A high energy, unstable molecular form, intermediate between the molecules of the reactants
and products of a chemical reaction. Also known as the transition state.
The atomic mass of an element (more correctly the
relative atomic mass, Ar) is the mass of one
atom of that element, divided by 1/12th of the mass of
one atom of the carbon isotope 12C. In older
textbooks, the relative atomic mass was called the atomic
weight.
Equal volumes of different gases, measured under the same
conditions of temperature and pressure, contain the same
number of molecules (also known as Avogadro's law).
The temperature at which the vapour pressure of a liquid is equal to the atmospheric pressure. A liquid is said to "boil" when its temperature is raised to its boiling point.
An average value for the energy required to break a covalent bond in such a way that each participating atom retains an unpaired electron. One can talk for example in general terms of the C-C bond energy as having an average value of 347 kJ.mol-1.
A calibrated tube fitted with a stopcock, from which
accurate volumes of a solution may be delivered. Burettes
are essential instruments used in titrations.
A temperature scale used in most countries, based on 100 degree difference between the melting point of ice, 0
ºC, and the boiling point of water, 100 ºC, at standard pressure.
An acronym for "chlorinated fluorocarbons",
which are substances consisting of chlorine, fluorine
and carbon. CFC's have been implicated in the destruction
of the earth's ozone layer, and their use is being
restricted.
Two substances are said to form a conjugate acid-base
pair if their molecular formulae differ by one proton, H+.
For example, H2SO4 and HSO4-
form a conjugate acid-base pair.
An industrial process for the production of sulphuric acid, based on the catalytic oxidation
of sulphur dioxide by atmospheric oxygen to sulphur trioxide.
The total pressure of a mixture of gases at a given temperature is equal to the sum of the
partial pressures
of the constituent gases. While this law applies strictly only for ideal gases, it is
a close approximation for real gases.
A phenomenon exhibited by certain substances whereby they become damp and ultimately
liquefy when exposed to water vapour in the atmosphere. See also efflorescence.
The energy required to break a specific covalent bond in such a way that each constituent atom
retains one unpaired electron. Not to be confused with bond energy.
The existence of a positive charge and a corresponding negative charge separated by a fixed distance constitute an electric
dipole. Many molecules are polar in that they form a
dipole.
A device which can generate an electric current by
exploiting the energy of a redox reaction between two
electrodes (usually metals) having different electrode
potentials.
A solution capable of conducting electricity, usually a
solution of a salt in water. Alternately, a substance
capable of conducting an electric current, such as a
molten salt.
Radiation whereby energy is transmitted in a wave form through space. Examples of electromagnetic radiation
would include X-rays, ultraviolet rays, visible light,infrared radiation, and radio waves.
Abbreviated e.m.f or emf. In a circuit in which a current is present, the total rate at
which energy is drawn from the source of current and dissipated in the circuit per
unit current is defined as the electromotive force in the circuit.
(1) A continuous emission spectrum is obtained from substances at a high temperature, which
emit white light, i.e. electromagnetic radiation with continuous frequencies of visible
light. (2) Line emission spectra are obtained when a vapor is excited in some way,
resulting in the emission of a few selected frequencies, which, in the visible region,
show up as lines of a single colour.
A formula for a substance which expresses the simplest relationship between its constituent elements. For
example, the empirical formula for ethanoic acid is CH2O, although its
molecular formula is C2H4O2.
The stage in a titration where enough solution has been delivered by the burette to
complete the reaction. The end points of acid-base titrations are normally
established by changes in the colour of indicators or pH measurements.
A reaction which takes place with an uptake of energy from the environment. In an endothermic reaction, the potential chemical energy of the products is greater than that of the reactants. See also exothermic reaction
An atom is said to be in an excited state if an electron has absorbed energy
sufficient to promote that electron to an energy level which is higher than that
in which it finds itelf in the ground state.
For example, the single electron of hydrogen in the ground state is in the level n = 1. If the electron is in the
level n = 2 or higher, the atom is in an excited state.
A reaction which takes place with a release of energy to the environment. In an exothermic reaction, the potential
chemical energy of the products is less than that of the reactants.
See also endothermic reaction.
A temperature scale still used in some countries, based on 180 degree difference between the melting point of ice, 32 ºF, and the boiling point of water, 212 ºF.
A group of atoms in an organic molecule, having certain
chemical properties, and which impart these properties to
the molecule. For example, the carboxyl group, -COOH,
imparts acidic properties to organic molecules.
The product of the pressure, p, and volume, v, of a sample of gas is directly proportional to its Kelvin
temperature, T: pv = nRT, where n is the number of moles of the gas, and R is the gas constant, 8.3143 J.mol-1.K-1
Also called "alkali metals", the group
comprises the elements Li, Na, K, Rb. Cs and Fr. As they all have
a single electron in their outer shell, their outer electron configuration is s1.
Also called "alkaline earths", the group
comprises the elements Be, Mg, Ca, Sr, Ba and Ra. As they all have two paired electrons in their outer shell,
their outer electron configuration is s2.
Also known as the inert gases or noble gases, this group
comprises the elements He, Ne, Ar, Kr, Xe and Rn. All
have have an outer electron configuration s2p6.
The cell potential of a cell consisting of a standard hydrogen electrode and the half-cell in question. This is
the standard electrode potential for the half-cell.
A redox reaction may be split into two half-reactions,
one describing the uptake of electrons (reducing
half-reaction) and the other describing loss of electrons
(oxidation half-reaction).
Hydrogen is the simplest element, a colourless, odourless
and non-poisonous gas. Produced in the laboratory by the
action of strong acids on metals such as zinc.
A proton H+ in aqueous solution. H+
is used as an abbreviation for the hydronium ion, H3O+,
as H+ never exists in the free form in the
presence of water.
(1) Reaction of cetain anions or cations with water, resulting
in the production of excess OH- or H3O+ ions. (2) The reaction of
certain subtances with water: hydrolysis of esters result in the formation of carboxylic acids and alkanols.
(1) Process whereby an atom acquires a net electrical
charge. (2) Process whereby an acid (or a base) react
with water to form hydrogen ions (or hydroxide ions).
The concept that gases consist of perfectly elastic molecules of negligible volume, moving randomly, and
creating pressure by collisions with the walls of the container.
The latent heat of fusion of a sample is the energy required to melt that
sample without a change in temperature. The latent heat of vaporization of a sample
is the energy required to vapourize the sample without a change in temperature. See also
specific latent heat.
When two elements form more than one compound, there is a simple numerical ratio
between the mass of one element which combines with a given mass of the other element.
A principle stating that if a stress is applied to a
system in a state of dynamic equilibrium, the system will shift in a direction which opposes the stress.
A measure of the temperature-dependent expansion or contraction of a solid. It is the fractional
increase/decrease in length for a rise/fall of 1 degree Kelvin.
A fundamental property of matter. (1) Gravitational mass:
the mass of a body influenced by gravitation. It is
obtained by dividing the force due to gravity on a object
by the gravitational acceleration at the location of the
object. (2) Inertial mass: the proportionality constant
which relates the acceleration of a body with the force
causing that acceleration. It can be shown that the
gravitational mass of an object is identical to its
inertial mass.
A table in which elements are arranged in increasing values of their atomic numbers. The rows in the table are called periods, while the columns are called groups.
(1) Two points on a wave train are said to be in phase if their displacements
and velocity are identical at any one time. (2) The phases of a substance are its different
physical states. Water for example can be in the solid phase (ice), liquid phase,
or vapour phase.
The loss of negative electrical charge, or gain of electrical positive charge, by an
insulated conductor when exposed to light of certain wavelengths. See also
work function.
The ratio of the sine of the angle of incidence to the sine of the angle of refraction for a ray of light passing through the surface separating two media. It is also equal to the ratio of the velocity of light in the first medium to the velocity of light in the second medium.
A mathematical relationship between the wavelengths, l, of the lines of the hydrogen spectrum:
1/l = RH(1/n12 - 1/n22), where n1 is
equal to 2 and n2 = 3, 4, 5... in the Balmer series. RH is known as Rydberg's constant.
For a ray of light refracted at the surface separating two media, the ratio of the sine
of the angle of incidence to the sine of the angle of refraction is constant and is
equal to the refractive index of the two media.
The specific latent heat of fusion of a substance is the energy required to melt
1 kg of that substance without a change in temperature. The specific latent heat of vaporization
of a substance is the energy required to vapourize 1 kg of that substance without a change in temperature.
See also latent heat.
An electrochemical cell
whose half-cells are both in the standard state of 25 ºC, 101.3 kPa and immersed
in electrolytes at a concentration of 1.0 mol.dm-3.
Two waves travelling in the same medium are said to be superposed if the displacement
of a particle at any given point is the algebraic sum of the displacement due to each wave.
A solution which contains a higher concentration of solute that would be present if the solution had been saturated
at the same conditions of temperature and pressure.
In chemistry, synthesis refers to the preparation of a compound from other, usually simpler
substances. Synthesis may take place in more than one step.
Transition elements are elements where the d electronic sublevel is gradually filled. They are called
transition elements because they form a transition between s-block elements and p-block elements.
The pressure exerted by a vapour. At the boiling point, the vapour pressure of a liquid is
equal to the pressure exerted on the liquid by the atmosphere.
Water molecules which form an integral part of the lattice of ionic crystals. The water of
crystallization is normally included in the formula for the substance, i.e.,
CuSO4.5H2O.
