Download CHAPTER-7 The p-Block Elements Block

CHAPTERCHAPTER-7
The pp-Block Elements
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
Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the
formation of OH ions.
NH3(g) + H2O(1) ⇌ NH4+ (aq) + OH- (aq)
It forms ammonium salts with acids, e.g. NH4Cl, (NH4)2 SO4etc. As a weak weak base,
b
it precipitates the hydroxides (hydrated oxides in case of some metals of many metals
from their salt solutions.
Sulphuric acid is manufactured by the contact process which involves three steps:
Burning of sulphur or sulphid ores in air to generate SO2.
Conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst
(V2O5), and
Absorption of SO3 in H2SO4 to give oleum (H2S2O7.)
With cold and dilute alkalies chlorine produces a mixture of chloride and hypochlorite
but with hot and concentrated alkalies it gives chloride and chlorate.
2naoh + Cl2 → nacl + naocl + H2O
(cold and cilute)
6 naoh + 3Cl2 → 5nacl + naclo3 + 3H2O
(hot and conc.)
With dry slaked lime it gives bleaching powder.
2Ca(OH)2 + 2Cl2 → Ca(ocl)2 + cacl2 + 2H2O
The composition of bleaching powder is Ca(ocl)2. Cacl2. Ca(OH)2. 2H2O
Nitrogen was discovered by Rutherford.
Nitrogen forms only trihalides (NCl3) but not pentahalides (NCl5).
Oxidation state of nitrogen varies form – 3 to + 5,
Nitrogen shows a maximum covalency of four (NH )
NCl3 is extremely explosive yellow liquid.
Nitrogen exhibits oxidation state of – 1/3 in N3H (hydrazoic acid).
Acidic character of the oxides increases with increasing oxidation number of nitrogen.
Nitric oxide forms reddish brown fumes on exposure to air due to formation of NO2.
Nitrogen is the third most electronegative elements.
N2O (nitrous oxide) is called laughing gas.
Nitrogen is neutral and diamagnetic.
N2O3 is called nitrogen sesquioxide.
NO is paramagnetic, while N2O2 is diamagnetic. Similarly NO2 is paramagnetic while
N2O4is diamagnetic.
Helium is a non- inflammable and light gas, Hence, it is used in filling balloons for
meteorological observations. It is also used in gas- cooled nuclear reactors.
Liquid helium (b.p. 4.2 K) finds use as cryogenic agent for carrying out various
experiments at low temperatures. It is used to produce and sustain powerful
superconducting magnets xhich form an essential part of modern NMR spectrometers and
magnetic Resonance Imaging (MRI) systems for clinical diagnosis it is used as a diluents
for oxygen in modern diving apparatus because of its very low solubility in blood.
Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes.
Neon bulbs are used in botanical gardens and in green houses.
Argon is used mainly to provide to provide an inert atmosphere in high temperature
metallurgical processes (are welding of metals or alloys and for filling electric bulbs. It is
also used in the laboratory for handling substances that are air sensitive. There are no
significant uses of Xenon and Krypton. They are used in light bulbs designed for special
purposes.
Groups 13to18 of the periodic table consist of p-block elements with their valence shell
electronic configuration ns2np1-6.
Group 15 consists of five elements namely. N, P, As. Sb and Bi which have general
electronic configuration ns2np3,nitrogen differs from other elements of this group due to
small size.
Carbon dioxide is also known as black damp. It is because it is a source of danger to coal
miners and well sinkers.
In p-block of the periodic table, noble gases (Group 18) show maximum similarity among
themselves. Halogens (Group 17) also show similarity among themselves bur in other
groups, the first member of each group differ from the other elements of the group.
B, C, N, O, F seldom forms compounds in which coordination number exceeds four.
Boron can also be produced by any one of the following methods:
By electrolytic reduction of fused borate r tetrafuroborates (KBF4) in molten
KCl/KF at 1073 K.
By the reduction of volatile boron compound by H2 at 1270K over heated
1270 K
tantalum filament.
3BCl3(g) + 3H2(g)
2B(s) + 6HCl(g)
Boron of high purity ( - 99.9% ) can be obtained by this method.
By thermal decomposition of bareness and boron halides at about 1173 K.
Diborance is manufactured by the reduction of BF3 with NaH.
2BF3 + 6NaH
450 K
B2H6 + 6NaF
Higher diboranes are formed when B2H6 id heated at 373 – 525 K.
Diborane is a colourless, highly toxic gas with b.p of 180 K.
Formation of Pπ-Pπmultiple bonds with itself and with highly electronegative atom like
O or C and nin-availability of d orbitals to expand its valence shell.
Elements of group 15 show gradation in properties they react with oxygen.
Hydrogen and halogens, They exhibit two important oxidation states. + 3 and + 5 but +3
oxidation is favored by heavier elements due to inert pair effect’.
Phosphorus exists as P4 in elemental form. It exists in several allotropic forms. It forms
hydride. PH3 which is a highly poisonous gas. It forms two types of halides as PX3 and
PX5.
Pcl3 is prepared by the reaction of which phosphorus with dry chlorine while pcl5 is
prepared by the reaction of phosphorus with SO2Cl2.
Phosphorus forms a number of oxoacids.
The group 16 elements have general electronic configuration ns2np4. They show
maximum oxidation state, +6.
The enzyme nitrogenase helps in t he fixation of atmospheric N2 in plants. In nitrogenase
the active sites contain Fe and Mo atoms.
Nitric oxide is very reactive and harmful in nature. It occurs in biological system in
traces. It acts as a neurotransmitter and plays a significant role I controlling blood
pressure by relaxing blood vessels. It also provides protection from bacterial infections.
Anhydrous NHO3 is obtained by distilling cone. Aqueous HNO3 with P4O10.
O2 is paramagnetic while O3 is diamagnetic.
Mixing conc. HNO3 with cone. H2SO4 results in the formation of N2O5,which in a
strongly polar solvent exists in the form of its solvated ions NO
and NO
C3O3 has a linear structure in which each carbon atom is sp2-hybridised. O = C = C = C =
O
Dinitrogen monoxide (N2O) is used as an anaesthetic.
Assuming that each isomer of B10C2H12 has a icosahedral structure, the number of
possible isomers are three. This is because (a) the two carbon atoms maybe at adjacent
positions, (b) may have one boron atom between them or (c) may be on opposite sides of
icosahedrons. The 12 positions of icosahedrons are all n herently equivalent.
Like B2H6, other borohydrides like B4H10 and B5H11 have 3c – 2e bonds.
Gradation in physical and chemical properties is observed in the group 16 elements. In
laboratory, dioxygen is prepared by heating kclo3 in presence of mno2. It forms a number
of oxides with metals.
Allotropic form of oxygen is O3 which is a highly oxidizing agent. Sulphur forms a
number of allotropes.
Group 17 of the periodic table consists of the following elements F, Cl, Br I and At.
These elements are extremely reactive and as such they are found in the combined state
only. The common oxidation state of these elements is -1.
Highest oxidation state can be +7. They show regular gradation in physical and chemical
properties. They form oxides, hydrogen halides, interhalogen compounds and oxoacids.
Chlorine is conveniently obtained by he reaction of hcl with kmno4.hcl is prepared by
heating nacl with concentrated H2SO4.
Group 18 of the periodic table consists of noble gases. They have ns2np6 valence shell
electronic configuration except he which has 1s2.
All the gases except Rn occur in atmosphere. Rn is obtained as the decay product of
266
Ra.
Properties nitrogen
Nitrogen is a colourless, tasteless and odourless gas. It is slightly lighter than air. It is
only slightly soluble in water. It is neither combustible nor supporter of combustion.
Like CO2, it is not poisonous but, animals die in an atmosphere of N2 for want of
oxygen.
With hydrogen, it forms ammonia (Haber’s process) under pressure of 100-1000
atmospheres in the presence of catalyst
Fe/Mo ; Fe/Mo
N2 + 3H2 400-550 0C ;
∆H f = - 92.2kJ mol-1
2NH3;
With oxygen, it forms nitric oxide. This reaction is used in Birkeland and Eyed
process for the manufacture of NHO3
3300K
N2 + O2
2NO
;
∆H f = + 135kJ mol-1
With metals, like Li, Mg, Ca, Al, Si, etc. if forms nitrides (N3-). The formation of
NO and nitrides are two important processes involved in the fixation of nitrogen.
Nitrides react with water to form NH3.
6Li + N2 →2Li3N,
3Ca + N2 → Ca3N2, 3Mg + N2 → Mg3N4
Uses of Nitrogen
In the manufacture of nitric acid, ammonia, calcium cyanamide and other nitrogen
compounds.
Liquid nitrogen is used as a refrigerant.
It provides inert atmosphere in metallurgical operations.
For preparing gas thermometers and for filling electric bulbs.
Ammonia, in general, is a non-combustible hydride.
Microcosmic salt [Na (NH4). HPO4]. 4H2O on heating liberates ammonia.
Ammonia is used for removing grease.
Liquid ammonia is commonly used in refrigeration due to its high heat of
vaporization.
Ammonia has dielectric constant value of 22 which is lower than water (82).
Liquor ammonia is concentrated solution of NH3 in water while liquid ammonia is
liquefied ammonia gas.
N3H is hydrazoic acid. It has following canonical structures
. .
. .
+
H–N=N =N:
. .
-
+
H–N–N ≡N:
. .
HNO3 only acts as oxidizing agent because in it nitrogen has +5 oxidation state which
is maximum.
HNO2 acts both as reducing and oxidizing agent because nitrogen is in +3 oxidation
state which can be increased or decreased.
All intrates are soluble in water.
HNO3 oxidizes sucrose to oxalic acid.
C12H22O11
+
18[O]
→
6[COOH]2 + 5H2O
from HNO2
HNO3 is yellow in colour due to the dissolution of NO2 gas formed during the
decomposition of some HNO3.
4HNO3 → 4NO2 + 2H2O + O2
The yellow colour of the acid is removed by passing CO2 gas through it when NO2
gets removed.
Fuming nitric acid is conc. HNO3 saturated with NO2.
Ordinary oxygen contains three isotopes of atomic weight 16, 17 and 18 in the ratio
of about 10, 000 : 1 : 8.
Oxygen is absorbed by alkaline pyrogallol.
Oxyacetylene flame is used for welding purposes.
In s and p-block elements basic character of oxides increases on moving down the
group.
Amongst non-metals, acidic character of oxides increases as the oxidation number of
non- metal increases.
Boron trifluoride is prepared by heating B2O3 with CaF2 and conc. H2SO4
B2O3 + 3CaF2 + 3H2SO4 → 2BF3 + 3CaSO4 + 3H2O
Hydrolysis of boron trihalides. Boron trihalides fumes in moist air and are highly
sensitive to moisture. Boron trifluoride undergoes slow and partial hydrolysis with water
while other halides react violently with water.
BCl3 + 3H2O → B(OH)3 + 3HCl
The basic structural unit in borate have one or more boron atoms linked together by
intervening oxygen atoms in the form of chains or rings. For example, correct structure of
borax Na2[B4O5(OH)4]8H2O is
OH
OH
OH
OH
Na O – B – O – B – O – B – O – B – O
Of the two naturally occurring isotopes of boron (10B and 11B) only 10B absorbs neutrons.
Boron nitride is used n nuclear industry as protective shields and control rods. This
special property of
10
B is of current intersest in the possible use of boron compounds in
cancer chemotherapy.
Borazinc on heating or passing electric discharge through it given a product similar to
naphthalene called inorganic naphthalene.
Dry powder type fire extinguishers contain said and NaHCO3. Acid type fire
extinguishers contain NaHCO3 and cone. H2SO4.
Fullerenes are insulators but potassium buckide (K3C60) is a stable metallic crystal which
is superconducting below 18 K.
The acidic strength o oxides of nitrogen increased in the order
N2O < NO < N2O3 < N2O4 < N2O5
Oxygen shows oxidation states of - 2 (in oxides), -1(in peroxides), - ½ (in superoxide), 0
(in dioxygen), + 2 (in OF2) and + 1 (in O2F2).
An aqueous solution of borax is alkaline due to hydrolysis.
Na2B4O7 + 7H2O
4H3BO3 + 2NaOH
Carnalite (KCl.MgCl2.6H2O) does not contain boron.
BBr3 > BCl3 > BF3. It can be explained on the basic of back donation of unshared
electron from halogen to the vacant p-orbital of boron.
Kernite is Na2B4O7.2H2O
Actually Diborance has two three centered B – H – B bonds.
Boric acid is not used n termite welding.
Formation trihalides does not support anomalous behavior of B.
Boric acid is polymeric due to hydrogen bonding (See answer to Q.17).
Boric acid is monobasic.
H3BO3 + H2O
H+ + [B(OH)4]-
Borax is not used as fuel in rockets.
B2H6 + 4BR3 → 3B2H2R4. Only four terminal hydrogen atoms are replaced by R group
whereas bridging hydrogen atoms cont get substituted.
Instead it contains B –H – B banana bond which is a three centered two electron bond.
Water gas is also known as blue gas.
Prussic acid is HCN.
Carbon suboxide (C3O2) is a colourless gas at room temperature. However, it readily
polymerises to coloured solids.
Carbon does not show inert pair effect.
Carbons differ from other elements of the group 14, because of its smaller size, high
electro negativity, catenation and non-availability of d-orbitals in its valence shell.
Colloidal solution of graphite in water is called aquadag.
Graphite is used as a lubricant.
The most stable allotropic form of carbon is graphite.
Each carbon atom in graphite is sp2hybridised.
Diamond is not oxidized by K2Cr2O7/H2SO4 at 2000C.
Carbon dioxide, CO2 is needed by plants for their growth.
Carbon dioxide molecule is linear O = C = O.
Carbon monoxide is not a good oxidizing agent.
Carbon in carbon dioxide is sp2-hybridised.
Carbon monoxide is absorbed by a mmoniacal cuprous chlored.
Carbon has no tendency to form complex compounds, as it has no d-orbitals in its valence
shell.
Graphite is similar to BN in structure.
Carbon atoms in graphite show sp2-hybridisation.
Catenation is formation of long chains (or rings) of similar atoms.
Carbon black is used for making printer’s ink, lack pain etc.
Diamond reacts with F2 at 1000 K to give CF4.
Coal gas is mainly a mixture of C114 and H2.
CO is often used in the preparation of metal carbonyl.
Na2CO3 does not decompose in heating.
Sn(OH)2, Pb(OH)2 and Cr(OH)3 are amphoteric oxides which react with HCl as well as
NaOH.
Al2O3, SO2 and ZnO react with aqueous solution of sodium hydroxide.
N2O and C3O2 are referred as suboxides as they contain relatively low proportions of
oxygen.
NO and N2O are neutral oxides.
O3 is readily absorbed by turpentine oil because turpentine contains unsaturated
compounds which form ozonides with ozone.
When O3 oxidises SnCl2 and SO2 no O2 is produced.
O3 is an allotrope of O2.
Energy is absorbed during the conversion of O2 into O3.
Properties of hydrides of group 15 Elements
Property
NH3
PH3
Ash3
Sbh3
Bih3
Melting point/k
195.2
139.5
156.7
185
-
Boiling point/k
238.5
185.5
210.6
245.6
290
(E-H) Distance/pm
101.7
141.9
151.9
170.7
-
HEH angle (o)
107.8
93.6
91.8
91.3
-
∆f Hθ / kj mol-1
-46.1
13.4
66.4
145.1
278
∆disshθ(E-H) /kjmol-1
389
322
297
255
-
Oxoacids of Phosphorus
Name
Formula
Oxidation
State of
Phosphors
Characteristic
bonds and their
number
Preparation
Hypophosphorous
H3PO2
+1
One P – OH
(Phosphinic)
White P4 + alkali
Two P – H
One P = O
Orthophosphorous
H3PO3
+3
Two P – OH
(Phosphonic)
O2O3 + H2O
One P – H
One P = O
Pyrophosphorous
H4P2O5
+3
Two P – OH
Pcl3+H3PO3
Two P – H
Two P = O
Hypophosphoric
H4P2O6
+4
Four P – OH
Red P4 + alkali
Two P = O
One P – P
Orthophosphoric
H3PO4
+5
Three P – OH
P4O10+H2O
One P = O
Pyrophosphoric
H4P2O7
+5
Four P – OH
Heat phosphoric acid
Two P = O
One P – O – P
Metaphosphoric
(HPO3)n +5
Three P – OH
Phosphorus
Three P = O
Br2, heat in a sealed
Thre P – O – P
tube
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acid
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