Isolation/preparation and properties of the following non-metals:
Properties of allotropes of
carbon (only diamond and graphite),
MAIN TOPICS IN TMH BOOK
ALLOTROPES OF CARBON
Compounds relevant to study of Boron
Borax (Na two B four O seven) Calamnite (Ca two B six O eleven)
Boron Trioxide (B two O three) Boric Acid (H three B O three)
Boron Nitride (B N)
Sodium Carbonate (Na two C O three)
Boron (B) - Preparation and Properties
atomic number electronic configuration
Z = 5, 1s²2s²2px¹
main ores of Boron
Borax, Kernite, Colemanite and Orthoboric acid.
methods of obtaining Boron
1. By the reduction of boric oxide y an electropositive metal like magnesium.
2. By the reductin of volatile boron compounds by dihydrogen at high temperatures (1270K).
3. By the electrolytic reduction of fused borates or other boron compounds (e.g., KBF-4, potasium tetrafluoroborate) in molten KCL/KF at 1073K.
4. By the thermal decomposition of boron tri-iodided over red hot tungsten.
5. By thermal decomposition of boron hydrides and boron halides at about 1173K.
physical properties of Boron
1. Boron is an extremely hard solid next to diamond.
2. Its melting point is 2450K and boiling point is 3925K.
3. It is a poor conductor of heat electricity.
4. It has two isotopic forms - B-10 and B-11. Relative abundance 19% and 81% respectively.
chemical Properties of Boron
1. Combination with nonmetals: At room temperature, it reacts with flourine. Superficial reaction with oxygen. At higher termperatures, it reacts directly with all nonmetals except H, Ge, Te and nobles gases.
2. With water: It does not react with water even in the form of steam.
3. Acids: HCL does not react with boron.
When heated with concentrated sulphuric acid or nitric acid, boron is oxidized to boric acid.
Boron does not react with nonoxidizing acids.
4. Boron react with fused caustic alkalies like NaOH and KOH forming borates.
It dissolves in fused Na-2CO-3 and NaNO-3 mixture at 1173K.
5. Boron comines directly with almost all metals (except heavy metals) at higher temperatures.
Heavy Metals not combining with boron include Ag, Au, Cd, Hg, Ga, In, Tl, Pb, Sn, Bi etc.
Boron belongs to 13th group.
It has a very high melting point(2453 K).
It is extracted from minerals Borax(Na-2 B-4 O-7) or Calamnite (Ca-2 B-6 O-11)
Process of extracting Boron from Borax:
i) Borax is treated with concentrated hydrochloric acid. Boric acid is precipitated.
ii) boric acid is strongly heated. Boron trioxide is obtained (B-2 O-3).
iii) Boron trioxide is heated with Na, K or Mg pieces. Amorphous form of boron is obtained.
Alternatively mixture of boron tribromide vapours and hydrogen are passed over electrically heated filament of tungsten at 1200 degree centigrade. Crystalline form of Boron is obtained.
In another way, Boron trioxide can react with Aluminium to give Boron.
JEE question 2006
1. B(OH)-3 +NaOH two way reaction NaBO-2 +Na[B(OH)-4] + H-2O
How can this reaction is made to proceed in forward direction?
(A) addition of cis 1, 2 diol
(B) addition of borax
(C) addition of trans 1, 2 diol
(D) addition of Na-2HPO-4
Compounds relevant to study of silicon
Silica (Si O two)
Silicon Halide (Si X four) X is for halogen
silicon (Si) - Preparation and Properties
electronic configuration of Silicon
Atomic Number is 14. 1s²2s²2p^63s²3p²
methods of obtaining Silicon
1. Heating finely divided silica with magnesium powder.
2. Heating potassium silicoflouride with potassium metal.
3. Heating potassium silicoflouride wtih Al or Zn in an iron crucible.
4. Passing a current of SiCl-4 over molten aluminium.
physical Properties of Silicon
Silicon is available in two allotropic forms. the amorphous silicon and the crystalline or admantine silicon.
amorphous silicon is a dark brown powder which is insoluble in water.
Crystalline silicon forms pale yellow crystals.
chemical properties of silicon
1. Silicon burns in air or oxygen forming silicon dioxide.
2. With halogens, it forms halides, SiX-4.
3. With fused acqueous caustic alkalies, silicon forms alkali silicates with liberation of hydrogen.
4. silicon decomposes on red heating liberating hydrogen.
5. Metals like Magnesium and nonmetal carbon form silicides with silicon.
Compound relevant to study of Sulphur
Hydrogen Sulphide (H two S)
Sulphur Dioxide (S O two)
Sodium Thio-Sulphate (Na two S two O three)
Ions formed by Sulphur
Sulphide ion (S O three (two-))
thiosulphate ion (S two O three (two-))
Sulphur occurs in the native as well as combined form.Large quantites of sulphur are obtained from underground deposits in USA.
Partial combustion of Hydrogen sulphide produce sulphur.
Reaction of Hydrogen Sulphide with Sulphur dioxide also gives sulphur.
Sulphur occurs as S eight. It is a puckered ring with crown conformation.
several allotropic forms of sulphur are available.
Rhombic sulphur (or alpha sulphur) is the stablest form. It is obtained by evaporation of solution of sulphur in carbon disulphide.
At about 95 C - 96 C rhombic sulphur is changed into another allotropic form monoclinic sulphur (als known as prismatic or beta sulphur).
Monoclinic sulphure has needle shaped crystals.
Other allotropic forms are amorphous (or colloidal) and plastic sulphur (or Gamma sulphur).
Amorphous sulphur is obtained by (1) the action of dilute Hydrochloric acid on sodium thio sulphate solution, and
(2)by passing Hydrogen sulphide through dilute nitric acid.
Plastic sulphur is obtained by pouring boiling sulphur in cold water. This results in rapid cooling. Plastic sulphur consists of a completely random arrangement of chains of sulphur atoms. On standing it passes over to the crystalline rhombic sulphur.
Sulphur is also active element like Oxygen. It combines with a large number of metals and nonmetals.
Sulphur is oxidized by concentrated nitric acid and sulphuric acid.
It also reacts with hot concentrated solution of alkalies.
See for some more details and diagrams on sulphur
Flourine (F), Chlorine (Cl), Bromine (Br), Iodine (I).
Chlorine is yellow green gas
Bromine is reddish brown liquid
Iodine is steel grey solid
• Allotropes are elements that can exist in two or more different physical forms
• Diamond, graphite and buckminster fullerine are allotropes of carbon.
• The allotropes of carbon are all the element carbon. The type of carbon is determined from the bonding that occurs.
The carbon atoms in graphite are arranged in flat sheets that slide easily over each other, while the atoms in diamond are bonded in a complex, honeycombed structure that makes the solid much harder. The difference between the allotropes may be in the bonding present as in the diamond carbon example or it may be due to the crystal form produced as is common with most metals.
When compounds show a simlar property - different characteristic forms they are termed polymorphic
In 1830, Swedish chemist Jöns Berzelius (1779-1848) questioned whether a given element could exist in two or more forms with different chemical and physical properties. A decade later he suggested the name allotropy for this phenomenon. Berzelius knew of elements with this property--carbon, phosphorus, and sulfur in particular.
The properties of allotropes can be very different. Atmospheric oxygen (O2) and ozone (O3) are allotropes of oxygen. As a gas diatomic oxygen is essential for life whereas triatomic ozone is poisonous. Both forms occur naturally in the atmosphere, but diatomic oxygen is present throughout the atmosphere and ozone is normally present only in the upper atmosphere, except when it occurs as a pollutant at ground level or when it is manufactured by electrical discharge during storms. Diatomic oxygen is a more stable form than ozone.
With the exception of nitrogen, all group V elements show allotropy. For example, phosphorous occurs in three forms--white, red, and black. White phosphorous is poisonous and very reactive, red phosphorous is not poisonous and it is only moderately reactive, and black phosphorous is nearly inert. Each of these allotropes of phosphorous also has its own subset of forms.
Group VI elements also show allotropism. The classic example of this is rhombic and monoclinic sulfur. Both forms are ring structures containing eight sulfur molecules. Rhombic sulfur is the most stable form, but after heating and allowing the sulfur to melt and then cool down monoclinic sulfur is formed. Both are crystalline forms of sulfur, but their structures are slightly different. If monoclinic sulfur is left standing it will slowly revert back to the rhombic form over several days. These different forms of sulfur have slightly different characteristics, such as altered melting points. There are several other allotropes of sulfur that exist as well.
Sulfur also shows allotropes in the liquid form. These are generally due to the breaking down of the ring structure and having different ratios of broken and unbroken rings. This effect is also enhanced by unpaired electrons at the broken ends of the ring structure. These broken ends can bond with other broken chains giving long chain molecules with a very high viscosity and decreased mobility. This latter effect is due to increased tangling of the molecules. Sulfur vapor also shows allotropic behavior.
Allotropes of Carbon
Carbon has two allotropic forms - diamond and graphite. Other amorphous carbons are actually micro crystals of graphite.
Diamond, an allotrope of Carbon is face centered crystal. It is the hardest natural substance. It is a nonconductor of electricity. It has high refractive inded (2.45) and much of the light that falls on it is internally reflected.
Diamond burns in air at 900 C and in oxygen at 700 C and forms carbon dioxide.
Graphite has a layer like structure in the three dimensional space and this gives it the lubricating property.
The carbon content of various amorphous forms of graphite; Peat (60%), Lignite (70%), Bituminous coal (78%) Semibituminous coal (83%) and Anthracite Coal (90%). The residue that remains after destructive distilation of coal in the absence of air is coke.
• Graphite is a slippery black powder.
• Graphite is the only nonmetallic substance that conducts electricity.
• Each C atom is bonded to 3 other C atoms, forming one double bond and 2 single bonds.
• Every C atom in the graphite structure is bonded in the same way, with each carbon atom having one double bond and 2 single bonds.
• Graphite consists of a two dimensional layer in which C atoms are arranged in a series of regular hexagons.
• There are weak attractions between layers and the layers can readily slide past one another.
• By experiment, it has been determined that the 3 bonds associated with the C atoms are identical. Thus, the fourth pair of electrons is really delocalized. This accounts for the electrical conductivity of graphite.
• Diamond is a colorless crystal.
• Diamond is the hardest naturally occurring substance.
• Each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement.
• The tetrahedral arrangement of carbon atoms gives diamond its characteristic
physical properties such as:
o diamond is very hard
o diamond has a high melting point
o diamond is not easily compressed
o when given a good cut, diamond reflects light off of its facets.
• Diamond is actually a giant interlocking group of tetrahedrally arranged atoms. The bond angle between any two bonds in the crystal is 109.5o.
Allotropes of Phosphorus
Phosphorus has three main allotropes: white, red and black.
White phosphorus is poisonous and can spontaneously ignite when it comes in contact with air. For this reason, white phosphorus must be stored under water and is usually used to produce phosphorus compounds.
Red phosphorus is formed by heating white phosphorus to 250°C (482°F) or by exposing white phosphorus to sunlight. Red phosphorus is not poisonous and is not as dangerous as white phosphorus, although frictional heating is enough to change it back to white phosphorus. Red phosphorus is used in safety matches, fireworks, smoke bombs and pesticides.
Black phosphorus is also formed by heating white phosphorus, but a mercury catalyst and a seed crystal of black phosphorus are required. Black phosphorus is the least reactive form of phosphorus and has no significant commercial uses.
Allotropes of sulphur
Sulfur exists as a number of different allotropes. Below 95.6°C, the stable crystal form is rhombic, while above this temperature the element changes to a triclinic form. Both these forms contain cyclic S8 molecules. At temperatures just above its melting point, sulfur is a yellow liquid also containing S8 molecules. At about 160°C, the sulfur atoms link together in chains and the liquid becomes dark brown and more viscuous. If the molten sulfur is quickly cooled, for example, by pouring it into cold water, the result is a reddish-brown solid called plastic sulfur. Above 200°C the viscosity decreases.
Sulfur vapor contains a mixture of S2, S4, S6, and S8 molecules. So-called "flowers of sulfur" is a yellow powder obtained by subliming the vapor.
Yellow transparent crystals
Melt pt. 113 oC
Obtained when sulphur crystallises
Amber coloured needles
Melt pt. 119 oC
Obtained when sulphur
solidifies above 95.6 oC.
At atmospheric pressure the rhombic form is stable below 95.6 oC and the monoclinic form above this temperature. Only at the transition temperature can the two allotropes exist in equilibrium with each other.
When sulphur is heated it melts and undergoes a series of changes as the temperature rises.
Ozone (O three) is an allotropic form of Oxygen.
It is naturally formed above 20 Km from the earth from oxygen by absorbing sunlight. Ozone layer protects earth from concentration of ultraviolet rays. chlorofluorcarbons (C Cl two F two) used as refrigerant releases active chlorine after absorbing sunlight and this active chlorine decomposes ozone leading to destruction of ozone layer.
Statement - 1
Boron always forms covalent bond
Statement - 2
The small size of B^3+favours formation of covalent bond.
(A) Statement – 1 is True, Statement – 2 is True; Statement – 2 is a correct explanation for statement – 1
(B) Statement – 1 is True, Statement – 2 is True; Statement – 2 is Not a correct explanation for Statement – 1.
(C) Statement – 1 is True, Statement – 2 is False
(D) Statement – 1 is False, Statement – 2 is True
This point is explicitly given in Jauhar's book
JEE Question 2007 Paper I Linked Comprehension
The noble gases have closed-shell electronic configuration and are monoatomic gases under normal conditions. The low boiling points of the lighter noble gases are due to weak dispersion forces between the atoms and the absence of other interatomic interactions. The direct reaction of xenon with fluorine leads to a series of compounds with oxidation numbers +2, +4 and + 6. 4 XeF reacts violently with water to give XeO-3 . The compounds of xenon exhibit rich stereochemistry and their geometries can be deduced considering the total number of electron pairs in
the valence shell.
1. Argon is used in arc welding because of its
(A) low reactivity with metal
(B) ability to lower the melting point of metal
(D) high calorific value
Argon provides inert atmosphere in welding due to low reactivity with metal.
2. The structure of XeO-3 is
Coorect choice: C
3. XeF-4 and XeF-6 are expected to be
(D) strong basic