CBSE Class 10 Science Carbon and its compounds Assignment Set 09

This is Assignment Set 09 for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds, with the marks of each question and a step-by-step answer with a Teacher's Note. It covers covalent bonding and the versatile nature of carbon, catenation and tetravalency, hydrocarbons, homologous series, isomers and functional groups.

Very Short and Short Answer Questions

Question 1. What is a saturated hydrocarbon? Write the formula of any one saturated hydrocarbon. [1 mark]
Answer:
A hydrocarbon in which all the carbon atoms are joined by single bonds only is called a saturated hydrocarbon (alkane). Example: methane, CH₄ (or ethane, C₂H₆).
In simple words: Only single C–C bonds, such as in methane.

Teacher's Note:
1. Saturated hydrocarbons are also called alkanes.
2. General formula: C_nH_2n+2.

Question 2. The formulae of four organic compounds are given as following: P = C₂H₄, Q = CH₃COOH, R = C₂H₅OH, S = C₂H₆. (A) Which one of these compounds P, Q, R or S is a saturated hydrocarbon? (B) Identify the organic acid and give its structural formula. [2 marks]
Answer:
(A) A saturated hydrocarbon has only carbon and hydrogen and only single bonds. S (C₂H₆, ethane) fits. P (C₂H₄) has a double bond, and Q and R contain oxygen.
(B) The organic acid is Q, ethanoic acid (CH₃COOH). Structural formula: CH₃–C(=O)–OH, that is, a methyl group joined to a carboxyl group, –COOH.
In simple words: Ethane is the saturated hydrocarbon and CH₃COOH is the acid.

Teacher's Note:
1. Check for oxygen first, then for double bonds.
2. The acid is recognised by the –COOH group.

Question 3. Two carbon atoms cannot be linked to each other by more than three covalent bonds. Why? [1 mark]
Answer:
Carbon has a valency of four. If two carbon atoms were joined by three bonds (a triple bond), each carbon would have only one valency left for another atom.
A fourth bond between the same two carbon atoms would use up the whole valency of both and leave no bond to attach any other atom, so no stable compound could be formed. Hence the maximum is a triple bond.
In simple words: Each carbon has only four bonds to give, so three bonds between two carbons is the limit.

Teacher's Note:
1. Ethyne, H–C≡C–H, has the triple bond.
2. Count the bonds on each carbon: it must be four.

Question 4. Catenation is the ability of an atom to form bonds with other atoms of the same element. It is exhibited by both carbon and silicon. Compare the ability of catenation of the two elements. Give reasons. [2 marks]
Answer:
Comparison: Carbon has a much greater ability of catenation than silicon. Carbon forms long chains, branched chains and rings, while silicon forms chains of only a few atoms (up to about seven or eight).
Reason: Carbon is a small atom, so the C–C bond is very strong. The silicon atom is larger, so the Si–Si bond is weaker and the chains break easily.
In simple words: Small carbon atoms make strong C–C bonds, so carbon forms long chains; silicon does not.

Teacher's Note:
1. Bond strength depends on atomic size.
2. Silicon compounds are therefore far fewer in number.

Question 5. Answer the following questions: (i) What are the two properties of carbon which lead to the huge number of carbon compounds we see around us? (ii) Which element exhibit the property of catenation to maximum extent and why? [3 marks]
Answer:
(i) Catenation: carbon atoms can link with other carbon atoms by strong covalent bonds to form long chains, branched chains and rings. Tetravalency: with a valency of four, carbon can bond with four other atoms such as carbon, hydrogen, oxygen, nitrogen, sulphur or chlorine.
(ii) Carbon shows catenation to the maximum extent, because its small size makes the C–C bonds very strong and stable.
In simple words: Catenation and tetravalency give the huge number of carbon compounds; carbon is the best catenator.

Teacher's Note:
1. Write both properties with a short reason.
2. Carbon–carbon bonds are among the strongest single bonds between identical atoms.

Question 6. Explain with the help of tetravalency of carbon, the bond formation in the CH₄ molecule. [2 marks]
Answer:
Step 1: Carbon has atomic number 6 (electronic configuration 2, 4), so it has four valence electrons and needs four more to complete its octet.
Step 2: Each of the four hydrogen atoms has one electron and needs one more to complete its duplet.
Step 3: Carbon shares one electron with each hydrogen atom, forming four C–H single covalent bonds. Carbon then has 8 electrons in its outer shell and every hydrogen has 2, so all atoms are stable.

In simple words: Carbon shares its four electrons with four hydrogens to form four C–H bonds.

Teacher's Note:
1. Tetravalency means carbon forms four bonds.
2. In a dot structure, show one shared pair for each bond.

Question 7. Mention the position of carbon on the basis of its electronic configuration and comment on the nature of compound formed by carbon. [2 marks]
Answer:
Position: Carbon has atomic number 6, so its electronic configuration is 2, 4. It has four valence electrons and two shells, so it is in group 14 and period 2 of the periodic table.
Nature of compounds: To gain or lose four electrons needs a very large amount of energy and would give highly unstable ions (C⁴⁺ or C⁴⁻). Carbon therefore shares its four electrons and forms covalent compounds.
In simple words: Carbon sits in group 14, period 2 and forms covalent compounds by sharing electrons.

Teacher's Note:
1. Group = number of valence electrons + 10 for p-block elements.
2. Period = number of shells.

Question 8. Explain the nature of the covalent bond using the bond formation in CH₃Cl. [3 marks]
Answer:
Step 1: Carbon (2, 4) has four valence electrons, hydrogen has one and chlorine (2, 8, 7) has seven. Carbon can neither lose nor gain four electrons easily, so it shares them.
Step 2: Carbon shares one electron pair with each of three hydrogen atoms and one electron pair with the chlorine atom, forming three C–H bonds and one C–Cl bond.
Step 3: In this way the carbon atom gets 8 electrons, each hydrogen atom gets 2 and the chlorine atom gets 8 electrons in the outer shell. The shared electrons belong to both atoms; this type of bond, formed by sharing electrons, is a covalent bond.
In simple words: Carbon shares electron pairs with three hydrogens and one chlorine, so all atoms complete their outer shells.

Teacher's Note:
1. Hydrogen needs only two electrons (duplet), the others need eight (octet).
2. The shared pair is counted for both atoms.

Question 9. What are covalent compounds ? Why are they different from ionic compounds? List their three characteristics properties. [3 marks]
Answer:
Covalent compounds are compounds formed by the sharing of electrons between atoms, usually between non-metals (for example, CH₄, H₂O, CO₂).
Difference: Ionic compounds are formed by transfer of electrons and consist of ions held by strong electrostatic forces, whereas covalent compounds consist of molecules held together by weak forces between them.
Properties of covalent compounds: (1) low melting and boiling points; (2) poor conductors of electricity, since they have no ions; (3) generally insoluble in water but soluble in organic solvents.
In simple words: Covalent compounds share electrons, are made of molecules and have low melting points and poor conductivity.

Teacher's Note:
1. Contrast with ionic compounds such as NaCl.
2. Weak intermolecular forces explain the low melting points.

Question 10. The table shows the electronic structures of four elements: P = 2, 6; Q = 2, 8, 1; R = 2, 8, 7; S = 2, 8, 8. (A) Identify which element(s) will form covalent bonds with carbon. (B) "Carbon reacts with an element in the above table to form several compounds." Give suitable reason. [3 marks]
Answer:
(A) Element P (2, 6, oxygen) needs two electrons and element R (2, 8, 7, chlorine) needs one electron, so both can share electrons with carbon and form covalent bonds. Q (2, 8, 1, sodium) is a metal which loses its one electron and forms ionic bonds, and S (2, 8, 8, argon) has a complete outer shell and does not react.
(B) Carbon forms several compounds with these elements because of its tetravalency and its ability of catenation. For example, carbon and chlorine give CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ (with hydrogen), and carbon with oxygen gives CO and CO₂.
In simple words: Oxygen and chlorine share electrons with carbon; carbon's four bonds and chain-forming ability give many compounds.

Teacher's Note:
1. Write the number of electrons each element needs to complete its octet.
2. Metals prefer to lose electrons, so they form ionic compounds.

Question 11. "Carbon forms strong bonds with most other elements making the compounds exceptionally stable." Give reason to justify this statement. [2 marks]
Answer:
Carbon is a very small atom, so its shared electron pairs are held close to the nucleus and the bonds it forms with other atoms (H, O, N, Cl and C itself) are very strong. A lot of energy is needed to break them.
Therefore carbon compounds do not break down easily and are exceptionally stable.
In simple words: Small size of carbon gives short, strong bonds and hence stable compounds.

Teacher's Note:
1. Link bond strength with atomic size.
2. Stable compounds are the reason why there are so many of them.

Question 12. Diamond is a poor conductor of electricity while graphite is a good conductor. Assign reason. [2 marks]
Answer:
Diamond: Each carbon atom is bonded to four other carbon atoms by strong covalent bonds, so all four valence electrons are used up. No free electrons are available, so diamond does not conduct electricity.
Graphite: Each carbon atom is bonded to only three others in a flat hexagonal layer. The fourth valence electron of each carbon atom is free to move between the layers, so graphite conducts electricity.
In simple words: Diamond has no free electrons; graphite has one free electron per carbon atom.

Teacher's Note:
1. Count the bonds on each carbon atom: 4 in diamond, 3 in graphite.
2. Graphite is used for electrodes.

Question 13. Why diamond has high melting point? [1 mark]
Answer:
In diamond, each carbon atom is joined to four other carbon atoms by strong covalent bonds in a rigid three-dimensional giant network. A very large amount of energy is needed to break these bonds, so diamond has a very high melting point.
In simple words: Diamond's strong network of bonds takes a lot of heat to break.

Teacher's Note:
1. It is a covalent solid but behaves differently from small covalent molecules.
2. The same structure makes diamond the hardest natural substance.

Question 14. Write the next higher order homologous of CH₂O, C₂H₂ and C₂H₅COOH. [3 marks]
Answer:
Successive members of a homologous series differ by one –CH₂– unit, so add CH₂ to each formula.
CH₂O (methanal, HCHO) → C₂H₄O (ethanal, CH₃CHO)
C₂H₂ (ethyne) → C₃H₄ (propyne)
C₂H₅COOH (propanoic acid) → C₃H₇COOH, or C₄H₈O₂ (butanoic acid)
In simple words: Add one CH₂ to each formula to get the next homologue.

Teacher's Note:
1. Add one carbon and two hydrogens.
2. Functional group stays the same in the series.

Question 15. State the meaning of functional group in an organic compound. Write the formula of the functional group present in alcohols, aldehydes, ketones and Carboxylic acid. [2 marks]
Answer:
A functional group is an atom or a group of atoms in a carbon compound that makes it reactive and decides its chemical properties.
Alcohols: –OH (hydroxyl). Aldehydes: –CHO (aldehyde group). Ketones: –C(=O)– (keto or carbonyl group, in the middle of the chain). Carboxylic acids: –COOH (carboxyl group).
In simple words: The functional group decides the chemical behaviour of a carbon compound.

Teacher's Note:
1. Learn the four groups with an example each: ethanol, ethanal, propanone, ethanoic acid.
2. The ketone group is always inside the carbon chain.

Question 16. What are isomers? Draw the structures of two isomers of butane, C₄H₁₀. Why can not we have isomers of first three members of alkane series? [3 marks]
Answer:
Isomers are compounds with the same molecular formula but different structural formulae.
Isomers of butane (C₄H₁₀): (i) n-butane: CH₃–CH₂–CH₂–CH₃; (ii) isobutane (2-methylpropane): CH₃–CH(CH₃)–CH₃.
Reason: In methane, ethane and propane, there are only one, two and three carbon atoms, so the atoms can be arranged in only one way. Branching is not possible, hence no isomers.
In simple words: Same formula, different arrangement; branching needs at least four carbon atoms.

Teacher's Note:
1. Isomers differ in chain structure but not in formula.
2. Pentane has three isomers and hexane has five.

Question 17. Write the structural formulae of all the isomers of hexane. [3 marks]
Answer:
Hexane has the molecular formula C₆H₁₄ and has five structural isomers:
1. n-hexane: CH₃–CH₂–CH₂–CH₂–CH₂–CH₃
2. 2-methylpentane: CH₃–CH(CH₃)–CH₂–CH₂–CH₃
3. 3-methylpentane: CH₃–CH₂–CH(CH₃)–CH₂–CH₃
4. 2,2-dimethylbutane: CH₃–C(CH₃)₂–CH₂–CH₃
5. 2,3-dimethylbutane: CH₃–CH(CH₃)–CH(CH₃)–CH₃
In simple words: Move the carbon branches along the chain to get five different structures of C₆H₁₄.

Teacher's Note:
1. Check each structure for 6 carbon and 14 hydrogen atoms.
2. Do not count the same structure twice by flipping the chain.

Long Answer Questions

Question 18. Answer the following questions: (i) What is meant by a functional group? Explain with an example. (ii) Write three common functional groups present in organic compounds. Give their symbols/formulae. (iii) Name the functional groups present in the following compounds: (a) CH₃COOH, (b) CH₃CH₂CHO, (c) C₂H₅OH, (d) CH₃COCH₂CH₃. (iv) Name the functional group which always occurs in the middle of a carbon chain. (v) Draw the structures for the following compounds: (a) Ethanal, (b) Propanal, (c) Butanal, (d) Pentanal. [5 marks]
Answer:
(i) A functional group is an atom or group of atoms that makes an organic compound reactive and decides its properties. Example: the –OH group in ethanol, C₂H₅OH.
(ii) Alcohol: –OH; Aldehyde: –CHO; Carboxylic acid: –COOH (also ketone: –CO–, and halo group: –Cl, –Br).
(iii) (a) Carboxylic acid group (–COOH); (b) aldehyde group (–CHO); (c) alcohol group (–OH); (d) ketone group (–CO–).
(iv) The ketone group (–CO–).
(v) (a) Ethanal: CH₃–CHO; (b) Propanal: CH₃–CH₂–CHO; (c) Butanal: CH₃–CH₂–CH₂–CHO; (d) Pentanal: CH₃–CH₂–CH₂–CH₂–CHO.
In simple words: A functional group decides properties; the aldehyde group –CHO sits at the end of the chain and the ketone group is in the middle.

Teacher's Note:
1. The names end in -al for aldehydes and -one for ketones.
2. Count carbon atoms including the carbon of –CHO.

Question 19. (A) What are hydrocarbons? Give examples. (B) Give the structural differences between saturated and unsaturated hydrocarbons with two examples each. (C) What is a functional group? Give examples of four different functional groups. [5 marks]
Answer:
(A) Compounds made of only carbon and hydrogen are called hydrocarbons. Examples: methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), ethyne (C₂H₂).
(B) Saturated hydrocarbons (alkanes) have only carbon–carbon single bonds, for example, ethane CH₃–CH₃ and propane CH₃–CH₂–CH₃. Unsaturated hydrocarbons have at least one carbon–carbon double or triple bond, for example, ethene CH₂=CH₂ and ethyne CH≡CH.
(C) A functional group is an atom or group of atoms that decides the chemical properties of a carbon compound. Four examples: alcohol (–OH), aldehyde (–CHO), ketone (–CO–) and carboxylic acid (–COOH).
In simple words: Hydrocarbons contain only C and H; single bonds make them saturated, double or triple bonds make them unsaturated.

Teacher's Note:
1. Give the formula of each example.
2. Alkenes and alkynes are the unsaturated hydrocarbons.

Question 20. Why certain compounds are called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur. [5 marks]
Answer:
Hydrocarbons: They are called hydrocarbons because they are compounds of only hydrogen and carbon.
General formulae and first members: Alkanes C_nH_2n+2, first member methane, CH₄. Alkenes C_nH_2n, first member ethene, H₂C=CH₂. Alkynes C_nH_2n−2, first member ethyne, HC≡CH.
Reaction: The conversion of an alkene into an alkane is hydrogenation (an addition reaction). It needs a catalyst such as nickel, palladium or platinum:
CH₂=CH₂ + H₂ → CH₃–CH₃ (catalyst: Ni or Pd, heat)
In simple words: Alkanes, alkenes and alkynes follow three general formulae; adding hydrogen over a nickel catalyst turns an alkene into an alkane.

Teacher's Note:
1. The first alkene is ethene because one carbon cannot form a double bond with itself.
2. Addition of hydrogen needs a catalyst.

Question 21. What is methane? Draw its electron dot structure. Name the type of bonds formed in this compound. Why are such compounds: (i) poor conductors of electricity and (ii) have low melting and boiling points? What happens when this compound burns in oxygen? [5 marks]
Answer:
Methane is the simplest hydrocarbon, an alkane with molecular formula CH₄. It is the main constituent of natural gas and biogas.
Electron dot structure:

Type of bonds: Four single covalent bonds (C–H), formed by sharing of electrons.
(i) Such compounds are poor conductors of electricity because they have no free electrons or ions; all electrons are tied up in the shared pairs.
(ii) They have low melting and boiling points because the forces of attraction between their molecules are weak, so little energy is needed to separate them.
Combustion: Methane burns in oxygen with a blue flame, giving carbon dioxide and water along with heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
In simple words: Methane has four C–H covalent bonds; it does not conduct, melts easily and burns to CO₂ and water.

Teacher's Note:
1. Write the balanced equation for combustion.
2. Covalent compounds have weak forces between molecules.