The molar volume
of an ideal gas is 22.7 dm3 mol-1 at 273 K and 1.00 atm,
which converts to approximately 24 dm3 mol-1 at 298 K
and 1.00 atm.
This refers to the concentration of a solution in mol dm-3. The term is now considered to be obsolete. The abbreviation M for the units mol dm-3 is still used, though it is not recommended.
Network covalent solids
Atoms bonded together covalently throughout the solid, eg. diamond, silicon
oxide. Also called giant molecules, macromolecular solids.
At one time this was taken as 18º C. However it has also been used to
mean standard temperature (0ºC) or standard ambient temperature
(25º C). Nowadays it is sometimes used loosely to refer to room temperature.
Because of this, it is recommended that the term be avoided.
Oxonium ion - see hydronium ion
The table in which the elements are arranged in groups and periods. Also sometimes
called the Periodic Classification.
Periodic Classification - see Periodic Table
An abbreviation commonly used to indicate an alkyl or aryl group in formulae
of organic compounds. Also used for the gas constant.
This term is sometimes called the velocity constant.
Rate equation - see rate expression
An equation relating the rate of the reaction to the rate (or velocity) constant,
and the concentrations of the reactants, also known as the rate law or rate
Rate law - see rate expression
A series of chemical elements (usually metals and hydrogen) arranged in order
of their tendency to lose electrons, ie. their strength as reducing agents,
also known as the activity series (or in the past electromotive series - this
term is no longer used). See also the closely related electrochemical series.
Alternative methods of representing the bonding in molecules or ions for a
given arrangement of atoms. If the various possible arrangements differ only
a littler in energy, than the actual bonding is a mixture of the various possible
representations, and will have a lower energy than any of the individual forms.
Also called canonical forms.
Room temperature and pressure - see standard ambient temperature and pressure
This is a term used to describe
(1) organic compounds that contain no multiple bonds (eg. C=C, C=O, C
(2) solutions of a solute and solvent in which, at a given temperature, no more solute will dissolve.
Simplest formula - see empirical formula
Standard ambient temperature and pressure
Standard ambient temperature is 298 K (25.0 ºC) and standard
pressure is one atmosphere (100.0 Pa). These are the standards most commonly
used for chemical reactions. Compare with standard temperature and pressure.
This is also called room temperature and pressure, and is the standard commonly
used in thermodynamics.
Standard electrode potentials
The electromotive force of a half-cell connected to a standard hydrogen electrode,
measured under standard conditions. The sign of the electrode potential being
measured is positive if reduction occurs at it, but negative if the reduction
occurs at the hydrogen electrode. Also called the standard reduction potential
or the standard redox potential.
This is usually taken by IUPAC convention to be the standard atmosphere or 100000 Pa (100.0 kPa).
Standard redox potential - see
standard electrode potential
Standard reduction potential
- see standard electrode potential
Standard temperature and pressure
Standard temperature is often taken as 273.15 K (0.0 ºC) and standard pressure as one atmosphere (100000 Pa). This is the standard commonly used in gas calculations. Compare with standard ambient temperature and pressure.
Standard state, standard conditions
This is used in thermodynamics as 298K (25ºC) and 1 atmosphere pressure.
It is represented by the superscript plimsole line. The standard state of a
substance is the state in which it exists under standard conditions.
An element the atoms of which have an incomplete set of d-electrons in their
penultimate shell in one or more of their oxidation states. This includes all
elements in the d-block except those with complete d-orbitals, such as zinc.
Scandium is sometimes excluded from the transition metals because its ions have
empty d-orbitals, and thus do not exhibit transition characteristics.
An unstable arrangement of atoms that exists for only a moment in the course
of a chemical reaction. It is often indicated by an asterix, TS*.
Universal gas law
Also known as the ideal gas equation. PV = nRT
This term is used to describe
(1) substances that contain multiple bonds, eg. C=C, C=O, C=N. The term is used most commonly in association with organic compounds;
(2) solutions of a solute and a solvent, at a given temperature, into which
more solute can be dissolved.
These are weak attractive forces between molecules (or intermolecular bonds).
The term is sometimes used to refer to all types of intermolecular forces, ie.
dispersion forces, dipole-dipole interactions and hydrogen bonds, or just the
first of these, or just the first two! The term dispersion forces is mostly
restricted to the first, ie. its recommended use is for dispersion forces.
Velocity constant - see rate constant
Abbreviation for valence shell electron pair repulsion theory.