Temperature is the thermodynamic property which defines the internal
energy of a system. The absolute scale of temperature results from
the physical properties of gases. There are a number of other
temperature scales in use.
The Celsius Scale of Temperature
uses the freezing point and boiling point of water as the two fixed points and assigns
the values 0 and 100 to these fixed points, respectively and divides the
scale between them into 100 degrees.
The influence of temperature on the rates of chemical reactions
depends on the Boltzman distribution of energies in the reactant
molecules. Thus, the higher the temperature, the greater the
fraction of molecules that is present in an excited (i.e. reactive)
state.
Temporary hardness is that fraction of the hardness of water that
is due to dissolved calcium bicarbonate.
Temporary hardness is removed on boiling, when the calcium bicarbonate
is converted to calcium carbonate.
Termination is the final step in a free radical
mechanism that results in the stopping of the free radical reaction. Normally, termination
occurs only after a significant number of
propagation steps in the
free radical process have occurred.
Tertiary alcohols are aliphatic alcohols
in which the hydroxyl group [-OH] is attached to a tertiary carbon atom (i.e. a carbon atom
which has no hydrogen atoms attached directly to it).
Tertiary treatment is the polishing process to which a sewage effluent
is subjected after it has received secondary treatment in order to
reduce further its nutrient content.
The theories of catalysis
explain the influence of the catalysts on the rate of a reaction by describing the detailed mechanism by
which the catalyst is involved in the steps of the chemical reaction.
Thermal pollution is the increase in temperature of natural waters
resulting from the discharge to these waters of hot effluents from
industrial and power plants. The higher temperatures reduce the
concentration of dissolved oxygen
in the receiving waters and promote eutophication.
Thermochemistry is the investigation of the transfer of energy in the
form of heat during the course of a chemical reaction, which are called
the heats of reaction.
In every chemical reaction, there is a difference between the
intrinsic energy in the reactants
and the intrinsic energy in the products. This energy, which is either adsorbed or
released during the course of the reaction, is the heat of reaction.
The heat of reaction (i.e. the change in the energy during the course
of a reaction) is expressed as dH, (i.e. "delta-H"), which has positive
values for endothermic reactions
and
negative values for exothermic
reactions .
The unit of heat is the joule. Another unit of heat which is widely
used in thermochemistry, the calorie (i.e. the amount of heat required
to raise 1 g of water through 10 C) which is equal to 4.18 joules.
Thermoplastic materials are polymeric materials which can be
repeatedly softened by heating and hardened again on cooling.
Thermosetting plastics are those rigid plastic which are molded into shape at the time of manufacture (i.e. at the time of
polymerisation) and they an
extensively cross-linked structure.
These plastics cannot be remolded to another shape as their
rigidity is due to the highly cross-linked structure formed between
the molecules at the time of polymerisation. "Baklite" is an example
of a thermosetting polymer.
Titration's involves taking a definite volume, usually 20 ml, of a
liquid whose concentration is unknown, and slowly adding a solution
of a reagent with which the solution reacts, until reaction is
complete. An indicator is used to denote the
end-point of the
titration. The volume of the solution added is noted, and allows
the concentration of the unknown to be calculated.
Titration curves are the plots of pH
versus the volume of reagent delivered during the course of a titration. The inflection point
in the titration curve marks the end-point
of the titration.
Different ions carry different fractions of the current because different
ions move at different speeds under the same potential gradient. The
transference number of an ion is the fraction of the total current that
is carried by that ion during electrolysis.
In general, a cation and an
anion differ in the amount of current
they can carry during electrolysis.
The transition metals are the elements found between the
Group II Elements and
Group III Elements
in The Periodic Table. The
transition elements are also known as the d-block elements, because
while the outermost level contains at most two electrons, their next
to outermost main levels have incompletely filled
d sub-orbitals, which
are filled-up progressively on going across the periodic table. The
filling of the d sub-orbitals of the transition elements across a row
of the periodic table is not always regular.
The series of 14 elements from Cerium to Lutetium is a series within a series
which is called an inner transition series. This one is the
Lanthanide series.
In the next period there is a similar inner transition series called the
Actinides series,
from Thorium to Lawrencium.
The transition elements readily form alloys (e.g. a copper-tin alloy is
used for mirrors, brass is a copper-zinc alloy). Tungsten,
is used to make tools and filaments in light bulbs. The atomic size is fairly constant
since the electrons in the outer most shells have similar environments. The low
ionisation potentials mean that the
elements show variable valency states by loss of electrons from the s and 3d
orbitals The elements in this group can have different oxidation
states makes them useful as catalysts.
Compounds of the transition elements can be paramagnetic (i.e. attracted by a
magnetic field) or diamagnetic (i.e. not attracted by a magnetic
field). Paramagnetism in the transition elements is caused by the
presence of unpaired electrons in the d sub-orbital. Diamagnetism
is characteristic of compounds where all the electrons are paired
in the d sub-orbitals.
The electronic configuration
of these elements are characterised as having full outer orbitals and as having the
second outermost orbitals incompletely filled. Thus, in the case of the first
transition metals series, the electronic configuration of the outer
orbitals is 4s2, while the second outermost orbitals (i.e. the 3d
level) are incompletely filled.
Because they are elements whose atoms have their next-to-outer most main
levels incomplete at the inner d sub-levels, but are filling up
progressively, while the outer most level contains at most two
electrons, the filling of the d orbitals is not always regular.
Irregularities are observed for chromium
and copper, because the
listed electronic configurations are energetically favoured
(i.e. 3d54s1 is more stable than 3d44s2, etc.).
Apart from copper, the transition metals are all white lustrous
metals. They vary widely in abundance (e.g. iron
and titanium are plentiful while
scandium is rare). They have high melting points
and high densities. This suggests that the electrons which enter
the d orbitals are being used to bind the atoms together in the
crystal lattice.
The transition elements form alloys readily with themselves and
with other elements. For example, a copper-tin alloy is used for
mirrors, and brass is a copper-zinc alloy.
Atomic size is fairly constant, since the electrons in the outer
most shells have similar environments.
The transition elements form complex ions. Examples of these compound
ions include
- ferrocyanide Ion, Fe(CN)6(4 -) in potassium ferrocyanide, K4Fe(CN)6,
and,
- chromate ion, CrO4(2 -), in barium chromate, BaCrO4.
Most compounds of the transition metal are coloured. There are
variations in colour for compounds of the same valency, and with
different valency (oxidation) states. For example,
Compound Formula Colour
Anhydrous Copper Sulphate Cu(II)SO4, White
Hydrated Copper Sulphate Cu(II)SO4.5H2O Light Blue
Ammoniacal Copper Sulphate [Cu(II)(NH3)4]SO4 Dark Blue
Potassium Permanganate KMnO4 Purple
Potassium DiChromate KCrO4 Orange
Potassium Chromate K2Cr2O4 Green
V(+++++) is pale yellow,
V(++++) is blue,
The colour exhibited by the transition metal ions is due to their ability to absorb light in the
visible region of the electromagnetic
spectrum. This causes unpaired d electrons to rise from a lower
to a higher energy state. Ions which have the electronic configuration
3d(10), including the scandium (III) ion and the copper (I) ion are
colourless, because they have no unpaired d electrons.
The treatment of drinking water involves the purification of raw
waters to render them suitable for human consumption. This involves the
removal of
- the colloidal matter which is responsible for colour, and
- the suspended solids responsible for turbidity.
Subsequently, this processed water has its pH
adjusted, to make it less corrosive to the metals of the distribution pipes. Finally,
fluoride is added to protect teeth from decay and the water is
disinfected with chlorine before being put into the distribution
system as drinking water.
The treatment of sewage is the purification of sewage and may involve
- primary treatment (i.e. the removal of suspended solids),
- secondary treatment (i.e. the removal of dissolved organic matter) and
- tertiary treatment (i.e. the removal of nutrients)
before it is discharged to the aquatic environment.
The triads are the three groups of elements, each of three metals, in
Group VII Elements of the periodic table.
Trihydric alcohols (i.e. Triols) are organic compounds containing
three hydroxyl groups. The simplest trihydric alcohol is
1,2,3-propane-triol, CH2(OH).CH(OH).CH2(OH), which is also
known as glycerol (from the Greek glykys meaning sweet) or glycerin.
Glycerol is commercially produced by the hydrolysis of Fats.
Fats + Alkali ==> Soap + Glycerol
Fats are triesters of glycerol, and when they are hydrolysed with
sodium hydroxide, yield glycerol, CH2(OH).CH(OH).CH2(OH), and
soap, R1COONa, which is a mixture of the sodium salt of the fatty acids.
CH2OOCR1 CH2OH R1COONa
CHOOCR2 + 3NaOH ==> CHOH + R2COONa
CH2OOCR3 CH2OH R3COONa
Fats Glycerol Sodium salts
of Fatty Acids
Glycerol is a by-product in the soap industry and is recovered
by suitable means.
The triple bond between carbon atoms in alkynes
involves a total of six electrons (i.e. three shared pairs of electrons). These bonds
are formed in different ways and the first bond behave differently in
chemical reactions from the second and third bonds.
Start of Hypertext ....
Elements ....
Compounds ....
Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.