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Daniell Cell
A type of primary cell consisting of two electrodes in different
electrolytes separated by a porous partition. The
cathode
is copper immersed in copper (II) sulphate solution.
The anode is zinc-mercury amalgam in either
dilute sulphuric acid or zinc sulphate solution. The voltage of this cell is approximately
1.1 volts.
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Density
The density of a substance is a physical property of the
substance. It is the mass of a substance occupying a certain volume.
The unit of density is usually expressed as the mass divided by the volume.
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Deuterium
This is a naturally occuring stable isotope of
hydrogen in which the nucleus contains one
proton and one
neurtron. The atomic mass is thus approximately
twice that of 1H. Deuterium is also known as "heavy hydrogen".
Chemically it behaves almost identically to hydrogen, forming analogous
compounds. But reactions of deuterium compounds are of slower than those of the
corresponding 1H compounds. This difference in reactivity is made use of in
kinetic studies where the rate of a reaction depends on transfer of a hydrogen atom.
This technique is known as a "kinetic isotope effect".
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Deuterons
The nucleus of the
deuterium atom, 2D+.
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Diatomic elements
The seven elements that exist as
diatomic molecules.
| The diatomic elements |
|---|
| H, N, O, F, Cl, Br, and I |
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Diatomic molecules
Molecules that are composed of two atoms.
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Dipole moment
The dipole moment is a measure of the extent
to which a separation of charge exists within a molecule.
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Dissociation
The process by which ionic compounds break up into
their individual ions in the presence of water molecules.
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Doping
The incorporation of impurities within the crystal lattice of a solid so as to alter
its physical properties. Examples are:
- silicon, when doped with boron, becomes semiconducting.
- arsenic is added to germanium to increase its semiconducting efficiency.
- neodymium and chromic oxide are used as activators for yttrium-garnet and ruby lasers.
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Downs Process
In the commercial preparation of sodium, molten sodium chloride NaCl is
electrolyzed in a specially designed cell called Downs cell.
Sodium chloride has a normal melting point of 804oC. By adding some calcium chloride,
CaCl2, the melting point of the cell medium is lowered such that the cell
can operate at around 600oC. The sodium metal and chlorine gas that are produced
in the electrolysis are kept from coming in contact and reforming NaCl. In addition, the sodium
must be prevented from contact with oxygen because the metal would quickly
oxidize under the high-temperature conditions.
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Electrolysis
A chemical reaction which is brought about by
the passage of current from an external energy source such as a battery.
Electrolyses of molten salts for the production of
sodium and
aluminum are important industrial processes for
obtaining metals.
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Electrolyte
These are substances that produce ions
in aqueous solution. It is also a solution that conduct an electric current.
Ionic compounds and strong acids and
bases are strong electrolytes.
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Electromagnetic spectrum
Electromagnetic radiation forms a whole electromagnetic
spectrum, depending on frequency ranging from high-frequency gamma rays to low-frequency
radio waves.
The approximate wavelengths of the electromagnetic spectrum are:
| Gamma | X-ray | Ultraviolet | Visible | Infrared | Microwave | Radio Wave |
|---|
| < 10-10 m | 10-10 to 10-9 m | 10-9 to 10-7 m | 10-7 to 10-6 m | 10-6 to 10-4 m | 10-4 to 10-1 m | > 10-1 m |
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Electron capture
A mode of radioactive decay in which an inner-shell electron is captured by the
nucleus.
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Electronegativity
is a measure of the
tendency of an atom to attract electrons to itself in a covalent bond.
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Electrons
Electrons are negatively charged fundamental particles.
They are found outside the nuclei of all atoms. The mass of an electron is 1/1835 of
the mass of a proton.
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Electron volt
This is a unit of energy. It is defined as the energy an
electron receives when it falls through a potential difference of 1 volt.
As a comparison, the breaking of chemical bonds yields from 5 to 10 electron volts, whereas the splitting
of an atomic nucleus releases about 200 million electron volts. The symbol for electron volt is
"eV".
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Element
A substance in which all the atoms have the same atomic number. It is a
substance that cannot be broken down into simpler substances.
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Empirical formula
Also known as the simplest formula. It is the chemical
formula that gives the simplest whole-number ratio of atoms of each element in a compound.
In some compounds the empirical formula is the same as the molecular formula .
For example,
- For water, H2O is the molecular formula. H2O is also the empirical formula because the
ratio of H to O in the molecular formula is also the simplest whole-number ratio.
- For benzene, C6H6 is the molecular formula. The empirical formula is CH, the simplest
ratio of C to H.
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Empirical formula unit
In benzene, the molecular formula
is C6H6. The empirical
formula is CH. Therefore, there are six empirical formula units in benzene.
(Molar mass of compound) / (Mass of the Empirical formula)
= Number of Empirical Formula Units |
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Energy
When samples of matter react chemically, energy is almost
always absorbed or released. So, the study of energy is an integral part of
the study of chemistry.
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Equilibrium
To denote an equilibrium reaction, double arrows are used instead
of a one-way arrow pointing to the product(s). For example,
The reactants A and B are forming the products C and D:
The products C and D also react to give the original reactants A and B:
The concentrations of A, B, C, and D change with time until a state is reached at
which both reactions are taking place at the same rate. When this occurs, the
system is said to be in a state of chemical equilibrium. What's important
is that the equilibrium is a dynamic one. The reactions still take place but at equal
rates.
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Fahrenheit
A temperature scale based on a value of 32 oF as the
melting point of ice and 212oF as the boiling point of water.
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Family of elements
See group of elements.
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Fission
A nuclear reaction brought about by direct impact of a neutron
upon the uranium-235 or upon the artificial
element plutonium-239. This causes
the nucleus to split or divide into two fragments (or fission products).
More important is the tremendous energy release for each nucleus split.
Fission reactions can be closely controlled and are finding increasing use ofr electric
power generation.
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Fluorescence
A kind of luminescence in which the emanation of light
continues for as long as the excitation lasts and in which the emission of radiation follows
the excitation almost instantaneously.
Among inorganic substances having the
property of fluroescence are sodium vapor and numerous compounds of zinc, cadmium, magnesium,
and calcium with sulfur, tungsten, silicon, boron, and others. Such materials
(called phosphors)
are utilized in fluorescent lights, television tubes, and scintillator counters.
Organic substances which fluoresce are anthracene, fluorescein, and other fused ring
structures, used in fluroescent dyes and as tracer materials.
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Formula mass
The formula weight of a substance is the sum of the atomic
masses of all atoms in a formula unit of the substance expressed in amu.
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Formula unit
The smallest unit of an ionically bonded substance.
For example, the formula unit of sodium chloride is NaCl.
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Fractional distillation
A method of separating volatile mixtures by utilizing
the fact that the vapour phase above a liquid mixture is richer in the more volatile component.
The method involves distillation carried out with partial
refluxing using a long vertical column
(or fractionating column). Fractionation is the fundamental process for producing petroleum
from crude oil.
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Gallium arsenide
A dark-grey crystalline compound, GaAs, is a material that is used
as semiconductors. One of the factors limiting the efficiency of computer chips is the
speed with which electrons can move. Through GaAs, electrons move 10 times faster than
through silicon-based chips.
GaAs chips also have optical properties such as absorbing light to create a current.
Light-emitting diodes (LED) makes use of this property. When a current is supplied,
the diode emits light. GaAs-based lasers are also much smaller and more powerful than normal types.
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Galvanizing
A process for coating a protective layer of
zinc to steel to prevent or inhibit corrosion. The
length and extent of the protection are dependent upon the weight and thickness
of the zinc coating.
Application of zinc is achieved by:
- the hot-dipping process by passing the metal through molten zinc, or
- electrodepositionof zinc by a plating process. This gives a white matte surface
that is bright and reflective.
The zinc protects the steel from corrosion because zinc is a more reactive metal than
the iron in the steel.
Galvanized metal is used chiefly on products such as pipe, nails, wire, container etc.
This is the largest commercial use of zinc.
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Gamma radiation
A form of highly energetic radiation emanating from unstable
atomic nuclei and characterized by very short wavelengths about 1
angstrom.
Gamma radiation is emitted slowly by decaying radioactive nuclei such as radium, and very rapidly
in the fission process. Gamma radiation is highly dangerous to body tissues
and may cause genetic damage or even death on prolonged exposure.
Cobalt-60 has been used in industry as a source of gamma rays.
Gamma radiation is used for
spectroscopic analysis and in cancer treatment.
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Garnet
The natural form (gemstone) is a silicate compound containing iron and aluminum.
Synthetic garnets contain iron oxide and one or more rare-earth oxides which have specific magnetic properties,
making them suitable for use in microwave equipment, laser, and similar devices. Their
unique magnetic function was discovered in 1956. They often contain
yttrium and
aluminum oxides.
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Glow discharge
When an electric current is passed through a tube which contains
a low pressure gas,
ionization of the gas molecules occurs. In the recombination
process, light energy is given off and the result is what is seen as a "glow discharge".
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Greenhouse effect
The great increase in our use of fossil fuels over the past
century has caused a significant increase in the CO2 concentration in the
atmosphere. It is projected that the concentration of atmospheric CO2
could double by early in the next century. This could result in a rise in the
average global temperature by 2 to 3 oC from the greenhouse effect
caused by the CO2 and H2O vapour in the atmosphere. Although
2 to 3 oC increase in the temperature of the atmosphere may not seem like
much, it is thought to be enough to cause a dramatic change inclimates and the partial
melting of the polar icecaps, raising the sea level to threaten some coastal cities.
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Ground state
The ground state of the electrons in an atom
is the energy associated with the arrangement of the electrons
such that they are at the lowest possible energy level. Because
electrons are constant moving, the ground state energy is not zero.
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Group of elements
A vertical column of elements in the
periodic table.
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Haber Process
An industrial process for the manufacture of ammonia.
The reaction is the equilibrium:
In practice, conditions of about 300 atmospheres and 500oC are
employed and about 20% conversion to ammonia is achieved in each cycle. The
catalyst is iron with small amounts of potassium and
aluminum oxides present. The ammonia is removed by liquefaction at -50oC and the remaining
nitrogen and hydrogen are made up to their original pressures and recycled over the
catalyst.
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Half life
It is the time required for one-half of the initially present reactant
or radioactive nuclei to disappear.
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Halogens
All the elements in Group VIIA of the periodic table.
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Heterogeneous mixture
Heterogeneous mixtures do not have
the same composition and properties throughout a sample. Some examples
of heterogeneous mixtures are: iced tea, blood, milk.
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Homogeneous mixture
Homogeneous mixtures are called
solutions. Homogeneous mixtures have the same composition and properties
that are uniform throughout a sample, but different samples may have
different properties. A homogeneous mixture can be separated into its components by appropriate
physical changes.
Some examples of solutions are: gasoline, seawater, tea.
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Hydrogenation
In organic chemistry, hydrogenation refers to the addition of
hydrogen to multiple bonds, usually with the aid of a
catalyst. For example, unsaturated natural liquid vegetable
oils can be hydrogenated to form saturated semisolid fats, a reaction used in making
margarine.
