CBSE Class 10 Science Carbon and its compound Notes

INTRODUCTION Carbon is the fourth most abundant element in the universe by mass. It is also the second most abundant element in the human body after oxygen. It is the 15th most common element in the Earth’s crust. Carbon was discovered in prehistory and it was known to the ancients. They used to manufacture charcoal by burning organic material.

Carbon is a non-metal. It belongs to the fourteenth group or IV A group in the modern periodical table. The elements of this group have four electrons in the valence shell.

Atomic Number: 6
Electronic configuration: 2, 4
Valence electrons: 4
Property: Non-metal

Compounds having carbon atoms among the components are known as carbon compounds. Previously, carbon compounds could only be obtained from a living source; hence they are also known as organic compounds.

BONDING IN CARBON: COVALENT BOND
Bond formed by sharing of electrons is called covalent bond. Two of more atoms sharen electrons to make their configuration stable. In this type of bond, all the atoms have similar rights over shared electrons. Compounds which are formed because of covalent bond are calledb COVALENT COMPOUNDS.

FORMATION OF COVALENT BONDS

1. Covalent band is the chemical bond formed through the sharing of electrons between two non-metal atoms.
2. Compounds which have covalent bonds are called covalent compounds.
3. Examples of covalent compounds or molecules are chlorine, Cl₂, carbon dioxide, CO₂, ammonia, NH₃, water, H₂O, and tetrachloromethane (carbon tetrachloride), CCl₄.
4. During the formation of covalent molecules, each non-metal covalent atom provides one, two or three electrons to be shared with other atoms. The bond formed is called a covalent bond.
5. Through this process, each non-metal atom in covalent molecules will achieve stable electron arrangement.
6. The type of covalent bond formed in a covalent compound depends on the number of electron pairs shared between non-metal atoms. Covalent bonds are of three types: Single, double and triple covalent bond.

CBSE Class 10 Science Chapter 4 Carbon and its compound Very Short Answer Type Questions 

Question. LPG (liquefied petroleum gas) is used as a fuel for cooking in homes. Why is LPG stored as a liquid?
Answer. LPG is stored as a liquid so that more fuel can be kept in a container. Liquids contain more particles per unit volume than gases. It is also easier to transport liquids than gases.

Question. Why carbon does not form the ionic compounds?
Answer. For formation of ionic compounds, carbon should either gain four electrons to form C^{4– i}ons or should lose four electrons to form C⁴⁺ ions but formation of both these ions is not possible due to high energy considerations.

Question. What are covalent compounds?
Answer. A covalent compound is a molecule formed by covalent bonds, in which the atoms share one or more pairs of valence electrons.

Question. Give different forms in which carbon occurs in nature.
Answer. Carbon occurs in free form e.g., graphite and diamond, in combined form like carbon dioxide, carbonates, etc.
In earth’s crust – 0.02% and in atmosphere – 0.03%.

Question. Write the molecular formula of first two members of homologous series having functional group —Cl.
Answer. The molecular formula of first two members of homologous series having –Cl functional group are CH₃Cl and CH₃CH₂Cl.

Question. Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is methane.
Answer. Methane, CH₄ is an alkane. Alkanes have general formula, C_nH_{2n + 2}. 2nd member of homologous series of alkanes is C₂H₆ i.e., ethane.
3rd member of homologous series of alkanes is C₃H₈ i.e., propane.

Question. Select saturated hydrocarbons from the following :
C₃H₆; C₅H₁₀; C₄H₁₀; C₆H₁₄; C₂H₄
Answer. Saturated hydrocarbons have general formula, C_nH_2n+2.Among the given compounds only C₄H₁₀ and C₆H₁₄ satisfy the above formula. Thus, these are saturated hydrocarbons.

Question. Write the name and formula of the 2nd member of homologous series having general formula C_nH_{2n – 2}.
Answer. General formula, C_nH_{2n + 2} belongs to alkyne series. The second member of this series is propyne i.e., (C₃H₄) or CH₃ — C ≡ CH.

Question. An organic compound burns with a sooty flame. Is it a saturated or an unsaturated compound?
Answer. It is an unsaturated compound. Unsaturated compounds have more percentage of carbon as compared to saturated compounds. Thus, unsaturated compounds undergo incomplete combustion in the presence of air producing a yellow flame with lots of black smoke.

CBSE Class 10 Science Chapter 4 Carbon and its compound Short Answer Type Questions 

Question. The molecular formulae of alkenes X and Y are CxH₈ and C₁₀Hy respectively. What are the relative molecular masses of X and Y?
Answer. The general formula of an alkene is C_nH_2n.
Thus, the molecular formula of X is C₄H₈, and the molecular formula of Y is C₁₀H₂₀.
Molecular mass of X = (4 × 12) + (8 × 1) = 56
Molecular mass of Y = (10 × 12) + (20 × 1) = 140

Question. What is homologous series of compounds?
List any two characteristics of homologous series.
Answer. A series of compounds having similar structures and similar chemical properties in which the successive member differs in their molecular formula by –CH₂ group is called homologous series.
Characteristics :
(a) All the members of homologous series have similar chemical properties.
(b) Any two consecutive members differ in their molecular formula by a –CH₂ group.

Question. A hydrocarbon molecule has 3 carbon atoms. Write down its molecular formula if it is an : (i) alkane (ii) alkene (iii) alkyne.
Answer. The number of carbon atoms in the molecule of this hydrocarbon is 3, that is, n = 3.
(i) The general formula of an alkane is C_nH_2n+2. On putting n = 3, we get C₃H_2×3+2 or C₃H₈ (propane).
(ii) For alkene having general formula C_nH_2n, we get,C₃H_{2 × 3} or C₃H₆ (propene).
(iii) For alkyne having general formula C_nH_2n–2 we get C₃H_{2 × 3 – 2} or C₃H₄ (propyne).

Question. Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Answer. The molecule of chloromethane (CH₃Cl) consists of three elements i.e., carbon (Z = 6), hydrogen (Z = 1) and chlorine (Z = 17). Carbon atom has four valence electrons (2, 4) ; hydrogen has one (1) while chlorine has seven electrons in the valence shell (2, 8, 7). In order to complete its octet, carbon shares three valence electrons with three hydrogen atoms while one is shared with the electron of chlorine atom.

Question. State two properties of carbon which lead to a very large number of carbon compounds.
Answer. Carbon forms a large number of carbon compounds like long chains which may be straight or branched chains or ring of different sizes due to its tetravalency and unique property of catenation. Carbon due to its small size forms exceptionally stable compounds by forming strong bonds.

Question. Two non-metals A and B combine with each other by sharing of electrons to form compound C.
(a) What is the nature of compound C ?
(b) Will it dissolve in water or organic solvents?
(c) Will it be a good or bad conductor of electricity?
(d) Will it have high or low melting/boiling point ?
Answer. (a) Covalent
(b) Usually soluble in organic solvents
(c) Bad conductor
(d) Usually low melting / boiling point.

Question. (i) Which allotrope of carbon is used for making cores of lead pencils?
(ii) Which allotrope of carbon is used for making expensive jewellery?
(iii) Name an allotrope of carbon which contains both single and double bonds.
Answer. (i) Graphite (ii) Diamond (iii) Graphite

Question. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties.
Answer. Covalent compounds are those compounds which are formed by sharing of valence electrons between the atoms e.g., Hydrogen molecule is formed by mutual sharing of electrons between two hydrogen atoms.
They are different from ionic compounds as ionic compounds are formed by the complete transfer of electrons from one atom to another e.g., NaCl is formed when one valence electron of sodium gets completely transferred to outer shell of chlorine atom.
The characteristic properties of covalent compounds are :
(i) They are generally insoluble or less soluble in water but soluble in organic solvents.
(ii) They have low melting and boiling points.
(iii) They do not conduct electricity as they do not contain ions.

Question. Why is homologous series of carbon compounds so called? Write the chemical formula of two consecutive members of any homologous series and state the part of these compounds that determines their (i) physical and (ii) chemical properties.
Answer. A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series are differ by a CH₂ unit or 14 mass units.

Question. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer. (i) Catenation : Carbon has the unique property of self linking which is known as catenation. In fact, any number of carbon atoms can be linked to one another by covalent bonds. This is on account of the stability of C — C bonds since the size of the carbon atom is quite small.
(ii) Linking of carbon with other atoms : Carbon is tetravalent in nature and can readily unite with atoms like hydrogen, oxygen, nitrogen, sulphur, etc. by electron sharing.

Question. What is a homologous series? Explain with an example.
Answer. A homologous series can be defined as a family of organic compounds having the same functional group, similar chemical properties and the successive members of which differ by a – CH₂ group or 14 mass units. For example, CH₃OH (methanol), CH₃CH₂OH (ethanol), CH₃CH₂CH₂OH (propanol), CH₃CH₂CH₂CH₂OH (butanol), etc. constitute a homologous series of alcohols. They have the same functional group, i.e., —OH (hydroxyl). Since they have the same functional group, they show similar chemical properties. The difference between any two successive members is a —CH2 group and 14 mass units. Their physical properties such as melting point and boiling point increase as the molecular mass increases. Their solubility in water, however, decreases with increase in molecular mass.

Question. (a) Why are most carbon compounds poor conductors of electricity?
(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound.
Answer. (a) Due to catenation, carbon forms covalent bonds with the constituent elements in the carbon compounds, hence it does not have mobile electrons and carbon compounds do not dissociate themselves into ions and hence, they are poor conductor of electricity.

Question. (a) What is a homologous series of compounds? List any two of its characteristics.
(b) What is the next higher homologue of C₃H₇OH? What is its formula and what is it called?
Answer. (a) A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series are differ by a CH₂ unit or 14 mass units.
Two characteristics of homologous series are :
(i) The successive compounds of the homologous series differ by —CH₂ unit i.e. 14 mass units.
(ii) Each homologous series belongs to similar class of compounds which shows the same properties.
(b) Next higher homologue of C₃H₇OH is C₄H₉OH i.e., butanol.

Question. Why are carbon and its compounds used as fuels for most applications?
Answer. Carbon burns in oxygen or air to form carbon dioxide gas. The reaction is highly exothermic. That is why different forms of coal are used as fuels. The most important compounds of carbon are hydrocarbons. Just like carbon, hydrogen also readily burns in oxygen or air to form water producing heat. The hydrocarbon methane (CH₄) is a constituent of natural gas. Propane (C₃H₈) and butane (C₄H₁₀) are present in liquid petroleum gas (L.P.G.). Petrol and kerosene also contain different hydrocarbons. Therefore, these are used as fuels.

Question. (a) Why two carbon atoms cannot be linked by more than three covalent bonds.
(b) Explain the following :
(i) Diamond is a covalent solid, yet has high melting point.
(ii) Graphite is a good conductor of electricity but diamond is not.
(iii) Diamond is used for making tools for cutting and drilling.
Answer. (a) Since the maximum angle strain is obtained when the two carbon atoms are linked by three covalent bonds, therefore, two carbon atoms cannot be linked to each other by more than three covalent bonds.
(b) (i) It is a giant molecule containing a large number of carbon-carbon single covalent bonds. To break these covalent bonds, a large amount of energy is needed and hence diamond has a high melting point.
(ii) Graphite has two-dimensional layered structure containing fused hexagonal rings of carbon atoms. Only three valence electrons are used for bond formation while the fourth electron is free to move.
Due to ability of this free electron to move, graphite is a good conductor of heat and electricity. Since in diamond, all the four electrons are involved in bond formation, therefore, diamond is a bad conductor of electricity.
(iii) Diamond is a good conductor of heat and is used for making cutting and drilling tools because the heat generated during cutting and drilling is easily absorbed by the network structure without overheating the diamond tool.

CBSE Class 10 Science Chapter 4 Carbon and its compound Case Based MCQs

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