Important trends and anomalous behavior of carbon

Introduction

Carbon is the first element in group 4A exhibits some unique features that differ from the rest of the elements of the group. This is because of the small size of carbon, it`s higher electro-negativity, higher ionization energy and also the fact that there is no d-orbital in its structure. A carbon atom can only hold a maximum of four pairs of electrons. Unlike other elements in group 4A, carbon is limited to a maximum of four covalences. Carbon is capable of forming pπ-pπ multiple bonds with each carbon atom and can also form with other atoms which have the small atomic size and higher electro-negativity. We should not that pπ-pπ bonds can only be formed in small atoms because the atomic size of heavier atoms is too large thus there is minimal overlapping. Carbon atoms can link with each other easily to form rings and chains because the C-C bond is a strong bond. This property is known as catenation. Carbon atoms, therefore, experiences allotropy because they can easily form pπ-pπ bonds. Catenation reduces as one moves down the group and lead doesn’t show catenation.

 

Allotropes of carbon

Allotropes are substances that can exist in 2 or more different forms. Carbon shows both crystalline and amorphous allotropism. The most common crystalline forms are diamond and graphite and let us learn more about them in this section.

 

1. Diamond

Diamond has a crystalline lattice and is one of the hardest element in the earth’s crust since it is not easy to break the extended covalent bonds that the diamond has. The carbon atoms in diamond undergo sp3 hybridization and are always attached to four other carbon atoms with the help of hybridized orbitals. Diamond forms a tetrahedral shape in which the C-C bond is approximately 154pm long. The covalent bonds in the structure of diamond are available throughout. Because of its hardness, diamond is mainly used as an abrasive for sharpening some metallic tools. It is also used in making dyes, production of tungsten filaments which are in turn used in electric bulbs.

 

2. Graphite

The structure of graphite contains multiple layers held together by weak van der Waals forces at a distance of 340pm between each layer.  The atoms in graphite are arranged in a hexagonal structure in which the C-C bond length is 141.5pm. In the graphite, each carbon atom undergoes hybridization forming sigma bonds with three available carbon atoms in the structure. The remaining 4th electron forms pie bond making graphite a good conductor of electricity (electrons are delocalized). Graphite is very slippery and soft because the layers in it slide over each other easily. This makes graphite a good lubricant in moving parts of a given machine mostly in areas where temperatures are high and oil can’t be used.

 

3. Fullerenes

Fullerenes are one of the latest forms of carbon that was recently discovered by scientists. They are obtained by heating graphite in an electric arc in the presence of either helium or argon. It is one of the purest forms of carbon known because it doesn’t have dangling bonds. Buckminsterfullerene is a C60 molecule that has a ball-like shape. The molecule comprises of twenty-six-membered rings and twenty-five member rings. In most cases, the twenty-six membered ring is fused with either five or six-membered rings. However, the five-membered ring is fused with a six-membered ring. In fullerene, all the carbon atoms are equal and they undergo sp2 hybridization. Each atom forms three sigma bonds with other three carbon atoms while the remaining electron at each carbon atom is delocalized in the molecular orbital giving fullerene the aromatic character. The C-C bond distance in fullerene is 143.5pm and that of the double bond, C=C is 138.3 pm.

 

Uses of carbon

  1. Graphite is used as an electrode in batteries because it is a good conductor of electricity.

  2. Activated charcoal is used in adsorbing poisonous gases because it is highly porous in nature.

  3. Charcoal is also used in filters to eliminate organic contaminators

  4. Coke is used as a fuel and as a reducing agent in metallurgy.

  5. Diamond is used in jewellery since it is precious.

 

Physical and chemical properties of carbon

Carbon reacts with excess air to form carbon dioxide. However, if the supply of oxygen is limited, it forms carbon monoxide as illustrated below.

C(s) + O2(g) → CO2(g)   (Excess air)

2C(s) + O2(g) → 2CO(g) (Limited air)

C(s) + 2S(s) → CS2(l)   

C(s) + Ca(s) → CaC2(s) 

Carbon acts as a reducing agent; it reduces steam to water gas.

C(s) + H2O(g)  → H2(g) + CO(g)

Carbon is not attacked by dilute acids, but both nitric acid and sulphuric acid react with carbon as illustrated below.

C(s) + 4HNO3(aq)  → 2H2O(l)  +4NO2(g)  + CO2(g)

C(s) + 2H2SO4(aq)  → 2H2O(l)  +2SO2(g)  + CO2(g)

 

Some important compounds of carbon

1. Carbon dioxide gas

Carbon dioxide can be prepared in the laboratory by reacting dilute HCl with marble chips.

CaCO3(s) + 2HCl(aq) → CaCl2(aq)  + H2O(l) + CO2(g)

 

Physical properties of carbon dioxide

  1. Carbon dioxide is a colourless, odourless and heavy gas.

  2. Carbon dioxide is soluble in water at STP

  3. Carbon dioxide gas easily liquefies at temperatures below 31.1.

 

2. Chemical properties of carbon dioxide

Carbon dioxide is the acidic anhydride of a weak acid, carbonic acid which ionizes as illustrated in the following steps:

H2CO3(aq) + H2O(l)   HCO3-(aq) + H3O+(aq)

H2CO3(aq) + H2O(l)   CO32-(aq) + H3O+(aq)   

 

The hydrogen carbonate buffer system is essential in the body of living things as it helps in maintaining the pH of the blood.

Excess carbon dioxide reacts with alkalies to yield hydrogen carbonates as shown below:

Ca(OH)2(aq) + CO2(g)    CaCO3(s+ H2O(l)

Ca(OH)2(aq) + H2O(l)+ CO2(g)    Ca(HCO3)2(aq)   (THIS EQUATION ACCOUNTS TO TEMPORARY HARD WATER)

 

Carbon dioxide gas is eliminated from the environment through the process of photosynthesis, the process through which green plants manufacture their own food by changing carbon dioxide gas into carbohydrates such as glucose.

6CO2(g) + 12 H2O(l)  → C6H12O6 + 6O2(g) + 6H2O(l)

 

Carbon monoxide

Laboratory preparation

In the laboratory, carbon monoxide gas is prepared by dehydrating methanoic acid with concentrated sulphuric acid.

HCOOH(l)   CO(g) + CO2(g)

Carbon monoxide can also be prepared by passing steam overheated coke. The mixture of carbon monoxide and hydrogen produced is referred to as water gas or simply synthesis gas.

C(S) + H2O(l)   CO(g) + H2(g)       (WATER-GAS)

 

Physical properties of carbon monoxide

  1. Carbon monoxide gas is colourless and odourless gas which burns in air with a blue flame to form carbon dioxide gas.

  2. Carbon monoxide is a poisonous gas and readily combines with haemoglobin in the blood thus causing suffocation.

 

Chemical properties of carbon monoxide

  1. Carbon monoxide is readily oxidized by both copper (II) oxide and manganese (IV) oxide to form carbon dioxide gas.

  2. Carbon monoxide gas reduces iron (III) oxide into iron metal during its extraction.

Fe2O3(S) + 3CO(G)   2Fe(S) + 2CO2(G)

 

  1. Carbon monoxide reacts with many transition elements to form carbonyls which are highly volatile in nature. Pure nickel can be obtained by reacting nickel with CO then decomposing the nickel carbonyl.

Ni(s) + 4CO(g)  Ni(CO)4(l)  Ni(s)  + CO2(g)

 

  1. Carbon monoxide is readily absorbed by an ammonical solution of copper (I) chloride to produce CuCl.CO. 2H2O. The gas reduces an ammonical solution of silver nitrate to silver solid.

 

 

Questions

  1. What are allotropes? Mention a few allotropes of carbon.

  2. How is carbon different from other elements within its group?

  3. Explain the bonding nature of carbon in the diamond.

  4. Explain the properties of Carbon monoxide.

  5. What are fullerenes? Explain.

  6. What do you mean by the anomalous behaviour of carbon?

 



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