Inorganic and analytical Chemistry

Lecture 5

Dr.: Sameh Araby El-Mekawy

P Block Elements
Group 15/V: The Nitrogen Family
Outlines
Group 16/VI: The Oxygen Family
Group 17/VII: The Halogens
Group 15/V: The Nitrogen Family
Physical Properties of Group 15 Elements
 Electronic Configuration.
 (ns2 np3) Valence electron configuration
 The valence shell electronic configuration of these elements is
ns2np3. The s orbital in these elements is completely filled and p
orbitals are half-filled, making their electronic configuration
extra stable.
Properties
 Physical state: Nitrogen is a gas consisting entirely of diatomic
molecules but the other elements are normally solids. Except
nitrogen, all the elements show allotropy.
 Metallic character:
 The metallic character of the group increases down the group
 Nitrogen and phosphorus are, in fact, typical non-metals, having
acidic oxides which react with alkalis to give salts. As and Sb are
metalloids and Bi is metal.
 Ionization energy: It goes on decreasing down the group due to
increase in atomic size. Group 15 elements have higher ionization
energy than group 14 elements due to smaller size of group 15
elements. Group 15 elements have higher ionization energy than
group 16 elements because they have stable electronic
configuration i.e., half filled p-orbitals.
 Oxidation states:
 The common oxidation states are +3, +5, –3.
 The tendency to show –3 oxidation state decreases down the group
due to decrease in electronegativity which is due to increase in
atomic size.
 The stability of +5 oxidation state decreases whereas stability of +3
 oxidation state increases due to inert pair effect.
 Nitrogen shows oxidation states from –3 to +5.
Nitrogen
 Nitrogen, the head element, shows many notable
differences from the other Group V elements, the
distinction arising from the inability of nitrogen to expand
the number of electrons in its outer quantum level beyond
eight. (The other Group V elements are able to use d
orbitals in their outer quantum level for further
expansion.)
 The behaviour of nitrogen differs from rest of the
elements. Because
 It has a small size.
 It does not have d – orbitals
 It has high electronegativity
 It has high ionization enthalpy
Occurrence & Extraction

 Industrially, elemental nitrogen is extracted from the air by the

fractional distillation of liquid air from which carbon dioxide and
water have been removed. The major fractions are nitrogen, b.p.
77 K and oxygen, b.p. 90 K, together with smaller quantities of
the noble gases.
 Liquid nitrogen is produced commercially from the cryogenic
distillation of liquefied air. An air compressor is used to compress
filtered air to high pressure; the high-pressure gas is cooled back to
ambient temperature, and allowed to expand to a low pressure. The
expanding air cools greatly (the Joule–Thomson effect), and
oxygen, nitrogen, and argon are separated by further stages of
expansion and distillation. Small-scale production of liquid
nitrogen is easily achieved using this principle.
 Liquid nitrogen may be produced for direct sale, or as a byproduct
of manufacture of liquid oxygen used for industrial processes such
as steelmaking. Liquid-air plants producing on the order of tons
per day of product started to be built in the 1930s but became very
common after the Second World War; a large modern plant may
produce 3000 tons/day of liquid air products.
Fractional distillation of air
4. Nitrogen boils at
Colder at the top -196oC so it can be
removed from the
top of the column
as a gas.
1. Air has to be
condensed into a
liquid. This happens
at -200oC.

2. This is done by
compressing or
squashing the air
and the use of cold
water. 5. At -185oC
oxygen is still a
3. At -200oC carbon liquid so can be
dioxide and water taken out the
are solids so can be bottom of the
easily taken out. Warmer at the bottom column.
 Cryogenics: The branches of physics and engineering that
study very low temperatures, how to produce them, and how
materials behave at those temperatures.

 Cryogens, like liquid nitrogen, are further used for specialty

chilling and freezing applications. Some chemical reactions,
like those used to produce the active ingredients for the
popular statin drugs, must occur at low temperatures of
approximately -100 °C. Special cryogenic chemical reactors
are used to remove reaction heat and provide a low
temperature environment. The freezing of foods and
biotechnology products, like vaccines, requires nitrogen in
blast freezing or immersion freezing systems.
 Reactivity of Nitrogen: The dissociation energy of the N≡N bond
is very large(946 kJ mol) and dissociation of nitrogen molecules
into atoms is not readily effected until very high temperatures,
being only slight even at 3000 K. It is this high bond energy
coupled with the absence of bond polarity that explains the low
reactivity of nitrogen, in sharp contrast to other triple bond
structures such as -C ≡ N,-C ≡ C-.
 Nitrogen is an inert molecule and will only react with other
elements, including oxygen, at very high temperature. Yet
nitrogen and oxygen form an array of oxides in which nitrogen
exhibits a whole range of oxidation state from +1 to +5: N2O,
NO, N2O3, NO2, N2O4, and N2O5. At high temperature, nitrogen
gas also reacts with H2, Li, the Group 2A elements, B, Al, C, Si,
Ge, and many transition elements.
Oxides of Nitrogen
The most hazardous of the nitrogen oxides are nitric oxide NO
and nitrogen dioxideNO2; the latter exists in equilibrium with
its dimer, nitrogen tetroxide. Nitric oxide is a colorless gas at
room temperature, very sparingly soluble in water. Nitrogen
dioxide is a colorless to brown liquid at room temperature and
a reddish-brown gas above 70°F poorly soluble in water. Nitric
oxide is rapidly oxidized in air at high concentrations to form
nitrogen dioxide.
 Ammonia: NH3
 Nitrogen is the starting point for an important group of compounds.
First, nitrogen is combined with hydrogen to make ammonia (NH3).
The production of ammonia is sometimes called industrial nitrogen
fixation.
 The formation of ammonia from nitrogen and hydrogen is very
difficult to accomplish. The two elements do not easily combine.
 German chemist Fritz Haber in 1905 found that nitrogen and
hydrogen would combine if they were heated to a very high
temperature with a very high pressure. He also found that a catalyst
was needed to make the reaction occur. The catalyst he used
was iron metal, though other metals are sometimes used
Haber Process
Ammonia, NH3, is produced commercially by the Haber Process.
Fe
N2(g) + 3H2(g) 2NH3(g) DH = -92 KJmol-1
460 oC
200 atm
Haber Process

Flow chart for the manufacture of ammonia

Raw Materials
 N2(g) is taken from the air via a process of fractional distillation.
 Modern ammonia-producing plant first converts natural gas (i.e.,
methane) or LPG (liquefied petroleum gases such as propane and
butane) or petroleum naphtha into gaseous hydrogen. The method
for producing hydrogen from hydrocarbons is known as steam
reforming.
CH4 + H2O ⇌ CO + 3 H2
Modern Method of Manufacturing Ammonia
The manufacturing process consists the following stages as shown from the block
diagram

Desulphurization
 Hydrocarbon feedstocks contain sulphur in the form of H2S,CS2 and
mercaptans.
 The catalyst used in the reforming reaction is deactivated
(poisoned) by sulphur.
 The problem is solved by catalytic hydrogenation of the sulphur
compounds as shown in the following equation:
H2+RSHRH + H2S(g)
 The gaseous hydrogen sulphide is then removed by passing it
through a bed of zinc oxide where it is converted to solid zinc
sulphide:
H2S+ZnO  ZnS+H2O
Primary (Steam) Reforming.
 Reforming is the process of converting natural gas or
naptha (CnH2n+2) into hydrogen, carbon monoxide
and carbon dioxide.
 Steam and natural gas are combined at a 3:1 ratio.
This mixture is preheated and passed through
catalyst-filled tubes in the primary reformer.
 Catalytic steam reforming of the sulphur-free
feedstock produces synthesis gas (hydrogen and
carbon monoxide). Using methane as an example:
CH4 + H2O ⇌ CO + 3 H2
Secondary reformer
 From the primary reformer, the mixture flows to the
secondary reformer.
 Air is fed into the reformer to completely convert
methane to CO in the following endothermic
reaction.
Ni/15-20atm/1000-1100oC
CH4 + Air CO + H2O + N2
Shift Conversion.
 The carbon monoxide is converted to carbon dioxide with the
assistance of catalyst beds at different temperatures.
CO+H2O = CO2+H2
 This water-gas shift reaction is favorable for producing carbon
dioxide which is used as a raw material for urea production. At
the same time more hydrogen is produced.
Purification
 CO is an irreversible poison for the catalyst used in the
synthesis reaction, hence the need for its removal
 The synthesis gas is passed over another catalyst bed in the
methanator, where remaining trace amounts of carbon
monoxide and dioxide are converted back to methane using
hydrogen.
CO+3H2CH4+H2O
CO2+4H2  CH4+2H2O
O2 + 2H2 2H2O
Note that the first equation is the opposite of the reformer reaction
 Ammonia Converter.
 After leaving the compressor, the gaseous mixture goes through
catalyst beds in the synthesis converter where ammonia is
produced with a three-to-one hydrogen-to-nitrogen stoichiometric
ratio.
 Not all the hydrogen and nitrogen are converted to ammonia. The
unconverted hydrogen and nitrogen are separated from the
ammonia in the separator and re-cycled back to the synthesis gas
compressor and to the converter with fresh feed.
Kinetics
A catalyst (iron) is used to speed up the rate of reaction
and to lower the high activation energy in breaking the
N2 triple bond. However, if the temperature is too
high, it begins to get destroyed and must be changed
more regularly.
The reaction mechanism, involving the heterogeneous
catalyst, is believed to be as follows:
1. N2 (g) → N2 (adsorbed)

2. N2 (adsorbed) → 2N (adsorbed)

3. H2 (g) → H2 (adsorbed)

4. H2 (adsorbed) → 2H (adsorbed)

5. N (adsorbed) + 3H (adsorbed)→ NH3 (adsorbed)

6. NH3 (adsorbed) → NH3 (g)

Ammonia Separation
The removal of product ammonia is accomplished via mechanical
refrigeration or absorption/distillation. The choice is made by
examining the fixed and operating costs. Typically, refrigeration is
more economical at synthesis pressures of 100 atm or greater. At lower
pressures, absorption/distillation is usually favored.
 Effect of temperature and pressure
on Haber Process
In the Haber Process for the production of ammonia,
based on the reversible reaction:
N2(g) + 3H2(g) 2NH3(g)
it is observed that:
• As the total pressure increases, the amount of ammonia
present at equilibrium increases.
• As the temperature decreases, the amount of ammonia at
equilibrium increases.
 Haber Process

N2(g) + 3H2(g) 2NH3(g)

Industrial importance of ammonia and nitrogen compounds
derived from ammonia
Ammonia is also used for the production of plastics, explosives, nitric
acid HNO3(via the Ostwald process) and intermediates for dyes and
pharmaceuticals.

1. Mixture of air & ammonia heated to 230oC and is passed through a

metal gauze made of platinum (90%) & Rhodium (10%).
Pt
4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l) DH = -909 KJmol-1
Conditions: 1100K and 4-10 atm. pressure

 About 95% of the ammonia is converted into nitrogen monoxide

 Reaction produces a lot of heat energy.
 Energy is used to keep reaction vessel temp at 800oC.
2. Colourless nitrogen monoxide gas produced from 1st stage is then
reacted with oxygen from the air to form brown nitrogen dioxide gas
(NO2).
2NO (g) + O2 (g) 2NO2 (g)
3. The nitrogen dioxide is then dissolved in water to produce nitric acid.
3NO2 (g)+ H2O(l)  2HNO3 (aq) + NO (g)
Oxidizing properties of HNO3
Properties of nitric(v) acid

Concentrated nitric acid (66%) is a strong acid:

HNO3(aq) H+(aq) + NO3-(aq)

It is also an oxidising agent and reacts with most metals to form nitrates
and nitrogen oxides:

Cu(s) + 2HNO3(aq) Cu(NO3)2(aq) + H2O(l) + NO2(g)

Commercial uses of nitric acid:
Nitric acid is used to make fertilizers, explosives and the polyamide, Nylon

Fertilizers
 Fertilizer is a substance added to soil to improve the growth and yield
of plants. Modern synthetic fertilizers are composed mainly of
nitrogen, phosphorus, and potassium compounds with secondary
nutrients added.
 Ammonium salts are used as fertilizers because they contain nitrogen
in a form that plants can use. The fertilizer Nitram is ammonium
nitrate and is made from a solution of ammonia in water and nitric
acid in an acid-base reaction:
NH3(aq) + HNO3(aq) NH4NO3(aq)

Similar neutralization reactions with phosphoric(V) and sulphuric acids:

2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)