10th Carbon and its compounds_ CBSE_NCERT Class Xth
(a) Alkanes are gas (C1to C4) ; liquid (C5to C17) and rest are solid at room temperature.
(b) The melting and boiling point of Alkanes increases with increase in number of C � atom in Alkanes .
Alkane | Formula | Boiling point [�C] | Melting point [�C] | State (at 20 �C) |
Methane | CH4 | -162 | -182 | gas |
Ethane | C2H6 | -89 | -183 | gas |
Propane | C3H8 | -42 | -188 | gas |
Butane | C4H10 | 0 | -138 | gas |
Pentane | C5H12 | 36 | -130 | liquid |
Hexane | C6H14 | 69 | -95 | liquid |
Heptane | C7H16 | 98 | -91 | liquid |
Octane | C8H18 | 126 | -57 | liquid |
Nonane | C9H20 | 151 | -54 | liquid |
Decane | C10H22 | 174 | -30 | liquid |
Undecane | C11H24 | 196 | -26 | liquid |
Dodecane | C12H26 | 216 | -10 | liquid |
Hexadecane | C16H34 | 281 | 18 | liquid |
Icosane | C20H42 | 343 | 37 | solid |
Triacontane | C30H62 | 450 | 66 | solid |
Tetracontane | C40H82 | 525 | 82 | solid |
Pentacontane | C50H102 | 575 | 91 | solid |
Hexacontane | C60H122 | 625 | 100 | solid |
Source: http://en.wikipedia.org/wiki/Alkane | ||||
Reason: Alkanes experience inter-molecular van der Waals forces. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes.
A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact.
Note: In Chemistry the Van der Waals forces include attractions and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces.
(c) The density of the alkanes usually increases with increasing number of carbon atoms
Alkane | Formula | Density |
Pentane | C5H12 | 0.626 (liquid) |
Hexane | C6H14 | 0.659 (liquid) |
Heptane | C7H16 | 0.684 (liquid) |
Octane | C8H18 | 0.703 (liquid) |
Nonane | C9H20 | 0.718 (liquid) |
(d) Alkanes are generally insoluble in polar compound like water but dissolve in organic solvents (Non polar compound) like benzene. The liquid alkanes are good solvents for many other covalent compounds.
Reason: Alkanes do not conduct electricity, nor are they polarized by electricity. For this reason they do not form hydrogen bonds and are insoluble in polar solvents such as water.
(e) Alkanes are stable and less reactive than alkenes
Reason: This is because saturated hydrocarbons contain only single bonds which are very stable and difficult to break. On the other hand, unsaturated hydrocarbons contain pie bonds, which can be easily broken as they are more strained.
(f) Alkanes are generally good combustible material due presence of the good percentage of Hydrogen.
Since Alkanes burn with non smoky and non sooty flame due to presence of the good percentage of Hydrogen, It is used as a fuel like LPG and CNG
2CH4 + 2O2 ? CO2 + H2O + Heat
2C2H6 + 7O2 ? 4CO2 + 6H2O + Heat
However, if the supply of air or oxygen is not sufficient for complete combustion, carbon monoxide is formed. Carbon monoxide (CO) is highly poisonous.
2CH4 + 3O2 ? 2CO + 4H2O
2C4H10 + 9O2 ? 8CO + 10H2O
(g) Alkanes do not undergo addition reaction due to strong van der wall force.
Unlike alkane, generally alkene Undergo addition reaction with hydrogen gas in the presence of Polladium catalyst and gives saturated hydrocarbon
Pd | ||||||
CH2 = CH2 | + | H2 | ---------------> | CH3 - CH3 | ||
ThIs addition reaction of hydrogen is helpful in converting vegetable Oil into saturated fat like vanaspati gee . This reaction is commonly called Hydrogenation of Oil.
Note: Butter contain saturated compound where as cooking oil contain unsaturated compound. If Alkaline KMno4 added to both, pink color of KMno4 disappear in cooking oil but remain in pink in butter.
Ni | ||||||
Vegetable Oil | + | H2 | ------------> | CH3 - CH3 | ||
(h) Alkanes undergo addition reaction because hydrogen attached to carbon easily replaced by atom more reactive than hydrogen like halo atom Cl,Br ,I etc.
Sunlight | ||||||
CH4 | + | Cl2 | ------------> | CH3Cl | + | HCl |
Sunlight | ||||||
CH4Cl | + | Cl2 | ------------> | CH2Cl2 | + | HCl |
Sunlight | ||||||
CH2Cl2 | + | Cl2 | ------------> | CHCl3 | + | HCl |
Sunlight | ||||||
CHCl3 | + | Cl2 | ------------> | CH3Cl | + | HCl |
If chlorine present in excess , then reaction does not stopin between but keep on reacting till it form carbon tetra chloride
Bromine and chlorine react with alkene to form vicinal dihalides (iodine does not undergo addition reaction under normal conditions).
CCl4 | ||||||
CH2 = CH2 | + | Cl2 | -----------> | CH2 | - | CH2 |
I | I | |||||
Br | Br | |||||
Ethene | 1,2-Dibromoethane | |||||
CH2 = CH2 | + | Cl2 | --------> | CH3 | - | CH | - | CH2 |
I | I | |||||||
Cl | Cl | |||||||
Ethene | 1,2-Dibropropane | |||||||
Note: In Alkanes, the four valencies of carbon atom are saturated and they have stable s (sigma) bonds. Any nuleophile comes, one hydrogen atom is replaced to accommodate the nucleophile.
A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction.
e.g. CH4 + Cl2 --------> CH3Cl + HCl
In case of Alkene or Alkyne, Carbon atom has double or triple bonds. In this, one is stable s (sigma) bond and the rests are unstable(or weak) p bonds. When nucleophile approches to the carbon atom that has double or triple bonds, they easily break to accommodate it.
e.g., H2C=CH2 + Cl2 --------> H2ClC-CH2Cl.
(i) Cracking (or pyrolysis): Higher alkanes undergo thermal decomposition to give lower alkanes. This process is called pyrolysis or cracking. In this process, vapour of higher alkanes is passed through a hot metal tube (5000C � 7000C). Propane on cracking gives,
D | C3H6 + H2 | |||||
C3H8 | ------------> | |||||
CH4 + C2H4 | ||||||
Cracking of hexane gives butane and ethane. | ||||||
D | ||||||
C6H14 | -------------> | C4H10 + C2H4 | ||||
Q. What's the difference between cracking and pyrolysis?
Pyrolysis : The decomposition of a compound on heating in absence of air is known as pyrolysis.
Cracking: is the breakdown of large organic compounds by use of a catalyst and low temperature to form fewer different compounds
Q. In cracking will the decomposed hydrocarbon always be a saturated and a unsaturated hydrocarbons? Can they both be either unsaturated or saturated hydrocarbons?
Cracking is defined as the process of breaking or decomposition of large hydrocarbons into smaller compounds. This process is widely used in the petroleum industry. Usually, we get a mixture of alkane and alkenes by the process of cracking of hydrcarbons. This is because the bonds in the hydrocarbons can be cleared in any way. As a result, we will not get alkane and alkene exclusively but a mixture of both.
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The ionization energy of an atom or molecule describes the amount of energy required to remove an electron from the atom or molecule in the gaseous state.
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