CBSE Class 10 Science
Ch-4 Carbon and its Compounds
Carbon and its Compounds – CBSE Notes for Class 10 Science
1. The earth’s crust, has only 0.02% carbon in the form of minerals (like carbonates^bicarbonates, coal, and petroleum).
2. The atmosphere has 0.03% of carbon dioxide.
3. Inspite of its small amount available in nature carbon is a versatile element as it forms the basis for all living organisms and many things which we use.
4. Bonding in carbon :
(a) Atomic number of carbon = 6
(b) Electronic configuration has 2 electrons in K shell and 4 electrons in L shell.
(c) In order to attain the noble gas configuration, carbon should either gain 4 electrons or lose 4 electrons or can share it’s 4 electrons with some other element.
(d) Gain of 4 electrons (to form octet, i.e., 8 electrons in C4- anion) is difficult because then a nucleus with 6 protons will have to hold extra four electrons.
(e) Loss of 4 electrons (to attain duplet, i.e., 2 electrons like He atom in C4+ cation) is difficult as it requires large amount of energy to remove four electrons.
(f) Carbon, hence, overcomes this difficulty by sharing it’s four valence electrons with other atoms of carbon or with atoms of other elements. These electrons contributed by the atoms for mutual sharing in order to acquire the stable noble gas configuration is called covalency of that atom. Hence, carbon shows TETRACOVALENCY.
(g) The simplest molecule formed by sharing of electrons (i.e., covalent bonds), can be represented by electron dot structure.
5. Allotropes of carbon: The phenomenon by means of which an element can exist in two or more forms, with similar chemical properties but different physical properties is called allotropy and the different forms are called allotropes. Carbon shows three allotropic forms :
» Diamond is the hardest substance whereas graphite is very soft.
» Diamond is used for grinding and polishing of ‘ hard materials and gnaphite is used as a lubricant.
» Diamond has three dimensional rigid structure but graphite has hexagonal sheet layer structure.
» Diamond is a bad conductor of electricity but graphite is very good conductor of electricity.
6. Fullerenes : A new category of carbon allotrope, fullerenes are spherical in shape or a soccer ball like. The first fullerene identified was C-60 with 60 carbon atoms arranged like the godesic dome designed by US architect, Buckminster Fuller, hence these are also known as Buckminster Fullerenes or Bucky Ball structures.
7. Cause of versatile nature of carbon : Four main reasons for versatile nature of carbon are:
(a) Catenation : It is the unique property of self linkage of carbon atoms by means of covalent bonds to form straight chains, or branched chains, or the rings of different sizes (as shown below):
(b) Tetracovalency: Due to small size, and presence of four valence electrons, carbon can form strong bonds with other carbon atoms, hydrogen, oxygen, nitrogen, or sulphur, etc. For example, compounds of carbon with hydrogen are called hydrocarbons.
(c) Multiple Bond Formation : Small size of carbon also enables it to form multiple bonds, (i.e., double bonds or triple bonds) with other elements as well as with its own atoms. This increases the number of carbon compounds.
» Compounds of carbon with double bonds and triple bonds are called as unsaturated compounds while those with carbon-carbon single bonds are called saturated compounds.
» Alkenes (with —C = C —) and Alkynes (with —C = C—) are hence unsaturated, whereas Alkanes (with — C — C—) are saturated compounds.
(d) Isomerism: The phenomenon by means of which the carbon compounds with same molecular formula show different structures, and properties, e.g., A chain of 4 carbon atoms can be written in two ways :
Hence, number of carbon compounds increases to a huge number.
8. Hydrocarbons : Large number of hydrocarbons can be classified as:
Note: In open chain, the name of parent chain is derived from the root word and suffix ane, ene or yne is added depending on the type of bond present in a chain :
Important: No alkene or alkyne is possible with single carbon atom because double or triple bond is not possible between carbon and hydrogen atom. It is only between two carbon atoms.
9. Functional Group:
» An atom or a group of atoms which when present in a compound gives specific properties to it, is called a functional group.
» A single line shown along with a functional group is called as its free valency by which it gets attached to a compound by replacing one hydrogen atom or atoms, e.g., -Cl.
» Functional group, replacing the hydrogen is also called as heteroatom because it is different from carbon, and can be nitrogen, sulphur, or halogen, etc.
» Important: Replacement of hydrogen atom by a functional group is always in such a manner that valency of carbon remains satisfied.
Note : Cl is named as prefix Chloro; Br as Bromo; NH2 as Amino and N02 as Nitro.
Important Note: Symbol ‘R’ in a formula represents an Alkyl Group which is formed by the removal of one hydrogen atom from an alkane.
10. Homologous series: A series of organic compounds in which every succeeding member differs from the previous one by -CH2 group or 14 a.m.u.
Note : As the molecular mass increases in a series, : so physical properties of the compounds
show a variation, but chemical properties which are determined solely by a functional group, remains same within a series.
11. Nomenclature of Organic Compounds
(I) Trivial or common names : These names were given after the source from which the organic compounds were first isolated, e.g., If a compound has one carbon atom, then its common name will have root word form and so on (see table).
(II) IUPAC name : International Union of Pure and Applied Chemistry gave following rules for naming various compounds :
(i) Identify the number of carbon atoms and write the word root corresponding to it. e.g., If number of carbon atoms are three, then word root is prop.
(ii) Presence of a functional group is indicated by prefix or suffix as given in table 2, and table 3.
(iii) If the name of functional group is to be given as a suffix, the last letter ‘e’ in the name of compound is deleted and suffix is added. e.g., ketone with three carbon atoms is named as :
Propane – e = Propan + ‘one’ = Propanone. Alcohol with three carbons is propanol. Carboxylic acid with three carbons is propanoic acid.
(iv) Halogens, in IUPAC, are written as Prefixes, e.g., Compound With two carbons and one chloro group is named as : chloroethane (CH3CH2CI).
12. Chemical properties of carbon compounds :
Main properties of carbon compounds are :
(a) Combustion Reaction
(b) Oxidation Reaction
(c) Addition Reaction .
(d) Substitution Reaction
(a) Combustion Reaction : A chemical reaction in which a substance burns in the presence of air or oxygen is called combustion reaction.
Note: Combustion is always a EXOTHERMIC reaction, e.g.,
» Saturated hydrocarbons generally give clean flame whereas unsaturated hydrocarbons give sooty flame (because carbon content is more than hydrogen content in these, and hence carbon shows incomplete combustion, and appears as soot).
» Saturated hydrocarbons can give sooty flame in limited supply of oxygen.
(b) Oxidation Reaction : The addition of oxygen in a compound upon combustion is called oxidation.
In addition to combustion, oxidation can also be : brought about by some substances which are
capable of giving oxygen to others, i.e., Oxidising agents, e.g., Acidified K2Cr207 (Potassium dichromate) and alkaline KMn04 (Potassium permanganate).
(c) Addition Reaction : Addition of a molecule in unsaturated compounds in the presence of a catalyst, to give saturated compound is called an addition reaction, e.g.,
Hydrogenation of vegetable oils as shown in the reaction below :
(d) Substitution Reaction : The reactions which involve the replacement of an atom or group of atoms from a molecule by another atom without any change in structure in the remaining part of the molecule.
13. Ethanol: (or alcohol)
Colourless liquid, soluble in water, and has a distinct smell and burning taste. Its consumption in small quantities causes drunkenness and can be lethal.
14. Ethanoic Acid : CH3COOH
Common Name : Acetic Acid.
5-8% solution of acetic acid in water is called Vinegar. And 100% pure acetic acid is called Glacial acetic acid because it has m.pt. 290 K and freezes forming glacier like crystals.
Reactions of ethanoic acid :
Saponification : Esters in the presence of acid or base react to give back alcohol and carboxylic acid is called saponification.
15. Soaps and Detergents :
Soaps and Synthetic Detergents : Soaps and detergents are substances used for cleaning.
Soap : Soaps are sodium or potassium salts of higher fatty acids, such as Oleic acid (C17H33COOH), Stearic acid (C17H35COOH), Palmitic acid (C15H31COOH), etc. These acids are present in the form of their esters along with glycerol (an alcohol containing three hydroxyl groups). These esters, called ‘glycerides’ are present in fats and oils of animal and vegetable origin.
Preparation of Soap: When an oil or a fat (glyceride) is treated with sodium hydroxide solution, it gets converted to sodium salt of the acid (soap) and glycerol. The reaction is known as saponification.
Detergents : Chemically, detergents are sodium salts of sulphonic acids, i.e., detergents contain a sulphonic acid group (—S03H), instead^of a carboxylic acid group (—COOH), on one end of the hydrocarbon.
The cleansing action of a detergent is considered to be more effective than a soap.
Cleansing Action of Soaps and Detergents : The cleansing action of soaps and detergents follows the same principle.
When a soap or detergent is dissolved in water, the molecules gather together as clusters, called micelles. The tails stick inwards and the heads outwards.
In cleansing, the hydrocarbon tail attaches.itself to oily dirt. When water is agitated, the oily dirt tends to lift off from the dirty surface and dissociates into fragments. This gives an opportunity to other tails to stick to oil. The solution now contains small globules of oil surrounded by detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and form aggregates. Thus, the oily dirt is removed from the object.
16. Scum : The insoluble precipitates formed by soap molecule when they react with calcium and magnesium ions present in hard water. Due to this, lot of soap gets wasted and cleansing action gets reduced to a larger extent.