10th Class Science Carbon and its Compounds Hydrocarbons


Category : 10th Class

*        Introduction


A hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls. Aromatic hydrocarbons (arenes), alkanes, alkenes, cycloalkanes and alkyne-based compounds are different types of hydrocarbons.

The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded, can catenate to form millions of chains.


*        Types of Hydrocarbons

The classifications for hydrocarbons defined as follows:


Saturated hydrocarbons (alkanes) are the simplest of the hydrocarbon species and are composed entirely of single bonds and are saturated with hydrogen. The general formula for saturated hydrocarbons is\[{{\mathbf{C}}_{n}}{{\mathbf{H}}_{2n+2}}\]. Saturated hydrocarbons are the basis of petroleum fuels and are either found as linear or branched species. Hydrocarbons with the same molecular formula buts’ different structural formulae are called structural isomers. Methylhexane and its higher homologues, branched hydrocarbons can be chiral. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with double bond are called alkenes. Those with one double bond have the formula \[{{\mathbf{C}}_{n}}{{\mathbf{H}}_{2n}}\]. Those containing triple bonds are called alkynes, with general formula \[{{\mathbf{C}}_{n}}{{\mathbf{H}}_{2n-2}}\].

Cycloalkanes are hydrocarbons; containing one or more carbon rings to which hydrogen atoms are attached. The general formula fora saturated hydrocarbon containing one ring is \[{{\mathbf{C}}_{n}}{{\mathbf{H}}_{2n}}\].


Molecular Name Number of Carbon atoms Prefix Melting Point (C) Boiling Point Physical State
Methane 1 Meth -183 -162 Gas
Ethan 2 Eth -172 -89 Gas
Propane 3 Prop -187 -42 Gas
Butane 4 But -138 0 Gas
Pentane 5 Pent -130 36 Liquid
Hexane 6 Hex -95 69 Liquid
Heptane 7 Hept -91 98 Liquid
Octane 8 Oct -57 126 Liquid
Nonane 9 Non -54 151 Liquid
Decane 10 Dec -30 174 Liquid


 Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring.                                               

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene).


*             General Properties

Because of differences in molecular structure, the empirical formula remains different between hydrocarbons; in linear, or straight-run alkanes, alkenes and alkynes.

The amount of bonded hydrogen lessens in alkenes and alkynes due to the self-bonding or catenation of carbon preventing entire saturation of the hydrocarbon by the formation of double or triple bonds.


*            Hydrocarbons and Formula

Sl. No. Alkanes Molecular Formula Alkenes Molecular Formula Alkynes Molecular Formula
1. Methan   *** *** *** ***
2. Ethane Ethene Ethne
3. Propane Propene Propyne
4. Butane Butane Butyne
5. Pentane Pentene Pentyne
6. Hexane Hexane Hexyn


*           Usage

Hydrocarbons are one of the Earth's most important sources of energy. The important use of hydrocarbons is as a combustible fuel. In their solid form, hydrocarbons take the form of asphalt. Mixtures of volatile hydrocarbons are now used in preference to the chlorofluorocarbons as a propellant for aerosol sprays, due to chlorofluorocarbon's impact on the ozone layer. Methane and ethane are gaseous at ambient temperatures and cannot be readily liquefied by pressure alone. Propane is however easily liquefied, and exists in 'propane bottles' mostly as a liquid. Butane is so easily liquefied that it provides a safe, volatile fuel for small pocket lighters. Pentane is a clear liquid at room temperature, commonly used in chemistry and industry as a powerful nearly odorless solvent of waxes and high molecular weight organic compounds, including greases. Hexane is also a widely used non-polar, non- aromatic solvent, as well as a significant fraction of common gasoline.


*           Burning Hydrocarbons

Hydrocarbons are currently the main source of the world's electric energy and heat sources, because of the energy produced when burnt. Often this energy is used directly as heat, such as, in home heaters, which use either oil or natural gas. The hydrocarbon is burnt and the heat is used to heat water, which is then circulated. A similar principle is used to create electric energy in power plants.


Common properties of hydrocarbons are the facts that they produce steam, carbon dioxide and heat during combustion and that oxygen is required for combustion to take place. The simplest hydrocarbon, methane, burns as follows:

\[C{{H}_{4}}+2{{O}_{2}}\to 2{{H}_{2}}O+C{{O}_{2}}+\]Energy 

Another example of this property is propane:                        

\[{{C}_{3}}{{H}_{8}}+5{{O}_{2}}\to 4{{H}_{2}}O+3C{{O}_{2}}+\]Energy 

\[{{C}_{n}}{{H}_{2n+2}}+\left( \frac{3n+1}{2} \right){{O}_{2}}\to (n+1)\,{{H}_{2}}O+n\,C{{O}_{2}}+\]Energy 

Burning of hydrocarbons is an example of exothermic chemical reaction.


*          Petroleum

Oil refineries are key to obtaining hydrocarbons. Crude oil is processed in several stages to form desired hydrocarbons, used as fuel and in other products. Extracted hydrocarbons in a liquid form are referred to as petroleum, whereas hydrocarbons in a gaseous form are referred to as natural gas. Petroleum and natural gas are found in the Earth's subsurface with the tools of petroleum geology and are a significant source of fuel and raw materials for the production of organic chemicals.

The extraction of liquid hydrocarbon fuel from sedimentary basins is integral to modern energy development. Hydrocarbons are mined from tar sands and oil shale, and potentially extracted from sedimentary methane hydrates.      

Hydrocarbons are economically important.                         

Because major fossil fuels such as coal, petroleum and natural gas, and its derivatives such as plastics, paraffin, waxes, solvents and oils are hydrocarbons. Hydrocarbons along with \[N{{O}_{2}}\] and sunlight contribute to the formation of tropospheric ozone and greenhouse gases.





         Which one of the following is not a hydrocarbon?

(a) \[C{{H}_{4}}\]

(b) \[{{C}_{2}}{{H}_{5}}OH\]

(c) \[C{{H}_{3}}COOH\]

(d) \[NaOH\]         

(e) None of these


Answer: (d)



          Which one of the following is a structure of alkenes?







(d) All of these  


(e) None of these  


Answer: (b)    



        The general formula for alkyne is:

(a) \[{{C}_{n}}{{H}_{2n+1}}\]

(b) \[{{C}_{n}}{{H}_{2n}}\]

(c) \[{{C}_{n}}{{H}_{2n-1}}\]

(d) \[{{C}_{n}}{{H}_{2n+2}}\]

(e) None of these


Answer: (c)



         The boiling point of methane is

(a) -162 °C

(b) -82 °C        

(c) -64 °C

(d) -120 °C

(e) None of these


Answer: (a)



        Which one of the following is liquid at room temperature?

(a) Methane

(b) Ethane       

(c) Butane

(d) Heptane

(e) None of these


Answer: (d)

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