CBSE Class 10 Science Chemistry Carbon and its compounds Assignment

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. What are covalent bonds? Show their formation with the help of electron dot structure of methane. Why are covalent compounds generally poor conductors of electricity?
Answer : Covalent bonds are those bonds which are formed by sharing the valence electrons between two atoms. Electron dot structure of methane is shown in the figure. Covalent compounds are generally poor conductors of electricity because they do not have free electrons or ions.

Question. Taking a suitable example show the formation of a molecule bonded by triple covalent bond.
Answer : Nitrogen (atomic number 7) atom has an electronic configuration of K L 2 5 indicating the presence of 5 electrons in the outermost L-shell. Thus, each nitrogen atom needs three electrons to attain the nearest stable noble gas electronic configuration of neon gas. So, two nitrogen atoms combine together by sharing 3 electrons each to form a molecule of nitrogen.

Question. Why are most carbon compounds poor conductors of electricity?
Answer : 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. 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 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 hydroca rbon 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. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint : The eight atoms of sulphur are joined together in the form of a ring).
Answer : The atomic number (Z) of sulphur is sixteen and its electronic configuration is 2, 8, 6. The sulphur atom has six valence electrons. The chemical formula of sulphur molecule
is S8. Each sulphur atom is linked to similar atoms on either sides by single covalent bonds and thus, completes its octet.
The molecule is in the form of a ring also represented by crown shape.

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. Consider the molecular models of the two organic compounds shown below: 

(a) Name the homologous series that compounds I and II belong to and give its general formula.
(b) Write the molecular formulae of next homologue of both compounds I and II.
Answer : (a) Alkenes with general formula C_nH_2n.
(b) Next homologue of compound (I) is C₃H₆ i.e., propene and next homologue of compound (II) is C₅H₁₀ i.e., pentene.