Chemistry Some important compounds of carbon and silicon

### Topics to be covered

=> Oxides of carbon
=> Carbon monoxide
=> Carbon dioxide
=> Silicon dioxide
=> Silicones
=> Silicates
=> Zeolites

### SOME IMPORTANT COMPOUNDS OF CARBON AND SILICON

So in this lecture we will discuss about the important oxides of carbon and silicon

### Oxides of Carbon

color{green}(★) Two important oxides of carbon are carbon monoxide, color{red}(CO) and carbon dioxide, color{red}(CO_2)

### Carbon Monoxide

color{green}(★) Direct oxidation of color{red}(C) in limited supply of oxygen or air yields carbon monoxide.

color{red}(2C(s) + O_2 (g) overset(Delta)→ CO (g))

color{green}(★) On small scale pure color{red}(CO) is prepared by dehydration of formic acid with concentrated color{red}(H_2SO_4) at 373 K

color{red}(HCOOH underset(Conc. H_2SO_4) overset(373K)→ H_2O + CO)

color{green}(★) On commercial scale it is prepared by the passage of steam over hot coke. The mixture of color{red}(CO) and color{red}(H_2) thus produced is known as water gas or synthesis gas.

color{red}(C(s) +H_2O(g) overset(473-1273K)→ underset("Water gas")(CO(g)+ H_2(g)))

color{green}(★) When air is used instead of steam, a mixture of color{red}(CO) and color{red}(N_2) is produced, which is called producer gas.

color{red}(2C(s) +O_2(g) + 4N_2 (g) overset(1273K)→ 2CO(g) + underset("Producer gas")(4N_2(g)))

color{green}(★) Water gas and producer gas are very important industrial fuels. Carbon monoxide in water gas or producer gas can undergo
further combustion forming carbon dioxide with the liberation of heat.

color{brown}(★) Carbon monoxide is a colourless, odourless and almost water insoluble gas.

color{brown}(★) It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminium and a few transition metals.

color{brown}(★) This property of color{red}(CO) is used in the extraction of many metals from their oxides ores.

color{red}(Fe_2O_3(s) +3CO(g) overset(Delta)→ 2Fe(s) + 3CO_2(g))

color{red}(ZnO(s) +CO(g) overset(Delta)→ Zn(s) +CO_2(g))

color{green}(★) In color{red}(CO) molecule, there are one sigma and two color{red}(π) bonds between carbon and oxygen, color{red}(:C ≡ O:) . Because of the presence of a lone pair on carbon, color{red}(CO) molecule acts as a donor and reacts with certain metals when heated to form metal carbonyls.

color{green}(★) The highly poisonous nature of color{red}(CO) arises because of its ability to form a complex with haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen round the body and ultimately resulting in death.

### Carbon Dioxide

color{green}(★) It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.

color{red}(C(s) + O_2 (g) overset(Delta)→ CO_2(g))

color{red}(CH_4(g) +2O_2(g) overset(Delta)→ CO_2(g) +2H_2O(g))

color{green}(★) In the laboratory it is conveniently prepared by the action of dilute color{red}(HCl) on calcium carbonate.

color{red}(CaCO_3(s) + 2HCl (aq) → CaCl_2 (aq) + CO_2 (g) + H_2O(l))

color{green}(★) On commercial scale it is obtained by heating limestone.

color{green}(★) It is a colourless and odourless gas. Its low solubility in water makes it of immense biochemical and geo-chemical importance.

=> With water, it forms carbonic acid, color{red}(H_2CO_3) which is a weak dibasic acid and dissociates in two steps:

color{red}(H_2CO_3(aq) + H_2O(l) ⇌ HCO_3^(–)(aq) + H_3O^(+)(aq))

color{red}(HCO_3^(–) (aq) + H_2O(l) ⇌ CO_3^(2–) (aq) + H_3O^(+)(aq))

color{red}(H_2CO_3//HCO_3^( –)) buffer system helps to maintain pH of blood between 7.26 to 7.42. Being acidic in nature, it combines with alkalies to form metal carbonates.

color{green}(★) Carbon dioxide, which is normally present to the extent of color{red}(~ 0.03 %) by volume in the atmosphere, is removed from it by the process known as photosynthesis. It is the process by which green plants convert atmospheric color{red}(CO_2) into carbohydrates such as glucose. The overall chemical change can be expressed as:

color{red}(6CO_2 +12 H_2O underset("Chlorphyll") overset(hv)→ C_6H_(12)O_6 + 6O_2+6H_2O)

color{green}(★) By this process plants make food for themselves as well as for animals and human beings. Unlike color{red}(CO), it is not poisonous. But the increase in combustion of fossil fuels and decomposition of limestone for cement manufacture in recent years seem to increase the color{red}(CO_2) 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.

color{brown}(★) Carbon dioxide can be obtained as a solid in the form of dry ice by allowing the liquified color{red}(CO_2) to expand rapidly.

color{brown}(★) Dry ice is used as a refrigerant for ice-cream and frozen food. Gaseous color{red}(CO_2) is extensively used to carbonate soft drinks.

color{brown}(★) Being heavy and non-supporter of combustion it is used as fire extinguisher. A substantial amount of color{red}(CO_2) is used to manufacture urea.

color{green}(★) In color{red}(CO_2) molecule carbon atom undergoes color{red}(sp) hybridisation. Two color{red}(sp) hybridised orbitals of carbon atom overlap with two p orbitals of oxygen atoms to make two sigma bonds while other two electrons of carbon atom are involved in color{red}(pπ– pπ) bonding with oxygen atom. This results in its linear shape [with both color{red}(C–O) bonds of equal length (115 pm)] with no dipole moment. The resonance structures are shown below:

underset("Resonance structures of carbon dioxide")( color{red}(text()^(-): underset(. .) overset(. .)O - C equiv O: text()^(+) ↔ : overset( . .)O =C=overset(. .)O: ↔ : text()^(+) O equiv C - underset(. .) overset(. .)O : text()^(-)))

### Silicon Dioxide, SiO_2

color{green}(★) 95% of the earth’s crust is made up of silica and silicates.

color{green}(★) Silicon dioxide, commonly known as silica, occurs in several crystallographic forms.

color{green}(★) Quartz, cristobalite and tridymite are some of the crystalline forms of silica, and they are interconvertable at suitable temperature.

color{green}(★) 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 as shown in diagram (Fig 11.6 ).

Each corner is shared with another tetrahedron. The entire crystal may be considered as giant molecule in which eight membered rings are formed with alternate silicon and oxygen atoms.

color{green}(★) Silica in its normal form is almost nonreactive because of very high color{red}(Si—O) bond enthalpy. It resists the attack by halogens, dihydrogen and most of the acids and metals even at elevated temperatures. However, it is attacked by color{red}(HF) and color{red}(NaOH).

color{red}(SiO_2 + 2NaOH → Na_2SiO_3 + H_2O)

color{red}(SiO_2 + 4HF → SiF_4 + 2H_2O)

color{green}(★) Quartz is extensively used as a piezoelectric material; it has made possible to develop extremely accurate clocks, modern radio and television broadcasting and mobile radio communications.

color{green}(★) Silica gel is used as a drying agent and as a support for chromatographic materials and catalysts. Kieselghur, an amorphous form of silica is used in filtration plants.

### Silicones

color{green}(★) They are a group of organosilicon polymers, which have color{red}((R_2SiO)) as a repeating unit.

color{green}(★) The starting materials for the manufacture of silicones are alkyl or aryl substituted silicon chlorides, color{red}(RnSiCl_(4–n)), where color{red}(R) is alkyl or aryl group.

color{green}(★) When methyl chloride reacts with silicon in the presence of copper as a catalyst at a temperature 573K various types of methyl substituted chlorosilane of formula color{red}(MeSiCl_3), color{red}(Me_2SiCl_2, Me_3SiCl) with small amount of color{red}(Me_4Si) are formed.

color{green}(★) Hydrolysis of dimethyldichlorosilane, color{red}((CH_3)_2SiCl_2) followed by condensation polymerisation yields straight chain polymers.

color{green}(★) The chain length of the polymer can be controlled by adding color{red}((CH_3)_3SiCl) which blocks the ends as shown below :

color{brown}(★) Silicones being surrounded by non-polar alkyl groups are water repelling in nature.

color{brown}(★) They have in general high thermal stability, high dielectric strength and resistance to oxidation and chemicals.

color{brown}(★) They have wide applications. They are used as sealant, greases, electrical insulators and for water proofing of fabrics.

color{brown}(★) Being biocompatible they are also used in surgical and cosmetic plants.
Q 3265723665

What are silicones ?

Solution:

Simple silicones consist of (- underset(|) overset(|)(Si) - O -)_n chains in which alkyl or phenyl groups occupy the remaining bonding positions on each silicon. They are hydrophobic (water repellant) in nature.

### Silicates

color{green}(★) A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos. The basic structural unit of silicates is color{red}(SiO_4^(4–)) (Fig.11.7) 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.

color{green}(★) When silicate units are linked together, they form chain, ring, sheet or three-dimensional structures.

color{green}(★) 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.

Two important man-made silicates are glass and cement.

### Zeolites

color{green}(★) If aluminium atoms replace few silicon atoms in three-dimensional network of silicon dioxide, overall structure known as aluminosilicate, acquires a negative charge.

color{green}(★) Cations such as color{red}(Na^(+), K^(+)) or color{red}(Ca^(2+)) balance the negative charge. Examples are feldspar and zeolites.

color{green}(★) Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation, e.g., color{red}(ZSM-5) (A type of zeolite) used to convert alcohols directly into gasoline. Hydrated zeolites are used as ion exchangers in softening of “hard” water.