Lecture 14
Liquid nitrogen
VA group. Nitrogen and
Phosphorous and their
compounds.
PhD Halina Falfushynska
Atomic and Physical Properties of Group 15 Elements
Electronic Configuration. The valence shell electronic configuration of these elements
is ns2np3. The s orbital in these elements is completely filled and p orbitals are halffilled, making their electronic configuration extra stable.
Atomic and Ionic Radii. Covalent and ionic (in a particular state) radii increase in size
down the group. There is a considerable increase in covalent radius from N to P.
However, from As to Bi only a small increase in covalent radius is observed. This is due
to the presence of completely filled d and/or f orbitals in heavier members.
Ionisation Enthalpy. Ionisation enthalpy decreases down the group due to gradual
increase in atomic size. Because of the extra stable half-filled p orbitals electronic
configuration and smaller size, the ionisation enthalpy of the group 15 elements is
much greater than that of group 14 elements in the corresponding periods. The order
of successive ionisation enthalpies, as expected is ΔH1 < ΔH2 < ΔH3
Physical Properties. All the elements of this group are polyatomic. Dinitrogen is a
diatomic gas while all others are solids. Metallic character increases down the group.
Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and
bismuth is a metal. This is due to decrease in ionisation enthalpy and increase in
atomic size. The boiling points, in general, increase from top to bottom in the group
but the melting point increases up to arsenic and then decreases up to bismuth.
Except nitrogen, all the elements show allotropy.
Preparation of dinitrogen
• In the laboratory, dinitrogen is prepared by treating an
aqueous solution of ammonium chloride with sodium
nitrite.
NH4CI(aq) + NaNO2 (aq) → N2 (g) + 2H2O(l) + NaCl (aq)
• It can be obtained by the thermal decomposition of
ammonium dichromate.
(NH4)2Cr2O7 (Heat) ⎯⎯⎯→ N2 + 4H2O + Cr2O3
• Very pure nitrogen can be obtained by the thermal
decomposition of sodium or barium azide.
Ba(N3)2 → Ba + 3N2
• Air (4N2 + O2) + C → 4N2 + CO2
• NH3 + 3O2 → 2N2 + 6H2O
• 2NH3 + 3Cl2 → N2 + 6HCl
Reactivity towards
hydrogen
All the elements of Group 15
form hydrides of the type EH3
where E = N, P, As, Sb or Bi.
N2(g) + 3H2(g) (773 k) ==>
2NH3(g); ΔH= –46.1 kJmol–1
Р4 + 6Н2 (heat, p) ==> 4РН3
The stability of hydrides decreases
from NH3 to BiH3 . the reducing
character
of
the
hydrides
increases. Ammonia is only a mild
reducing agent while BiH3 is the
strongest reducing agent amongst
all the hydrides. Basicity also
decreases in the order NH3 > PH3
> AsH3 > SbH3 > BiH3.
Reactivity towards
oxygen
All these elements form two
types of oxides: E2O3 and
E2O5. The oxide in the higher
oxidation state of the
element is more acidic than
that of lower oxidation state.
Their acidic character decreases
down the group. The oxides
of the type E2O3 of nitrogen
and phosphorus are purely
acidic.
N2 (g)+ O2 (g) (heat) ==> 2NO (g)
P4 + 5O2 (heat) ==> 2P4O10
Reactivity
towards halogens
These elements react to form
two series of halides: EX3 and
EX5. Nitrogen does not form
pentahalide. Pentahalides are
more covalent than trihalides. All
the trihalides of these elements
except those of nitrogen are
stable.
In case of nitrogen, only NF3 is
known to be stable.
P4 + 6Cl2 ==> 4PCl3
3PCl5 + 2P ==> 5PCl3
3PCl5 + P2O5 ==> 5POCl3
Reactivity towards
metals
All these elements react with
metals to form their binary
compounds exhibiting –3
oxidation state, such as,
Ca3N2
(calcium
nitride)
Ca3P2 (calcium phosphide),
Na3As2 (sodium arsenide),
Zn3Sb2 (zinc antimonide) and
Mg3Bi2
(magnesium
bismuthide).
3Mg + N2 ==> Mg3N2
Mg3N2 + 6H2O ==> 3Mg(OH)2 +
2NH3
Ammonia
• Ammonia is present in small quantities in air and soil
where it is formed by the decay of nitrogenous organic
matter e.g., urea.
• NH2CONH2 +2H2O → (NH4)2CO3 → 2NH3 + H2O+ CO2
• On a small scale ammonia is obtained from ammonium
salts which decompose when treated with caustic soda
or lime.
2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2
(NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4
• On a large scale, ammonia is manufactured by Haber’s
process.
• N2(g) + 3H2(g) → 2NH3(g); ΔH = – 46.1 kJ mol-1
Flow chart for the manufacture of ammonia
Phosphine
• Phosphine is prepared by the reaction of calcium
phosphide with water or dilute HCl.
• Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3
• Ca3P2 + 6HCl → 3CaCl2 + 2PH3
• In the laboratory, it is prepared by heating white
phosphorus with concentrated NaOH solution in
an inert atmosphere of CO2.
• Р4 + 3КОН + 3Н2О → РН3 + 3КН2РО4
• PH4I (phosphonium iodide)+KOH→KI + H2O+PH3
Properties of Ammonia
• Ammonia gas is highly soluble in water. Its aqueous solution
is weakly basic due to the formation of OH– ions.
NH3(g) + H2O(l) → NH4+ (aq) + OH– (aq)
• 3CuO + 2NH3 → 3Cu + 3H2O + N2
• It forms ammonium salts with acids, e.g., NH4Cl, (NH4)2SO4,
etc. As a weak base, it precipitates the hydroxides of many
metals from their salt solutions.
2FeCl3+3NH4OH→Fe2O3xH2O (brown ppt)+ 3NH4Cl
• The ammonia molecule can act is a Lewis base
Cu2+ (aq, blue) + 4 NH3(aq, deep blue) → [Cu(NH3)4]2+(aq)
Ag+ (aq, colourless) Cl- → AgCl (s, white ppt)
AgCl (s, white ppt)+2NH3 (aq) →[Ag(NH3)2]Cl)(aq,colourless)
Properties of Ammonia
Ammonia Fountain. Demonstration Preparation of Potassium Amide.
Potassium amide is prepared by
of the high solubility of gaseous
dissolution of potassium in liquid
ammonia in water
ammonia.
Nitrogen oxides
Nitric acid
• In the laboratory, nitric acid is prepared by heating
KNO3 or NaNO3 and concentrated H2SO4 in a glass
retort: NaNO3 + H2SO4 → NaHSO4+ HNO3
Preparation of Nitric acid
Nitric acid
• In aqueous solution, nitric acid behaves as a
strong acid giving hydronium and nitrate ions.
• HNO3(aq) + H2O(l) → H3O+ (aq) + NO3– (aq)
• Concentrated nitric acid is a strong oxidising
agent and attacks most metals except noble
metals such as gold and platinum.
• 8HNO3 (dilute)+ 3Cu → 3Cu(NO3)2 + 2NO + 4H2O
• 4Sn + 10HNO3  4Sn(NO3)2 + NH4NO3 + 3H2O
• Some metals (e.g., Cr, Al) do not dissolve in
concentrated nitric acid because of the formation
of a passive film of oxide on the surface.
• Concentrated HNO3 also oxidises non–metals and
their compounds. Iodine is oxidised to iodic acid,
carbon to carbon dioxide, sulphur to H2SO4, and
phosphorus to phosphoric acid.
• I2 + 10HNO3 → 2HIO3 + 10 NO2 + 4H2O
• C + 4HNO3 → CO2 + 2H2O + 4NO2
• S8 + 48HNO3(conc.) → 8H2SO4 + 48NO2 + 16H2O
• P4 + 20HNO3(conc.) → 4H3PO4 + 20 NO2 + 4H2O
Brown Ring Test
• The familiar brown ring test for nitrates depends
on the ability of Fe2+ to reduce nitrates to nitric
oxide, which reacts with Fe2+ to form a brown
coloured complex.
Devarda's test
• Devarda's alloy (Cu/Al/Zn) is a reducing agent.
When reacted with nitrate in sodium
hydroxide solution, ammonia is liberated.
• 3 NO−3 + 8 Al + 5 OH− + 18 H2O → 3 NH3 + 8
[Al(OH)4]−
• Aluminium is the reductant in this reaction.
Test for Ammonia
Test for Ammonia using
Nessler's Agent. Ammonia is
tested in a 1:10 dilution row
using K2[HgI4].
Phosphorus. Allotropic Forms
3s2 3p3
15P
3d0
2s2 2p6
1s2
• White phosphorus is a translucent white waxy
solid. It is poisonous, insoluble in water but
soluble in carbon disulphide and has
chemiluminescence.
• Р4 + 3NaOH + 3H2O → PH3 + NaH2PO2
• Р4 + 3О2 → 2Р2О3
• Р2О3 + О2 → Р2О5
• 3Mg + 1/2P4 → Mg3P2
• 2P + 3Cl2 → 2PCl3
White phosphorus
White phosphorus exposed to air glows
in the darkness
Red phosphorus
• It is obtained by heating white phosphorus at 573K
in an inert atmosphere for several days. When red
phosphorus is heated under high pressure, a series
of phases of black phosphorus are formed.
• Red phosphorus possesses iron grey lustre. It is
odourless, non-poisonous and insoluble in water as
well as in carbon disulphide.
• Chemically, red phosphorus is much less reactive
than white phosphorus. It does not glow in the
dark.
Black phosphorus
• It has two forms α-black phosphorus and β-black
phosphorus.
• α-Black phosphorus is formed when red
phosphorus is heated in a sealed tube at 803K. It
can be sublimed in air and has opaque monoclinic
or rhombohedral crystals. It does not oxidise in
air.
• β-Black phosphorus is prepared by heating white
phosphorus at 473 K under high pressure. It does
not burn in air up to 673 K.
Phosphorus Tri- and Pentachlorides
They are obtained by the action of thionyl chloride
with white phosphorus.
• P4 + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2
• P4 + 10SOCl2 → 4PCl5 + 10SO2
Reaction of Phosphorus Halides
PCl3 + 3H2O → H3PO3 + 3HCl
PCl5 + H2O → POCl3 + 2HCl
POCl3 + 3H2O → H3PO4 + 3HCl
CH3COOH + PCl5 → POCl3 + HCl + CH3COCl
H2SO4 + PCl5 → POCl3 + HCl + SO2(OH)Cl
POCl3 + 3H2O → H3PO4 + 3HCl
POCl3 + 3ROH → PO(OH)3 + 3RCl
2Ag + PCl5 → 2AgCl + PCl3
Sn + 2PCl5 → SnCl4 + 2PCl3
Oxoacids of Phosphorus
Properties of phosphorus oxoacids
and their salts
• The acids which contain P–H bond have strong reducing
properties.
• 4 AgNO3 + 2H2O + H3PO2 → 4Ag + 4HNO3 + H3PO4
• Acids in +3 oxidation state of phosphorus tend to
disproportionate to higher and lower oxidation states.
4H3PO3 → 3H3PO4 + PH3
• 3H(PH2O2) + 2HNO3  3H2(PHO3) + 2NO + H2O
• 5H4P2O6 + 3H2SO4 + 2KMnO4 + 2H2O  10H3PO4 +
2MnSO4 + K2SO4
• Salts of phosphorus oxoacids hydrolyze and base or
neutral medium occurs
Applications of nitrogen compounds
• As a modified atmosphere, pure or mixed with
carbon dioxide, to preserve the freshness of
packaged or bulk foods
• Nitrogen can be used instead of CO2 to pressurize
kegs of some beers, in particular, stouts and British
ales, due to the smaller bubbles it produces, which
make the dispensed beer smoother and headier
• Liquid nitrogen is used in the cryopreservation of
blood, reproductive cells (sperm and egg), and
other biological samples. It is used in the clinical
setting in cryotherapy to remove cysts and warts on
the skin.
Applications of nitrogen compounds
• Nitrous oxide (N2O), "laughing gas“, was discovered
early in the 19th century to be a partial anesthetic,
though it was not used as a surgical anesthetic until
later.
• Nitrogen-containing drugs are drugs derived from
plant alkaloids, such as morphine (there exist many
alkaloids known to have pharmacological effects; in
some cases, they appear natural chemical defenses
of plants against predation). Drugs that contain
nitrogen include all major classes of antibiotics and
organic nitrate drugs like nitroglycerin and
nitroprusside that regulate blood pressure and heart
action by mimicking the action of nitric oxide.
Applications of phosphorous compounds
• White phosphorus, called "WP" (slang term "Willie
Peter") is used in military applications as incendiary
bombs, for smoke-screening as smoke pots and
smoke bombs, and in tracer ammunition.
• The spontaneous combustion of phosphine is
technically used in Holme’s signals. Containers
containing calcium carbide and calcium phosphide
are pierced and thrown in the sea when the gases
evolved burn and serve as a signal.
• Phosphine is also used in smoke screens.