The Haber Process is the industrial method for the production of
ammonia by reacting nitrogen with hydrogen.
N2 + 3H2 ==> 2 NH3
The process is reversible and
The process operates at high temperatures because at low temperature the rate of
reaction would be too slow for equilibrium to be reached in a
reasonable time. Thus, high temperature, 450 degC, and
high pressure, 250 atmospheres, is used to increase the
yield. An iron catalyst is used.
The process was invented by the German chemist,
Fritz Haber in 1909AD.
However, Carl Bosch
developed it for industrial use, leading
to the alternative name Haber-Bosch Process.
A half-cell is an electrode in contact with a solution of
ions, and it forms one half of a cell. Gas half cells have
a gold or platinum plate in a solution with gas bubbled
over the metal plate. The hydrogen half-cell is used as
a reference half cell and is assigned the electrode
potential of zero. Common half cells include the zinc and copper
Halides are compounds which contain one of the halogen elements
in chemical combination with another element. The halides of
typical metals are ionic. Metals form halides in which the
chemical bonding is largely covalent.
The halide salts are the fluorides,
chlorides, bromides or iodides.
The halogens (from the Greek hals, salt) are the non-metallic
elements in group VII of the
periodic table (i.e. fluorine, chlorine, bromine, iodine and astatine). They are
highly reactive and are not found in the elemental state in nature. The halogens are
used as oxidising agents in many
The electronic configuration of the
halogens has one electron short of the stable configuration of its neighboring Nobel Gases in the
The halogen atoms can acquire a Nobel Gas structure in either of two ways.
Thus, the halogens exhibit an electrovalence and a covalence
of 1. Positive oxidation states of III, V, VII are also
known for all the halogens, with the exception of
- by accepting an electron from a donor atom to form an
ionic bond. Example of this behaviour
includes sodium chloride, potassium bromide and sodium iodide, or
- by forming a covalent bond by either
- overlap of p orbitals of the atoms. Example of this
behaviour includes chlorine and bromine, or
- by overlap of a p of the halogen and the s orbital of
another atom. Example of this behaviour includes hydrogen
chloride and hydrogen bromide.
The melting points and boiling points of the halogens increase
with atomic number.
The electron affinity is at a maximum
in chlorine. The halogens are oxidising agents, (i.e. they readily gain electrons
to form negatively charged ions).
Fluorine has the highest electrode potential and
is therefore the strongest oxidising agent. The
high electrode potentials illustrate high activity in
solution. Fluorine with its high electronegativity, is the
most reactive non-metal in the Periodic Table.
The hardness of water results from dissolved salts of
calcium and magnesium, which are introduced into the water
in the environment when it passes through limestone areas. The
nature of the materials dissolved will depend on the
geology of the region. However, calcium and magnesium
salts are the source of the hardness of water. Total hardness
and calcium hardness can be distinguished chemically.
The hardness may be classified as temporary
hardness which is removed on boiling, or
permanent hardness which is due to dissolved salts.
The heat of combustion of a substance is the heat evolved when one gram-molecule
of the substance combines with oxygen. When a substance burns in air, combustion
is the process of reacting a substance with oxygen. In everyday life, we make use of
the combustion of coal or gas in air as a source of heat. Chemical reactions
accompanied by an evolution of heat are called
The heat of formation of a substance is the heat evolved
when one gram-molecule of the substance is produced
from its elements.
The heat of neutralisation is the quantity of heat
liberated when one mole of a strong acid is neutralised
by one mole of strong base.
The heat Of neutralisation is independent of the nature
of the acid or base. For this reason, it is assumed to
be the heat released due to the recombination of the
hydrogen ion from the acid and hydroxide ion from the base
to form water.
H(+) + OH(-) ==> H2O
The heat of neutralisation is the amount of heat evolved
when one gram equivalent of an acid is neutralised by one gram
equivalent of a base to give one gram equivalent of a neutral
salt. Thus, in dilute solution, an acid or base is considered
to be completely ionized, and the neutralisarion reaction is
Na(+) + OH(-) + H(+) + Cl(-) ==> Na(+) + Cl(-) + H2O
The value of H should always be - 57.26 kj per mole for
neutralisation of any strong acid and strong base.
The heat of reaction is a difference between the intrinsic
energy in the products of a chemical reaction and the intrinsic
energy in the reactants, and it is either adsorbed or released
during the course of the chemical reaction.
The heat of solution is the amount of heat evolved or absorbed
when one gram molecule of a compound is dissolved in a
large quantity of water.
The Heisenberg Uncertainty Principle describes the uncertainty
with which the velocity and position of an electron in an
orbital can be known. Only the wave function for the electron is
known with certainty, and the electron density at any point
is the square of the wave function at that point. Thus, the
probability of finding an electron at any point is
proportional to the electron density at that point.
Henry's Law states that the mass of gas which is dissolved by
a given volume of liquid at a fixed temperature, is
proportional to the pressure of the gas.
In the case of water, the law only applies to those
gases which are slightly soluble in it, as the more soluble gases
react with water to form ionic species in solution.
Heptane, C7H26, is the seventh member of the alkane
series of hydrocarbons. It is a liquid which is obtained
from petroleum. Its relative density is 0.684, its melting
point is -90.6 degC, and its boiling point is 98.4 degC.
Hertz, Hz, is the unit of frequency, and is the number of
cycle per second. It is called after a German
physicist, Heinrich Hertz.
Hess's Law of Heat Summation states that the internal energy
of a substance is independent of the process by which it was made.
Thus, if a chemical reaction takes place in stages, the algebraic
sum of the amount of heat evolved in each separate stage is
equal to the total amount of heat that would be evolved if
the reaction took place in one stage.
Heterocyclic compounds are ring compounds in which the ring
contains carbon and other elements, the commonest being oxygen,
nitrogen and sulphur.
High density polyethylene, HDPE, is an important plastic which
is manufactured industrially by the polymerisation of
A Hofmann Voltmeter is an apparatus for the
volumetric analysis of gases
produced during electrolysis. For example, the hydrogen
and oxygen produced during electrolysis can be collected in
separate graduated tubes, and volumes produced compared.
A Homologous Series is a family of organic compounds which have
- a common general formula,
- similar methods of preparation,
- similar chemical properties,
- a regular trend in physical properties, and
- a molecular weight difference of 14 between adjacent members of the series, due to each
member of the series containing an extra -CH2- group (i.e. a methylene group).
The Hund's Rule of Maximum Multiplicity states that when
electrons occupy the orbitals about a nucleus, and two or
more orbitals are at the same energy level, each orbital is filled
singly, before any is filled doubly.
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