The Elements of Group 15 (5A, V, VA)
The Nitrogen Group
(Pnictogens/Pnicogens)
Pnictogens/Pnicogens)
N, P, As, Sb, Bi
Valence electron configuration: ns2np3
Standard States
Nitrogen is the most abundant element in our
atmosphere (78%), found as a diatomic gas.
N2(g) exhibits low reactivity, and is isolated from
condensed air. N2 boils at 77K(-196oC) and is can thus
serve as a refrigerant.
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Standard States
Hydrides
Phosphorus is a tetrameric solid (white phosphorus) in its
standard state (P4(s)), although it exists as many allotropes.
White phosphorus reacts with oxygen (combusts), so must be
stored under water. Formerly used in matches.
Ammonia (NH3) is produced in higher molar quantities than any other
chemical. It is mainly used for fertilizer production (80%), and for
explosives, textiles (e.g. rayon), and chemical synthesis.
ammonium
NH4+
2 NH4NO3(s) 2 N2(g) + O2(g) + 4 H2O(g)
∆H < 0
Ammonia is produced industrially by the Haber process:
380oC, 200 atm, Fe(s)
3 H2(g) + N2(g) 2 NH3(g)
This reaction must be run at high temperatures, catalyzed by iron filings.
Interestingly, it can be done at ambient temperatures, pressures by
nitrogenase, as follows (loosely):
Arsenic, antimony, and bismuth tend to exist as
rhombohedral solids, although (hot) gaseous Sb is a tetramer
(like P).
The Haber Process – Using Le Châ
Châtelier
to Our Advantage
hot zone: 400oC
This apparatus helps
push the equilibrium to
the right by removing the
ammonia (NH3) from the
system as a liquid.
Hydrides
Other hydrides of nitrogen.
Combustion of N2H4 (hydrazine)
is highly exothermic (used as
rocket fuel). It is also a good
reducing agent.
cooled zone: -50oC
b.p.(NH3) = -33oC
b.p.(H2) = -253oC
b.p.(N2) = -196oC
3 H2(g) + N2(g)
⇌ 2 NH (g)
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2
Hydrides
Phosphine (PH3) is a highly toxic, volatile gas. Its melting and
boiling points are lower than for NH3 due to lack of H-bonding.
Halides
Halides are known for all Group 15 elements. They are
commonly in the +3 or +5 oxidation state.
For nitrogen, the most commonly encountered form is NF3,
which is not reactive, and not a Lewis base like other halides.
Fluorines, being very electronegative, render the lone pair on
nitrogen inaccessible.
F
Phosphines of the form (R3P, R = alkyl or aryl group) are
important in coordination chemistry (Lewis bases towards
metal cations).
Analogous Sb and As compounds are frequently studied in
coordination chemistry.
The pentacoordinate forms are known for P, As, Sb, and Bi,
but not for N.
F
F
F P
F
F
These are bridged trigonal bipyramidal structures
F
F
F
F
SbF5 is a tetramer with
bridging fluoride groups
F
F
F
Sb
F
F
F
Other halides of this form are well known also (PF3, AsF3, BiF3)
Synthesis:
2As(s) + 3F2(g) 2AsF3(l)
F
F
Sb
F
F
F
F
F
F
N2O is a strong oxidizing agent, but is kinetically
slow:
N2O + 2H+ + 2e- N2 + H2O
Sb
F
Nitrogen can be found in a variety of different
oxidation states. Most oxo-compounds can be found
in the gaseous state
+1 oxidation state: N2O (nitrous oxide)
A colorless, unreactive gas that is used as an
anesthetic and as a propellant
Synthesis:
NH4NO3 N2O + 2H2O
Sb
F
F
Oxides
Halides
N
F
3
Oxides
Oxides
+3 oxidation state: N2O3 (dinitrogen trioxide)
N2O3 NO + NO2 (disproportionation)
+2 oxidation state: NO (nitric oxide)
A neutral, paramagnetic compound that exists as a
colorless gas
Extra electron exists in π* orbital, and bond order
is 2.5.
NO is generated in the body and serves to control
blood pressure and aid in neurotransmission
+4 oxidation state: NO2 (nitrogen dioxide), N2O4
(dinitrogen tetroxide). These interconvert at T> -11oC.
NO2 is a strong oxidizing agent
Disproportionates in basic solution
2NO2(aq) + 2OH- NO2-(aq) + NO3-(aq) + H2O
+2
+4
+3
+5
Oxides
+5 oxidation state: N2O5 (dinitrogen pentoxide)
Colorless, ionic solid but monomeric in gas phase
m.p. = 32oC
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Frost Diagram
Oxides
Phosphorus: commonly P4O10 and P4O6
P4O10 is obtained by burning elemental phosphorus in air
P4 + 5O2 P4O10
Hydrolysis yields acids:
P4O6 + 6H2O H3PO3 (phosphorous acid)
P4O10 + 6H2O H3PO4 (phosphoric acid)
P
O
O
O
P
P
O
O P O
O
P
O
O
O
P
O
O P O
P
O
O
O
These compounds are useful
dehydration reagents in the lab
P4O6
OxoOxo-ions
Nitrogen-group elements have many oxoanions and oxocations. Again,
surveying as a function of their oxidation states:
+1 oxidation state: N2O22- (hyponitrile)
OxoOxo-ions
+1 oxidation state: H2PO2- (hypophosphite)
(good reducing agent)
+4 oxidation state: P2O64- (hypophosphate)
(Bronsted base)
(good reducing agent)
+3 oxidation state: NO2- (nitrite)
(reducing agent; mild Lewis base)
+3 oxidation state:
NO+
(nitrosonium)
(oxidizing agent and strong Lewis acid)
Nitrous acid converts to NO+ in a stronger acid
HNO2 + H+ NO+ + H2O
P4O10
+5 oxidation state: NO3- (nitrate ion)
3-
O
O
+5 oxidation state: NO2+ (nitronium)
(good oxidizing agent; good Lewis acid)
O
P
P
4O
O
+5 oxidation state: PO43- (phosphate) and P2O74(diphosphate)
(oxidizing agent; very weak base)
O
O
O
P
O
O
O 4P
P
O O O O
O
O
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