CBSE Class 10 Science HOTs Carbon And its Compounds Set 05

Bonding in Carbon - The Covalent Bond

Question. The correct representation of covalent bonding in an oxygen molecule is
(a) Oxygen atoms sharing one electron each.
(b) Oxygen atoms sharing three electrons each.
(c) Oxygen atoms sharing two electrons each (double bond).
(d) Oxygen atoms with a single electron shared.
Answer: (c)

Question. The electron dot structure of chlorine molecule is
(a) Cl-Cl with 4 electrons in overlap.
(b) Cl-Cl with 1 electron in overlap.
(c) Cl-Cl with 2 electrons in overlap.
(d) Cl-Cl with 6 electrons in overlap.
Answer: (c)

Question. Assertion (A) : Carbon has a strong tendency to either lose or gain electrons to attain noble gas configuration.
Reason (R) : Carbon has four electrons in its outermost shell and has the tendency to share electrons with carbon or other elements.
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (d) (A) is false, but (R) is true.

Question. Covalent compounds have low melting and boiling points. Why?
Answer: Covalent compounds have low melting and boiling points because the intermolecular forces between the molecules are weak, and a small amount of energy is required to break these bonds.

Question. Write the name of an allotrope of carbon.
Answer: Diamond (or Graphite/Fullerene).

Question. How are covalent bonds formed?
Answer: Covalent bonds are formed by the sharing of valence electrons between two atoms so that both can achieve a stable noble gas configuration.

Question. Covalent compounds are generally poor conductors of electricity. Why?
Answer: Covalent compounds are poor conductors of electricity because they are formed by the sharing of electrons and do not have free ions or electrons to carry an electric charge.

Question. “Carbon prefers to share its valence electrons with other atoms of carbon or with atoms of other elements rather than gaining or losing the valence electrons in order to attain noble gas configuration.” Give reasons to justify this statement.
Answer: Carbon cannot gain four electrons to form \( C^{4-} \) because it would be difficult for the nucleus with six protons to hold ten electrons. It cannot lose four electrons to form \( C^{4+} \) because it requires a huge amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding onto just two electrons. Therefore, carbon shares its electrons.

Question. Carbon forms compounds mainly by covalent bonding. Why?
Answer: Carbon has four electrons in its outermost shell. To attain stability, it needs to gain or lose four electrons, which is energetically unfavorable. Thus, it attains stability by sharing its four valence electrons with other atoms, forming covalent bonds.

Question. State the reason why (i) carbon compounds have low melting and boiling points. (ii) carbon compounds do not conduct electricity. (iii) carbon can form only covalent compounds.
Answer: (i) Because of weak intermolecular forces. (ii) Because they do not form ions. (iii) Because gaining or losing four electrons requires too much energy; sharing is energetically more feasible.

Question. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties.
Answer: Covalent compounds are formed by the sharing of electrons between atoms. They differ from ionic compounds because ionic compounds are formed by the transfer of electrons. Characteristics: 1. Low melting and boiling points. 2. Generally insoluble in water but soluble in organic solvents. 3. Non-conductors of electricity.

Question. Elements forming ionic compounds attain noble gas electronic configuration by either gaining or losing electrons from their valence shells. Explain giving reason why carbon cannot attain such a configuration in this manner to form its compounds. Name the type of bonds formed in ionic compounds and in the compounds formed by carbon. Also explain with reason why carbon compounds are generally poor conductors of electricity.
Answer: Carbon has 4 valence electrons. Losing 4 electrons (\( C^{4+} \)) requires immense energy, and gaining 4 electrons (\( C^{4-} \)) is difficult for a 6-proton nucleus to handle. Ionic compounds have ionic bonds; carbon compounds have covalent bonds. They are poor conductors because no free ions or electrons are produced.

Question. State the reason why carbon can neither form \( C^{4+} \) cations nor \( C^{4-} \) anions, but forms covalent compounds. Also state reasons to explain why covalent compounds : (i) are bad conductors of electricity? (ii) have low melting and boiling points?
Answer: Carbon cannot lose or gain 4 electrons due to energy constraints and nuclear charge limitations. (i) Bad conductors as they lack free charge carriers. (ii) Low melting/boiling points due to weak intermolecular forces between neutral molecules.

Versatile Nature of Carbon

Question. Assertion (A) : Following are the members of a homologous series : \( CH_3OH \), \( CH_3CH_2OH \), \( CH_3CH_2CH_2OH \)
Reason (R) : A series of compounds with same functional group but differing by \( -CH_2 \) unit is called homologous series.
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Question. Name a cyclic unsaturated carbon compound.
Answer: Benzene (\( C_6H_6 \)).

Question. Write the molecular formula of first two members of homologous series having functional group \( -Cl \).
Answer: \( CH_3Cl \) (Chloromethane) and \( C_2H_5Cl \) (Chloroethane).

Question. Write the molecular formula of first two members of homologous series having functional group \( -OH \).
Answer: \( CH_3OH \) (Methanol) and \( C_2H_5OH \) (Ethanol).

Question. Write the molecular formula of the \( 2^{nd} \) and \( 3^{rd} \) member of the homologous series whose first member is ethene.
Answer: \( 2^{nd} \) member: Propene (\( C_3H_6 \)); \( 3^{rd} \) member: Butene (\( C_4H_8 \)).

Question. Write the molecular formula of the \( 2^{nd} \) and \( 3^{rd} \) member of the homologous series whose first member is methane.
Answer: \( 2^{nd} \) member: Ethane (\( C_2H_6 \)); \( 3^{rd} \) member: Propane (\( C_3H_8 \)).

Question. Write the next homologue of each of the following : (i) \( C_2H_4 \) (ii) \( C_4H_6 \)
Answer: (i) \( C_3H_6 \) (ii) \( C_5H_8 \).

Question. Name the following compounds : (a) \( CH_3 - CH_2 - OH \) (b) \( CH_3 - CHO \)
Answer: (a) Ethanol (b) Ethanal.

Question. Select saturated hydrocarbons from the following : \( C_3H_6 \); \( C_5H_{10} \); \( C_4H_{10} \); \( C_6H_{14} \); \( C_2H_4 \)
Answer: \( C_4H_{10} \) and \( C_6H_{14} \).

Question. Write the name and structure of an alcohol with three carbon atoms in its molecule.
Answer: Propanol (\( C_3H_7OH \)). Structure: \( CH_3 - CH_2 - CH_2 - OH \).

Question. Write the name and structure of an alcohol with four carbon atoms in its molecule.
Answer: Butanol (\( C_4H_9OH \)). Structure: \( CH_3 - CH_2 - CH_2 - CH_2 - OH \).

Question. Write the name and structure of an aldehyde with four carbon atoms in its molecule.
Answer: Butanal (\( C_3H_7CHO \)). Structure: \( CH_3 - CH_2 - CH_2 - CHO \).

Question. Which element exhibits the property of catenation to maximum extent and why?
Answer: Carbon exhibits catenation to the maximum extent because of its small size and the high strength of the C-C bond.

Question. Write the name and molecular formula of the fourth member of alkane series.
Answer: Butane (\( C_4H_{10} \)).

Question. What is homologous series of carbon compounds?
Answer: A series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, where successive members differ by a \( -CH_2 \) unit.

Question. Write the name and formula of the \( 2^{nd} \) member of homologous series having general formula \( C_nH_{2n} \).
Answer: Propene (\( C_3H_6 \)).

Question. Write the name and formula of the \( 2^{nd} \) member of homologous series having general formula \( C_nH_{2n+2} \).
Answer: Ethane (\( C_2H_6 \)).

Question. Write the name and formula of the \( 2^{nd} \) member of homologous series having general formula \( C_nH_{2n-2} \).
Answer: Propyne (\( C_3H_4 \)).

Question. Write the number of covalent bonds in the molecule of ethane.
Answer: Seven covalent bonds (6 C-H bonds and 1 C-C bond).

Question. Write the number of covalent bonds in the molecule of butane, \( C_4H_{10} \).
Answer: Thirteen covalent bonds (10 C-H bonds and 3 C-C bonds).

Question. Write the name of each of the following functional groups : (a) \( -OH \) (b) \( >C=O \)
Answer: (a) Alcohol group (b) Ketone group.

Question. Write the name and molecular formula of the first member of the homologous series of alkynes.
Answer: Ethyne (\( C_2H_2 \)).

Question. (a) Write the molecular formula of the following carbon compounds : (i) Methane (ii) Propane (b) Carbon compounds have low melting and boiling points. Why?
Answer: (a) (i) \( CH_4 \), (ii) \( C_3H_8 \). (b) Due to weak intermolecular forces between the covalent molecules.

Question. Consider the carbon compounds having following molecular formula: (i) \( C_3H_6 \) (ii) \( C_3H_8 \) (iii) \( C_4H_6 \) (iv) \( C_6H_6 \) (v) \( C_6H_{12} \) (a) State the number of double covalent bonds present in \( C_3H_6 \). (b) Write the formula of first member of the homologous series to which the carbon compound \( C_4H_6 \) belongs. (c) Which one of the above compounds forms ring structure of carbon atoms? (d) Identify, which of the above compounds, is a member of alkane series.
Answer: (a) One double bond. (b) Ethyne (\( C_2H_2 \)). (c) \( C_6H_6 \) (Benzene) or \( C_6H_{12} \) (Cyclohexane). (d) \( C_3H_8 \) (Propane).

Question. The molecular formulae of two alkynes, A and B are \( C_xH_2 \) and \( C_3H_y \) respectively. (a) Find the values of x and y. (b) Write the names of A and B.
Answer: (a) Using \( C_nH_{2n-2} \): For A, \( 2n-2 = 2 \implies 2n = 4 \implies n=2 \), so x = 2. For B, \( n=3 \implies y = 2(3)-2 = 4 \). (b) A is Ethyne, B is Propyne.

Question. What is a homologous series ? Find the difference in molecular mass between the two consecutive members of a homologous series. State how in a homologous series of carbon compounds the following properties vary with increase in molecular mass : (i) Melting and boiling points (ii) Chemical properties
Answer: A series of related organic compounds. Mass difference is 14 u. (i) Melting and boiling points increase with molecular mass. (ii) Chemical properties remain similar as the functional group is the same.

Question. Carbon, a member of group 14, forms a large number of carbon compounds estimated to be about three million. Why is this property not exhibited by other elements of this group? Explain.
Answer: Due to catenation and tetravalency. Other elements of group 14 have larger atomic size, making their bonds weaker and less stable than carbon-carbon bonds.

Question. What is a homologous series of carbon compounds? Give an example and list its three characteristics.
Answer: Example: Alkanes (\( CH_4, C_2H_6, ... \)). Characteristics: 1. Successive members differ by \( -CH_2 \). 2. Same functional group. 3. Gradation in physical properties.

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) Because they do not form ions. (b) Cyclohexane (\( C_6H_{12} \)). It has 18 single bonds (6 C-C and 12 C-H).

Question. An aldehyde as well as a ketone can be represented by the same molecular formula, say \( C_3H_6O \). Write their structures and name them. State the relation between the two in the language of science.
Answer: Aldehyde: Propanal (\( CH_3CH_2CHO \)); Ketone: Propanone (\( CH_3COCH_3 \)). They are functional isomers.

Question. What is meant by functional group in carbon compounds? Write in tabular form the structural formula and the functional group present in the following compounds : (i) Ethanol (ii) Ethanoic acid
Answer: A functional group is an atom or group of atoms that defines the chemical properties of an organic compound. (i) Ethanol: \( CH_3CH_2OH \), Alcoholic group (\( -OH \)). (ii) Ethanoic acid: \( CH_3COOH \), Carboxylic acid group (\( -COOH \)).

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: "Homos" means same; members share same functional group. Example: \( CH_3OH \) and \( C_2H_5OH \). (i) The carbon chain/molecular mass determines physical properties. (ii) The functional group determines chemical properties.

Question. State the meaning of the functional group in an organic compound. Write the formula of the functional group present in alcohols, aldehydes, ketones and carboxylic acids.
Answer: Alcohols: \( -OH \); Aldehydes: \( -CHO \); Ketones: \( >C=O \); Carboxylic acids: \( -COOH \).

Question. Explain why carbon forms compounds mainly by covalent bonds. Explain in brief two main reasons for carbon forming a large number of compounds. Why does carbon form strong bonds with most other elements?
Answer: Carbon forms covalent bonds because its tetravalency makes losing or gaining 4 electrons difficult. Two reasons: Catenation and Tetravalency. It forms strong bonds because of its small atomic size, which allows the nucleus to hold onto the shared pair of electrons strongly.