We shall discuss about p-block elements for chemistry 11th. Carbon, silicon, germanium, tin and lead are the members of group 14 .Carbon is the seventeenth most abundant element by mass in the earth crust. It is widely distributed in nature in free as well as in the combined state. In elemental state, it is available as coal, graphite and diamond. However, in combined state it is present as metal carbonates, hydrocarbons and carbon dioxide gas (0.03%) in air.
Naturally occurring carbon contains two stable isotopes 12C and 14C. Silicon is the second (27.7% by mass) most abundant element on the earth’s crust. It is present in nature in the form of silica and silicates. Germanium exists only in traces. Tin occurs mainly as cassiterite, SnO2 and lead as galena, PbS.
Atomic and Physical properties of P-Block Elements for Chemistry 11th group 14
Electronic Configuration:
The valence shell electronic configuration of group 14 elements is ns2np2. The inner core of the electronic configuration of elements in this group also differs.
Covalent Radius:
There is a considerable increase in covalent radius from C to Si. Thereafter from Si to Pb a small increase in radius is observed. This is due to the presence of completely filled d and f orbitals in heavier elements
Ionisation enthalpy
The first Ionisation enthalpy of group 14 elements is higher than the corresponding members of group 13. In general the Ionisation enthalpy decreases down the group. Small increase in Ionisation enthalpy from Si to Ge to Sn and slight increase from Sn to Pb is the consequence of poor shielding effect intervening d and f orbitals and increase in size of the atom.
Electronegativity:
Due to small size, the elements of this group are slightly more electronegative than group of 13 elements. The electronegativity values for elements from Si to Pb are almost the same.
Physical properties:
All the elements of this group are solid. Carbon and silicon are non metals, germanium is a metalloid and tin and lead are soft metals with low melting and boiling points of group 14 elements are much higher than those of corresponding elements of group 13.
Chemical properties:
Oxidation states:
The elements of this group have four electrons in outermost shell. The common oxidation state of these elements are +4 and +2. Carbon also exhibits negative oxidation state since the sum of the first four Ionisation enthalpies is very high.
In heavier elements, the tendency to show +2 oxidation state increases in the sequence Ge < Sn < Pb . It is due to increase in inert pair effect. Carbon and silicon mostly show +4 oxidation state. Germanium forms stable compounds in +4 oxidation state and only few compounds in +2 state. Tin forms compounds in both oxidation states but in +2 state is a reducing agent. Lead compounds in +2 state are stable and in +4 state are strong oxidising agent.
Reactivity towards oxygen:
The elements of this group form two types of oxides as monoxide MO and MO2. SiO only exists at high temperature. Oxides in higher oxidation states are generally more acidic than those in lower oxidation states. The dioxides of carbon, silicon and germanium are acidic, whereas of tin and lead are amphoteric in nature. Among monoxides , CO is neutral, GeO is acidic whereas SnO and PbO are amphoteric.
Reactivity towards water:
Carbon, silicon and germanium are not affected by water. Tin decomposes steam to form dioxide and dihydrogen gas
Sn + 2H2O → SnO2 + 2H2
Lead is unaffected by water due to formation of a protective oxide film.
Reactivity towards halogens:
The elements of this group form halides of formula MX2 and MX4 (where X= F, Cl, Br, I). Except , all other members react directly with halogens under suitable conditions to make halides. Most of the MX4 are covalent in nature. Exceptions are SnF4 and PbF4, which are Ionic in nature. Stability of dihalides increases down the group. If we consider the thermal and chemical reactivity, GeX4 is more stable than GeX2 whereas PbX2 is more than PbX4. Except CCl4, other tetra chlorides are easily hydrolysed by water by accepting lone pair of electrons from water molecules in d orbitals.
Anomalous behaviour of carbon:
Carbon also differs from rest of the members of its group due to its smaller size, high electronegativity, high Ionisation enthalpy and absence of d-orbitals.
- The maximum covalency of carbon is four whereas other elements can expand their covalency due to the presence of d-orbitals.
- Carbon also has unique ability to form pπ-pπ multiple bonds with itself and with other atoms of small size and high electronegativity. Ex:- C=C, C=O, C=S and C≡N. Heavier elements do not form pπ-pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.
- Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called catenation. Down the group the tendency to show catenation decreases due to increase in size and decrease in electronegativity.
- Due to property of catenation and pπ-pπ bond formation, carbon is able to show allotropic forms.
Allotropes of carbon:
Carbon exhibits two types of allotropic forms as crystalline as well as amorphous. Crystalline forms are diamond and graphite. The third form of carbon as Fullerenes was discovered by H.W. Kroto, E. Smalley and R. F. Curl. For this discovery they were awarded the Noble Prize in 1996.
Diamond:
In diamond each carbon atom undergoes sp3 hybridisation and linked to four other carbon atoms by using hybridized orbitals in tetrahedral fashion. The C-C bond length is 154 pm. The structure extends in space and produces a rigid three dimensional network of carbon atoms. It is very difficult to break extended covalent bonding and therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dies and in the manufacture of tungsten filaments for electric light bulbs.
Graphite:
It has layered structure. Layers are held by van der Waals forces and distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. C – C bond length within the layer is 141.5 pm. Each carbon atom in hexagonal ring undergoes sp2 hybridisation and makes three sigma bonds with three neighboring carbon atoms. Fourth electrons form a π bond. The electrons are delocalized over the whole sheet. Electrons are mobile and hence, graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and, therefore, it is very soft and slippery.
Fullerenes:
Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium and argon. The sooty material formed by condensation of vapourised 11C small molecules consists of mainly C60 with smaller quantity of C70 and traces of fullerenes consisting of even number of carbon atoms up to 350 or above.
Fullerenes are the only pure form of carbon because they have smooth structure without having any dangling bonds. C60 molecules have a shape like soccer ball and called Buckminsterfullerene. This shape contains six membered rings twelve five membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings. All the carbon atoms are equal and they undergo sp2 hybridisation. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalized in molecular orbitals. Which in turn give aromatic character to molecule.
Other forms of elemental carbon like carbon black, coke and charcoal are all impure form of graphite and fullerenes. Carbon black is obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke are obtained by heating wood or coal respectively at high temperature in the absence of air.
Some important compounds of carbon:
Carbon monoxide (CO):
Preparation of carbon monoxide
(1) Direct oxidation of carbon in limited supply of oxygen or air produces carbon monoxide.
2C(s) + O2(g) → 2CO(g)
(2) On small scale pure CO is prepared by dehydration of formic acid with concentrated H2SO4 at 373K.
HCOOH → H2O + CO
(3) On commercial scale, it is prepared by the passage of steam over hot coke. The mixture of CO and H2 thus produced is known as water gas or synthesis gas.
C(s) + H2O (g) → CO(g) + H2(g)
When air is used instead of steam, a mixture of CO and N2 is produced, which is called producer gas.
2C(s) + O2(g) + N2(g) → 2CO(g) + N2(g)
Water gas or producer gas on further combustion produces CO2 gas with the liberation of heat.
Properties of carbon monoxide:
(1) carbon monoxide is a colourless, odourless and almost water insoluble gas.
(2) It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminum and a few transition metals. Therefore, CO is used in the extraction of many metals from their oxides ores.
ZnO(s) + CO(g) → Zn(s) + CO2(g)
Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g).
(3) CO molecules acts as a donar of lone pair of electrons and reacts with a metal to form metal carbonyl
(4) The highly poisonous nature of CO arises because of its ability to form a complex with hemoglobin. Which is about 300 times more stable than the oxygen- hemoglobin complex. This prevents hemoglobin in the red blood corpuscles from carrying oxygen round the body and ultimately resulting in death.
Carbon dioxide (CO2):
Preparation of carbon dioxide:
(1) It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.
C(s) + O2(g) → CO2(g)
CH4(g) + 2O2(g) → CO2(g) + 2H2O (g)
(2) In the laboratory, it is easily prepared by the action of dilute HCl on calcium carbonate.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
(3) On commercial scale, it is obtained by heating limestone.
Properties of carbon dioxide:
(1) It is a colourless and odourless gas. It is low soluble in water.
(2) It forms carbonic acid which is a weak dibasic acid and dissociate into two steps.
H2CO3 (aq) + H2O (l) ↔ HCO3–(aq) + H3O+(aq)
HCO3–(aq) + H2O (l) ↔ CO32- (aq) + H3O+(aq)
H2CO3/HCO3– buffer system helps to maintain pH of blood between 7.26 to 7.42.
(3) In photosynthesis, CO2 is converted into glucose. By this process plants plants makes food for themselves as well as for animals and human beings.
6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
Green house effect:
The increase in combustion of fossil fuels and decomposition of limestone for cement manufacture in recent years seem to increase the CO2 content of the atmosphere. This may lead to increase in green house effect and thus raise the temperature of the atmosphere which might have serious consequences.
Application of carbon dioxide:
Carbon dioxide can be obtained as a solid in the form of dry ice that is used as a refrigerant for ice cream and frozen food. Gaseous CO2 is extensively used to carbonate soft drinks. Being heavy and non- supporter of combustion it is used as fire extinguisher.
Structure of carbon dioxide:
In CO2 molecule, carbon atom undergoes sp hybridisation. Two sp hybridised orbitals of carbon atom overlap with two p-orbitals of oxygen atoms to make two sigma bonds while other two electrons are involved in pπ-pπ bonding. The resonance structures are shown below.
Compounds of silicon:
Silicon dioxide (SiO2)
Silicon dioxide is commonly known as silica. Quartz, cristobalite and tridymite are some of the Crystalline form of silica. and they are inter convertible at suitable temperature. Silicon dioxide is a covalent, three dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom in turn covalently bonded to another silicon atoms. Each corner is shared with another tetrahedron. The entire crystal may be considered as giant molecule in which eight membered ring are formed with alternate silicon and oxygen atoms.
Properties of silica:
Silica in its normal form is almost non- reactive because of very high Si-O bond enthalpy. It resists the attack by halogens, dihydrogen and most of the acids and metals even at high temperature. However it is attacked by HF and NaOH.
SiO2 + 2NaOH → Na2SiO3 + H2O
SiO2 + 4HF → SiF4 + 2H2O
Silicones:
They are a group of organosilicon polymers. Which have (R2SiO-)- as a repeating unit. The starting materials for the manufacture of silicones are alkyl or aryl substituted silicon chloride, RnSiCl(4-n), where R is alkyl or aryl group.
When methyl chloride reacts with silicon in the presence of Copper as a catalyst at a temperature 573K various types of methyl substituted chlorisilane of formula MeSiCl3, Me2SiCl2, Me3SiCl with small amount of Me4SiCl are formed. Hydrolysis of dimethyl dichlorosilane followed by condensation polymersiation yields straight chain polymers.
The chain length of the polymer can be controlled by adding (CH3)3SiCl which blocks the ends as shown below.
Silicones being surrounded by non polar alkyl groups are water repelling in nature. They have in general high thermal stability, high dielectric strength and resistance to oxidation and reduction. They have wide application as sealant, grease, electrical insulators and for water proofing of fabrics.
Silicates:
A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos. The basic structural unit of silicate is SiO4+. In which silicon atom is bonded to four oxygen atoms in tetrahedron fashion. In silicates either the discrete unit is present or a number of such units are joined together via corners by sharing 1,2,3 or 4 oxygen atoms per silicate units. When silicate units are linked together, they form chain, ring, sheet or three dimensional structures.
Negative charge on silicate structure is neutralised by positively charged metal ions. If all the four corners are shared with other tetrahedral units, three dimensional network is formed.
Zeolite:
If aluminum atoms replace few silicon atoms in three dimensional network of silicon dioxide, overall structure known as aluminosilicate, acquires a negative charge. Cations such as Na+, K+ or Ca2+ balance the negative charge. Examples are feldspar and zeolite. ZSM-5 as a zeolite used to convert alcohols directly into gasoline. Hydrated zeolites are used as ion exchanger in softening of hard water.