Introduction:

Carbon is a non-metal which has been acknowledged for a very long time under the names soot, charcoal and diamond. It is the 6th richest element in the universe. Carbon exists naturally in the elemental states as graphite, diamond and coal; and in the combined state as petroleum, wood, natural gases and mineral deposits. The common mineral deposits of carbon are the metallic trioxocarbonates (IV) like limestone [calcium trioxocarbonates (IV) and dolomite (Magnesium trioxocarbonates (IV)]. Carbon as carbon (IV) oxide, CO₂, comprises 0.03% of the atmosphere. The carbon found in petroleum, coal, wood and natural gases are employed essentially as source of fuels for energy supply.

Carbon is as well a necessary constituent of all living things. The study of the millions of carbon compound in living (that is, organic) organism makes a separate branch of chemistry termed as "Organic Chemistry". The organic compounds will be treated in separate units of this program.

We are familiar that carbon consists of 6 electrons having an electronic configuration of 1s² 2s² 2p⁴ and is thus positioned in group 4 and period 2 of the periodic Table. The energy needed to lose its 4 outermost electrons or gain four electrons to make a stable configuration is enormous; thus carbon joins essentially by making four covalent bonds by other elements. Chemically, carbon is not extremely reactive element and that is why it exists naturally as elemental carbon in different forms example - graphite, diamond, coal, charcoal and carbon black.

Allotropes of Carbon:

The existence in the similar physical state of two or more structural form of an element is termed as allotropy. The carbon represents allotropy. Graphite and Diamond are the two allotropic forms of crystalline carbon. The others such as coke, coal, charcoal, sugar charcoal, lamp-black and animal charcoal are amorphous or non-crystalline forms of carbon.

Diamond:

Diamond exists naturally in the Brazil, South Africa, India and Venezuala. This is the purest form of naturally occurring carbon and is generally obtained as a colorless solid that can be converted into luminous gem. The structure of diamond comprises of an infinite array of carbon atoms covalently bonded. The diamond crystal is octahedral in shape.

Fig: Structure of Diamond

The network structure provides diamond its great strength and high melting temperature. Diamond is the hardest substance acknowledged. This is a poor conductor of heat and electricity. Diamond is resistant of chemical attack. As diamonds are hard and dense, they are utilized industrially in drills for mining, as abrasives sharpen extremely hard tools and for cutting glass and metals. They are as well employed as pivot supports in valuable instruments and as dies for drawing wires. Its high refractive index and dispersion power give it a sparkling brilliance whenever it is cut and polished making it precious as jewellery.

Graphite:

The structure of graphite comprises of hexagonal sheets of covalently bonded carbon atoms. The sheets are held altogether through weak intermolecular forces in the parallel layers. Graphite takes place naturally as an opaque black solid termed as plumbago that is formed through the action of volcanic heat on coal deposits. This is found as deposit in China, Austria, West Germany, Madagascar and Sri-lanka.

Fig: Structure of Graphite

Graphite can as well be artificially made by heating coke at a high temperature in an electric furnace. As graphite is in sheets, it is soft and slippery. However it consists of a high melting point, it is less dense than diamond. Graphite is comparatively inert chemically however can be oxidized under appropriate conditions. Dissimilar diamond, graphite is the good conductor of electricity due to the presence of mobile electrons in the sheets. The electrons exist as just three of the four valence electrons of each and every carbon atom in the graphite crystal are comprised in the bond formation.

Graphite is the outstanding dry lubricant. This is due to its layered structure which allows one layer to slide over the other easily. Dissimilar oil, it is non volatile and not sticky. This is generally employed on bicycle chains and for the bearings of several motor cars. Whenever mixed by oil it forms a high temperature lubricant. Being a good conductor of electricity and is often employed as electrodes in electroplating and batteries. A non-conductor can be made conductive through coating it by graphite. As graphite is soft and marks paper, it is employed to make pencils. This is used as the black pigment in paint.

Amorphous (or non-crystalline) carbon:

Carbon exists in various other forms that have no definite crystalline structure. They are not considered as the true allotropes of carbon. Despite from coal, which is naturally occurring, all the other amorphous forms of carbon can be made in different manners.

i) Coal was made up from vegetation protected from complete decay via water-washed earth deposit. Decomposition occurred slowly under pressure and in the absence of air. Carbon (IV) oxide, methane and water were discharged, leaving behind a material which contained a very high percentage of carbon. Throughout this method of carbonization, the vegetable material was transformed in phases into peat, lignite (or brown coal), bituminous (or soft) coal and lastly anthracite (or hard coal) that is around 95% pure carbon. Impurities present might comprise sulphur, nitrogen and phosphorus. Coal is employed mostly as a fuel to produce power for steam engines, factories and electrical plants. This is as well employed for making different chemicals example: benzene and methane.

ii) Coke is obtained through heating bituminous coal to extremely high temperatures in the lack of air to burn off the volatile constituents. This method is termed to as the destructive distillation of coal. Coke is employed mostly as a fuel as it burns by practically no smoke and leaves extremely little residue. This is a significant industrial reducing agent, employed in the extraction of metals, example: iron from their ores. This is as well employed in the production of gaseous fuels, such as water gas and producer gas and for the manufacture of calcium carbide, graphite, silicon carbide and carbon (IV) sulphide.

iii) Carbon black (or soot) is prepared by heating carbon having materials in a limited supply of air to give finely divided particles of carbon. Lamp-black is achieved from vegetable of lamp oils, whereas carbon black itself gets from coal gas, natural gas and fuel oils. Carbon black is employed in manufacturing tuber tyres, printer's ink, black shoe polish, typewriting ribbons and carbon paper.

iii) Charcoal can be made up by nut shells bones, burning wood, sugar and even blood. Wood charcoal, the most general, is prepared through burning wood in a limited supply of air. Sugar charcoal is made up whenever sugar is dehydrated that is, the hydrogen and oxygen it contains are eliminated in the form of water either through burning the sugar in a limited supply of air or through the action of concentrated tetraoxosulphate(VI) acid. This is the purest form of amorphous carbon.

Animal charcoal is generated whenever bones and animal refuse are burnt in limited supply of air. Charcoal consists of an extremely porous structure and lets molecules of gases and dyes to adsorb or hold on to its internal surfaces. Therefore, this is a good adsorbent specifically when activated. Wood charcoal is employed in gas-masks for adsorbing poisonous gases. This is as well employed for the purification of the noble gases and the recovery of industrial solvents. Likewise, animal charcoal is utilized in eliminating the brown color from crude sugar and colorizing petroleum jelly. Similar to coal, wood charcoal is as well utilized mostly as a domestic fuel.

Common Properties of Carbon:

1) All the various allotropes of carbon are black solids or grayish-black apart from diamond that is colorless whenever pure.

2) All the allotropes encompass high melting point, around 3,500°C.

3) Carbon is insoluble in the entire common solvents example: water, alkalis, petrol, acids and carbon (IV) sulphide.

This feature describes why carbon deposit in motor engines has to be mechanically eliminated-decarbonization of the motor engines.

Chemical Properties of Carbon:

Carbon is not an extremely reactive element. Its compounds are made through covalent bonding and most of them are stable. Carbon makes single or multiple covalent bonds by itself and other elements like hydrogen, oxygen, nitrogen and sulphur. The unique capability of carbon to form long chains and rings of carbon-carbon bonds (termed as catenation) allows it to encompass many compounds. The molecules range from small to extremely large ones and most of them are organic in nature.

Each and every allotropes of carbon contain similar chemical properties; though graphite and diamond are less reactive than the amorphous form.

Combustion reactions:

Carbon burns in surplus oxygen to generate carbon (IV) oxide only, however the temperature needed vary for the allotropic forms.

C (s) + O₂ (g) → CO₂ (g)

Whenever the supply of air is limited, combustion might not be complete. Carbon (II) oxide is formed rather than carbon (IV) oxide

2C (s) + O₂ (g) → CO (g)

Charcoal Fires:  In many countries, charcoal is utilized widely for making fires. As the charcoal burns, carbon (IV) oxide and carbon (II) oxide are made at different levels within the charcoal pot, based on the level of oxygen supply.

Several reactions comprised are:

2CO + O₂ → CO₂

CO₂ + C → 2CO (g)

C + O₂ → CO₂

The carbon (II) oxide which is not oxidized and escapes to the atmosphere is hazardous if the area around the fire is not well ventilated as the gas is toxic.

Combination reactions:

Carbon joins directly by some elements like sulphur, calcium, hydrogen and aluminium at extremely high temperatures generally greater than 500°C.

C(S) + S(s) → CS₂ (l) carbon (IV) sulphide

C(S) + 2H₂ (g) → CH₄ (g) methane

2C(s) + Ca (g) → CaC₂ (s) Calcium Carbide

3C(s) + 4Al (s) → Al₄C₃ (g) aluminum carbide

Carbon as reducing agent:

Carbon is a strong reducing agent that is, as oxygen acceptor. It decreases the oxides of less active metals like copper and iron; at high temperatures.

Fe₂O₃ + 3C → 2Fe + 3CO

2CuO + C → 2Cu + CO₂

At high temperatures, carbon can as well reduce steam to hydrogen and carbon (IV) oxide.

C + H₂O → CO + H₂

Reactions having strong oxidizing agents:

Whenever carbon is heated by concentrated trioxonitrate (V) acid or concentrated tetraoxosulphate (VI) acid, this is oxidized to carbon (IV) oxide.

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

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

Oxides of Carbon:

Carbon forms two kinds of oxides, carbon (IV) oxide, CO₂, and carbon (II) oxide CO. Both oxides are obtainable as the products of combustion of carbon.

Carbon (IV) Oxide:

Carbon (IV) oxide comprises around 0.03% by volume of air and is as well found, in very small quantity, dissolved in water. In the earth crust, carbon (IV) oxide is found as the metallic trioxocarbonates (IV) and hydrogen trioxocarbonate (IV) example: in limestone areas and coral reefs. Carbon (IV) oxide is extremely significant to green plants that employ it to produce food substances example: starch through the process of photosynthesis.

1) Preparation: Carbon (IV) oxide is made in the laboratory through the action of dilute hydrochloric acid or trioxonitrate (V) acid on metallic trioxocarbonate (IV) or hydrogen trioxocarbonate.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

CaCO₃ + 2HNO₃ → Ca (NO₃) + H₂O + CO₂

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Action of heat on metallic trioxocarbonate (IV) apart from those of sodium and potassium or the hydrogen trioxocarbonates (IV) of sodium and potassium; is as well employed for the production of carbon (IV) oxide

CaCO₃ → CaO + CO₂

2NaHCO₃ → Na₂O + H₂O + 2CO₂

Carbon (IV) oxide is acquired industrially as by-product in the fermentation process and manufacture of quicklime (CaO) from limestone (CaCO₃).

2) Physical properties:

• CO₂ is a colorless, odorless and tasteless gas.
• It is around 1.5 times denser than air.
• This is moderately soluble in water.
• It turns damp blue litmus paper red as CO₂ dissolves in water to form an acidic oxide, trioxocarbonate (IV) acid.
• This can readily be liquefied and solidified (- 78°C). Solid CO₂ is termed as cardice or dry-ice.

3)  Chemical properties:

a) CO2 doesn't burn nor does it support the combustion. Though burning magnesium decomposes CO₂ leaving at the back carbon deposit and magnesium (II) oxide ash.

CO₂ + 2Mg → 2MgO + C

b) CO dissolves in the water to form trioxocarbonate (IV) acid (that is, soda water). This is a weak acid.

CO₂ + H₂O ↔ H₂CO₃ ↔ 2H⁺ + CO₃^2-

c) CO₂ reacts directly by alkalis example: NaOH, to form trioxocarbonates (IV).

Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃

Whenever CO₂ is passed via the alkali, calcium hydroxide (that is, limewater), the lime water turns milky due to the precipitation of insoluble calcium trioxocarbonate (IV). This reaction is employed to test for CO₂.

Ca (OH)₂ + CO₂ → CaCO₃ + H₂O

Limewater            Insoluble

Though, whenever excess gas is bubbled, the milkiness disappears leaving a clear solution as the Soluble trioxocarbonate (IV) is transformed to soluble hydrogen trioxocarbonate (IV).

CaCO₃ + H₂O + CO₂ → Ca (HCO₃)₂

4) Uses:

a) Most of the fire extinguishers make use of carbon (IV) oxide to put out fires as the gas doesn't support combustion. Being heavier than air, it wraps the burning material and cuts off the supply of oxygen. Carbon (IV) oxide is more efficient than water in putting out petrol or oil fires since such materials float on water.

b) Carbon (IV) oxide is employed in the manufacture of:

- Sodium trioxocarbonate (IV) (that is, washing soda) through the Solvay method.

- Sodium hydrogentrioxocarbonates (IV).

- Lead (II) trioxocarbonate (IV).

- Urea and ammonium tetraoxosulphate (VI) - significant fertilizers.

c) Yeast and baking powder are employed in baking to produce carbon (IV) oxide that causes the dough to rise, making dough light.

d) Solid carbon (IV) oxide that is, dry ice is employed as a refrigerant for perishable goods, example:  ice-cream. On warming, it sublimes and gives a lower temperature. Gaseous carbon (IV) is employed to preserve the fruits. Carbon (IV) oxide is as well employed as a coolant in the nuclear reactors.

e) Carbon (IV) oxide is employed to give carbonated (or aerated) drinks a pleasant and refreshing taste example: Coca-cola, beer, Pepsi-cola, cider, wine and champagne. Soda-water drink is the solution of CO₂ gas in water pressure.

f) Green plants make use of CO₂ throughout photosynthesis.

Carbon (II) Oxide:

CO is made by the incomplete combustion of carbon compounds, like octane, C₈H₁₈, found in petrol.

2C₈H₁₈ (l) + 17O₂ (g) → 16 CO (g) + 18 H₂O (l)

CO takes place in traces as an impurity in the air. The percentage might increase in cities where the gas is discharged in the exhaust fumes of motor cars, and in industrial areas due to the combustion of fuels. This is a poisonous gas and as little as 0.5% of it in the air might lead to the death. As the gas consists no color or odor, its presence is difficult to detect, thus it is extremely dangerous.

a)  Preparation: Carbon (II) oxide is made by passing carbon (IV) oxide over red-hot carbon

CO₂ + C → 2CO

This can as well be made by dehydrating methanoic acid, HCOOH, or ethanedioc acid, C₂H₂O₄, by utilizing concentrated tetraoxosulphate (VI) acid, that act as the dehydrating agent.

Fig: Preparation of Carbon (II) Oxide

The preparation of CO should be done in the fume cupboard as the gas is poisonous. In the later reaction, the CO₂ is eliminated by passing gaseous products via concentrated sodium hydroxide.

b) Physical Properties:

• Carbon (II) oxide is a colorless, tasteless and odorless gas.
• It is insoluble in water, however dissolves in a solution of ammoniacal copper (I) chloride.
• It is to some extent less dense than air.
• It consists of no effect on litmus

c)  Chemical Properties:

i) As a reducing agent: Carbon (II) oxide is a strong reducing agent. It decreases some metallic oxides to metals and is itself oxidized to the carbon (IV) oxide.

PbO (s) + CO (g) → Pb(s) + CO₂ (g)

It as well decreases steam to hydrogen

ii) It burns in air having a blue flame to provide carbon (IV) oxide

2C0 + O₂ → 2CO₂

iii) The poisoning nature of Carbon (II) oxide is as a result of its reaction having haemoglobin in the red blood cells and therefore prevents the haemoglobin from transporting oxygen in our body. Death resultant from CO poisoning take place whenever the supply of oxygen to the body becomes not enough as the carrier haemoglobin is not available for this function. 

d)  Uses:

• CO is utilized as a reducing agent in the extraction of metals example: iron from its ores.
• CO is the significant constituent of gaseous fuels such as producer gas (CO + N₂) and water gas (CO + H₂).

e) Fuel Gases:

These are the gas mixtures generated by heating coke with oxygen or steam. All the raw materials coke, air and water are economical and readily available.

i) Whenever hot coke combines with oxygen of the air, the product is a gaseous mixture, of (1/3)rd carbon (II) oxide and (2/3)rd nitrogen by volume, known as producer gas. The formation of producer gas is exothermic.

2C + O₂ + 4N₂ → 2CO + 4N₂ + heat

Coke  form air     producer gas

Producer gas consists of a low heating power as it contains around 67% non-combustible nitrogen and 33% carbon (II) oxide. Though, it is cheap and is broadly utilized to heat furnaces retorts example: in the manufacture of zinc, steel and glass. It is as well a source of nitrogen for the manufacture of ammonia (that is, Haber method).   

ii) Whenever hot coke combines with steam, the product is a gaseous, mixture, of equivalent volumes of carbon (II) oxide and hydrogen, known as water gas. The formation of water gas is endothermic.

C + H₂O → CO + H₂ + heat

Throughout the method, the coke rapidly cools to a temperature too low for reaction if heat is not supplied externally. Industrially both the producer gas and water gas are made in similar plant, termed as the producer, by passing air and steam alternately via the heated coke. The heat generated whenever producer gas is made is adequate for the formation of water gas.

Water gas is employed to manufacture hydrogen and as a significant industrial fuel (both CO and H₂ burn in air discharging a lot of heat). 

Fig: production of fuel gases

Trioxocarbonates (IV) and Hydrogen Trioxocarbonate (IV):

Trioxocarbonates (IV) and hydrogen trioxocarbonates (IV) are the inorganic compounds of carbon. Whenever carbon (IV) oxide dissolves in water to generate trioxocarbonates (IV) acid, is a weak dibasic acid.

CO₂ + H₂O → H₂CO₃ (trioxocarbonate (IV) acid)

H₂CO₃ ↔ 2H⁺ + CO₃^2- weak acid

The acid reacts by some free metals example: zinc; metallic oxides example: CaO and alkalis example: NaOH to form trioxocarbonate (IV) salts. Metallic trioxocarbonates are generally found as natural ores or deposits example: limestone (CaCO₃) and dolomite (MgCO₃).

Zn + H₂CO₃ → ZnCO₃ + H₂

CaO + H₂CO₃ → CaCO₃ + H₂O

2NaOH + H₂CO₃ → Na₂CO₃ + 2H₂O

Trioxocarbonate (IV) salts are of two kinds:

• Normal trioxocarbonate (W) MCO₃ and
• Acidic hydrogen trioxocarbonate (IV) MHCO₃

Preparation of soluble trioxocarbonate (IV) salt:

The water soluble trioxocarbonate (IV) salts are sodium, potassium and ammonium trioxocarbonate (IV). They are generally made by passing carbon (IV) oxide via a solution of corresponding alkali.

2MOH + CO₂ → M₂CO₃ + H₂O

(Here M = Na, K or NH₄).

Preparation of insoluble trioxocarbonate (IV) salts:

Most of the metallic trioxocarbonate (IV) are insoluble in water. The common methods for preparing them are:

a) Reaction of a base example: Ca (OH)₂; metals example: Zn; or metallic oxide example: MgO; having trioxocarbonate (IV) acid

Ca(OH)₂ + H₂CO₃ → CaCO₃ + 2H₂O

Zn + H₂CO₃ → ZnCO₃ + H₂

MgO + H₂CO₃ → MgCO₃ + H₂O

(b) Displacement of trioxocarbonates of the water soluble sodium trioxocarbonate example:

Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl

Na₂CO₃ + 2AgNO₃ → 2Na₂NO₃ + Ag₂CO₃

The insoluble trioxocarbonates are precipitated out.

Hydrogen trioxocarbonate (IV) salts:

These are the acid salts of trioxocarbonate (IV) acid made whenever a metal or ammonium radical substitutes one of the two hydrogen atoms in the molecule. They are soluble in water. Though, the hydrogentrioxocarbonates (IV) of ammonium, potassium and sodium can be isolated as solids. Calcium hydrogentrioxocarbonate (IV) is responsible for the hardness in water.

Hydrogentrioxocarbonates (IV) might be prepared through passing CO₂ via a solution of the corresponding hydroxides or trioxocarbonates (IV) example: NaOH or Na₂CO₃

NaOH + CO₂ → NaHCO₃

Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃

Properties of trioxocarbonates (IV) salts:

i) Action of heat: All trioxocarbonate (IV) salts apart from those of sodium, potassium and barium, decompose on heating to release carbon (IV) oxide.

ZnCO₃ → ZNO + CO₂

2Ag₂CO₃ → 4 Ag + 2CO₂ + O₂

(NH₄)₂CO₃ → 2NH₃ + CO₂ + H₂O

The entire hydrogen trioxocarbonates (IV) as well decompose on heating the solid or solution to provide carbon (IV) oxide, water and the corresponding trioxocarbonates (IV).

2KHCO₃ → K₂CO₃ + H₂O + CO₂

ii) All the trioxocarbonate (IV) and hydrogen trioxocarbonate (IV) salts react by dilute acids to form carbon (IV) oxide water and a salt.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Ca(HCO₃)₂ + 2HCl → CaCl₂ + 2H₂O + 2CO₂

Uses of some significant trioxocarbonate (IV) salts:

i) Na₂CO₃ is utilized in the manufacture of glass, soap and detergents, and also in the production of paper and pulp.

ii) NaHCO₃ is employed in baking powder to discharge CO₂ gas that helps the dough to rise throughout baking. It is as well employed as medicine to relieve indigestion example: in Andrews liver salts. This is utilized in dry-powder fire extinguishers.

iii) CaCO₃ is utilized in the production of cement, chalk and significant industrial chemicals example: CaO and NH₃.

Carbon Cycle in Nature:

Carbon is being constantly circulated in nature through a series of changes termed as the Carbon Cycle. Atmospheric carbon (IV) oxide forms the vital link among the different carbon compound in the cycle. The level of carbon (IV) oxide in the air is kept at around 0.03% through volume and this is achieved via a natural balance between its rate of formation and its rate of elimination from the atmosphere.

Some of the methods that release CO₂ into the atmosphere are as follows:

i) The respiration of living organism in which glucose is transformed to CO₂.  

C₆H₁₂O₆ + 6O₂ → 6CO₂ + H₂O + energy (respiration method)

Glucose

ii) Combustion of carbon having substances example: petrol, wood, coal and butane gas.

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Petrol

iii) Decomposition of the organic materials example: plants and animals and trioxocarbonates (IV) salts.

Several methods that remove CO₂ from the atmosphere are:

a) Photosynthesis in plants for the production of food

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂ (photosynthesis method)

b) The sea and other water bodies act as reservoirs for carbon (IV) oxide whenever it dissolves in them.

c) This is stored as part of bone, shells in living organism and in trioxocarbonate ores or deposits.

Increased human activities are upsetting the fragile natural balance that keeps the CO₂ level fairly constant in the atmosphere. Such activities are:

• The increase in the combustion of fossil fuels in industries, homes and vehicles
• Deforestation so that the forests that discharge carbon (IV) oxide to the atmosphere are much decreased in size.

Fig: Carbon cycle

The effect of disturbing the CO₂ balance is the greenhouse effect, a slow warming of our planet that will lead to the melting of the polar cap and submergence of the coastal lands on earth.

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