CBSE Class 10 Science HOTs Carbon And its Compounds Set 04

Question. (a) Draw the structure of butanone molecule, \( CH_3COC_2H_5 \). (b) Draw the structure of the hexanal molecule, \( C_5H_{11}CHO \).
Answer: (a) Structure of butanone: \( CH_3-C(=O)-CH_2-CH_3 \)
(b) Structure of hexanal: \( CH_3-CH_2-CH_2-CH_2-CH_2-CHO \)

Question. An organic compound burns with blue flame. It is saturated or unsaturated compound. Give reason.
Answer: It is a saturated compound. Saturated hydrocarbons generally burn with a clean blue flame because they have a low carbon content and undergo complete combustion in the presence of sufficient air.

Question. Explain why carbon generally forms compounds by covalent bonds.
Answer: Carbon has 4 valence electrons. To attain stability, it would need to lose 4 or gain 4 electrons. Gaining 4 electrons to form \( C^{4-} \) is difficult because 6 protons cannot hold 10 electrons. Losing 4 electrons to form \( C^{4+} \) requires a very high amount of energy. Therefore, carbon shares its valence electrons with other atoms to complete its octet, resulting in the formation of covalent bonds.

Question. Write the name and molecular formula of an organic compound having its name suffixed with ‘–ol’ and having two carbon atoms in the molecule. With the help of a balanced chemical equation indicate what happens when it is heated with excess of conc. \( H_2SO_4 \).
Answer: The compound is Ethanol. Its molecular formula is \( C_2H_5OH \). When it is heated with excess concentrated \( H_2SO_4 \) at 443 K, it undergoes dehydration to form ethene.
\( CH_3CH_2OH \xrightarrow{\text{Hot conc. } H_2SO_4} CH_2=CH_2 + H_2O \)

Question. Match the following reactions in Column I with their names in Column II:
Column I:
(i) \( CH \equiv CH + Br_2 \rightarrow CHBr_2-CHBr_2 \)
(ii) \( CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2 \)
(iii) \( CH_2=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3CH_2OH \)
(iv) \( CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O \)
Column II:
A. Combustion reaction
B. Test for carboxylic acid
C. Bromine water test
D. Hydration
Answer: (i) - C, (ii) - B, (iii) - D, (iv) - A

Question. Write the molecular formula of an alkene and an alkane with twenty carbon atoms.
Answer: Alkene: \( C_{20}H_{40} \)
Alkane: \( C_{20}H_{42} \)

Question. Select alkene and alkyne from the following: \( C_6H_{12} , C_3H_4 , C_2H_4 , CH_4 , C_4H_8 , C_5H_8 \)
Answer: Alkenes: \( C_6H_{12} , C_2H_4 , C_4H_8 \)
Alkynes: \( C_3H_4 , C_5H_8 \)

Question. What is the IUPAC name of (a) \( CH_3-CH_2-CH=CH_2 \) (b) \( CH_3CHO \)?
Answer: (a) But-1-ene (b) Ethanal

Question. Write IUPAC names of:
(a) \( CH_3COCH_2CH_3 \)
(b) \( CH_3-CH(OH)-CH_3 \)
(c) \( HCOOH \)
(d) \( CH_3COOCH_3 \)
Answer: (a) Butanone, (b) Propan-2-ol, (c) Methanoic acid, (d) Methyl ethanoate

Question. What is the function of conc. \( H_2SO_4 \) in the formation of ethene from ethanol?
Answer: Concentrated sulphuric acid acts as a dehydrating agent as it removes a water molecule from ethanol to form ethene.

Question. A carbon compound ‘A’ having melting point 156K and boiling point 351K, with molecular formula \( C_2H_6O \) is soluble in water in all proportions.
(a) Identify ‘A’ and draw its electron dot structure.
(b) Give the molecular formulae of any two homologues of ‘A’.
Answer: (a) ‘A’ is Ethanol. [Electron dot structure consists of two carbon atoms shared with six hydrogen atoms and one oxygen atom, with an O-H bond].
(b) Methanol (\( CH_3OH \)) and Propanol (\( C_3H_7OH \)).

Question. What happens when hydrogen is added to a vegetable oil in the presence of nickel? Name the reaction and write one difference between the physical property of the vegetable oil and the product obtained in this reaction. Write the role of nickel in this reaction.
Answer: When hydrogen is added to vegetable oil in the presence of nickel, the unsaturated vegetable oil is converted into saturated vegetable ghee.
Reaction name: Addition reaction (Hydrogenation).
Difference: Vegetable oil is a liquid at room temperature, whereas the product (vegetable ghee) is a solid.
Role of nickel: Nickel acts as a catalyst.

Question. (a) Define the term functional group. Identify the functional group present in (i) H-CHO (ii) \( CH_3-COOH \). (b) What happens when 5% alkaline \( KMnO_4 \) solution is added drop by drop to warm ethanol taken in a test tube? State the role of alkaline \( KMnO_4 \) solution in this reaction.
Answer: (a) A functional group is an atom or group of atoms that defines the chemical properties of an organic compound. (i) Aldehyde group (—CHO), (ii) Carboxylic acid group (—COOH).
(b) Ethanol is oxidized to ethanoic acid. Alkaline \( KMnO_4 \) acts as an oxidising agent.

Question. Two carbon compounds X and Y have the molecular formula \( C_4H_8 \) and \( C_5H_{12} \) respectively. Which one of these is most likely to show addition reaction? Justify your answer. Also give the chemical equation to explain the process of addition reaction in this case.
Answer: Compound X (\( C_4H_8 \)) is an alkene (unsaturated), so it will show addition reaction.
\( C_4H_8 + H_2 \xrightarrow{Ni} C_4H_{10} \)

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) Most carbon compounds are covalent and do not dissociate into ions, hence they lack free charge carriers.
(b) Cyclohexane. Structure is a hexagonal ring of 6 carbon atoms, each attached to 2 hydrogen atoms. It has 18 single bonds.

Question. Write the molecular formula of the following compounds and draw their electron-dot structures: (a) Ethane (b) Ethene (c) Ethyne
Answer: (a) Ethane: \( C_2H_6 \), (b) Ethene: \( C_2H_4 \), (c) Ethyne: \( C_2H_2 \). [Electron dot structures involve sharing of electrons between C-C and C-H atoms].

Question. Write the structural formula of ethanol and list its two physical properties. What happens when it is heated with excess of conc. \( H_2SO_4 \) at 443 K? State the role of conc. \( H_2SO_4 \) in this reaction.
Answer: Structural formula: \( CH_3-CH_2-OH \).
Physical properties: 1. It is a liquid at room temperature. 2. It has a burning taste.
When heated at 443 K with conc. \( H_2SO_4 \), it forms ethene. The acid acts as a dehydrating agent.

Question. An organic compound ‘P’ is a constituent of wine. ‘P’ on reacting with acidified \( K_2Cr_2O_7 \) forms another compound ‘Q’. When a piece of sodium is added to ‘Q’ a gas ‘R’ evolves which burns with a pop sound. Identify P, Q and R and write the chemical equations of the reactions involved.
Answer: P is Ethanol (\( C_2H_5OH \)), Q is Ethanoic acid (\( CH_3COOH \)), and R is Hydrogen gas (\( H_2 \)).
\( CH_3CH_2OH + 2[O] \xrightarrow{\text{Acidified } K_2Cr_2O_7} CH_3COOH + H_2O \)
\( 2CH_3COOH + 2Na \rightarrow 2CH_3COONa + H_2 \)

Question. Write IUPAC names of: (a) \( HC \equiv CH \) (b) \( CH_3CH_2CH_2OH \) (c) \( CH_3-CHO \)
Answer: (a) Ethyne, (b) Propan-1-ol, (c) Ethanal

Question. 3 mL of ethanol is taken in a test tube and warmed gently in a water bath. A 5% solution of alkaline potassium permanganate is added first drop by drop to this solution, then in excess. (a) How is 5% solution of \( KMnO_4 \) prepared? (b) State the role of alkaline potassium permanganate in this reaction. What happens on adding it in excess? (c) Write chemical equation of this reaction.
Answer: (a) By dissolving 5g of \( KMnO_4 \) in 100 mL of water.
(b) It acts as an oxidising agent. When added in excess, the pink color of the solution persists.
(c) \( CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4} CH_3COOH \)

Question. Complete the following reactions:
(a) \( CH_3COOH + NaOH \rightarrow \)
(b) \( R_2C=CR_2 + H_2 \xrightarrow{Ni} \)
(c) \( CH_3OH + O_2 \xrightarrow{MoO_3, 873-923 K} \)
(d) \( CH_2=CH_2 + Br_2 \rightarrow \)
(e) \( CH_3COOC_2H_5 + NaOH \rightarrow \)
(f) \( CH_2=CH_2 + H_2O \xrightarrow{H_2SO_4} \)
Answer: (a) \( CH_3COONa + H_2O \)
(b) \( R_2CH-CHR_2 \)
(c) \( HCHO + H_2O \)
(d) \( CH_2Br-CH_2Br \)
(e) \( CH_3COONa + C_2H_5OH \)
(f) \( CH_3CH_2OH \)

Question. (a) Why do we add ethanol to petrol in these days? (b) Give one use of ethanol in medicines. (c) Why is vinegar used in pickles?
Answer: (a) Ethanol is a clean fuel and burns to produce only carbon dioxide and water, hence it is used as an additive to petrol to reduce pollution.
(b) It is used as a solvent in tincture of iodine.
(c) Vinegar acts as a preservative in pickles.

Question. Give the IUPAC name of the following: (a) \( CH_3-CH_2-CH_2-OH \) (b) \( CH_3-CH(OH)-CH_2-CH_3 \) (c) \( CH_3-CH(OH)-CH_3 \)
Answer: (a) Propan-1-ol (b) Butan-2-ol (c) Propan-2-ol

Question. (a) Write two characteristics of homologous series. (b) Name the functional group present in: (i) HCHO (ii) \( CH_3CHO \) (iii) \( CH_3COOH \) (iv) \( CH_3OH \).
Answer: (a) 1. Members have the same functional group. 2. Successive members differ by a \( -CH_2- \) group. (b) (i) Aldehyde, (ii) Aldehyde, (iii) Carboxylic acid, (iv) Alcohol.

Question. You are provided with an organic compound having the molecular formula \( C_2H_6 \). Give the name and formula of the compound formed when: (a) One H atom of \( C_2H_6 \) is replaced by an OH group. (b) One H atom of \( C_2H_6 \) is replaced by a CHO group. (c) One H atom of \( C_2H_6 \) is replaced by a COOH group.
Answer: (a) Ethanol, \( C_2H_5OH \) (b) Propanal, \( C_2H_5CHO \) (c) Propanoic acid, \( C_2H_5COOH \)

Question. An organic compound ‘X’ on heating with conc. \( H_2SO_4 \) forms a compound ‘Y’ which on addition of one molecule of hydrogen in the presence of nickel forms a compound ‘Z’. One molecule of compound ‘Z’ on combustion forms two molecules of \( CO_2 \) and three molecules of \( H_2O \). Identify the compounds ‘X’, ‘Y’ and ‘Z’. Write the chemical equations involved.
Answer: X is Ethanol, Y is Ethene, and Z is Ethane.
\( CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4} CH_2=CH_2 + H_2O \)
\( CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3CH_3 \)
\( 2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O \)

Question. A compound (A) \( C_2H_4O_2 \) reacts with sodium metal to form a compound ‘B’ and evolves a gas which burns with a ‘pop’ sound. Compound ‘A’ on treatment with alcohol ‘C’ in the presence of acid forms a sweet smelling compound ‘D’ (\( C_4H_8O_2 \)). On addition of NaOH to D gives back ‘B’ and ‘C’. Identify A, B, C and D and write the reactions.
Answer: A is Ethanoic acid (\( CH_3COOH \)), B is Sodium ethanoate (\( CH_3COONa \)), C is Ethanol (\( C_2H_5OH \)), and D is Ethyl ethanoate (\( CH_3COOC_2H_5 \)).
\( 2CH_3COOH + 2Na \rightarrow 2CH_3COONa + H_2 \)
\( CH_3COOH + C_2H_5OH \xrightarrow{H^+} CH_3COOC_2H_5 + H_2O \)
\( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \)

Question. (a) Identify ‘A’, ‘B’ and ‘C’ in the following: (i) \( C_2H_5OH + CH_3CH_2COOH \xrightarrow{\text{Conc. } H_2SO_4} \text{A} + H_2O \) (ii) \( CH_3COOC_2H_5 + NaOH \rightarrow \text{B} + C_2H_5OH \) (iii) \( CH_3COONa + NaOH \xrightarrow{CaO, heat} \text{C} + Na_2CO_3 \). (b) A cyclic compound ‘X’ has molecular formula \( C_6H_6 \). Identify ‘X’ and write its structural formula. Will it decolourise bromine water?
Answer: (a) A: Ethyl propanoate, B: Sodium ethanoate, C: Methane. (b) ‘X’ is Benzene. It will not decolourise bromine water due to its aromatic stability.

Question. An organic compound with molecular formula \( C_2H_4O_2 \) produces brisk effervescence on addition of sodium carbonate /bicarbonate. (a) Identify the organic compound. (b) Name the gas evolved. (c) How will you test the gas evolved? (d) Write the chemical equation for the reaction. (e) List two important uses of the above compound.
Answer: (a) Ethanoic acid. (b) Carbon dioxide. (c) Pass it through lime water; it turns milky. (d) \( 2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \). (e) Preservative, solvent.

Question. (i) Draw two structural isomers of butane. (ii) Draw the structures of propanol and propanone. (iii) Name the third homologue of (a) alcohols (b) aldehyde. (iv) Name the following: (a) Benzene structure (b) \( CH_3-CH=CH_2 \).
Answer: (i) n-Butane and Isobutane. (ii) \( CH_3CH_2CH_2OH \) and \( CH_3COCH_3 \). (iii) (a) Propanol, (b) Propanal. (iv) (a) Benzene, (b) Propene.

Soaps and Detergents and their Cleaning Action

Question. The ionic part of synthetic detergent is
(a) —OSO₃^– Na⁺
(b) —COO^– Na⁺
(c) —COO^– H⁺
(d) —COO^– CH₃⁺
Answer: (a) —OSO₃^– Na⁺

Question. Soaps are sodium or potassium salts of
(a) alcohol
(b) fatty acids
(c) sulphonic acid
(d) esters
Answer: (b) fatty acids

Question. Which of the following is correct about detergents?
(a) They are 100% biodegradable.
(b) They work well with hard water.
(c) They do not create pollution.
(d) They do not work with hard water.
Answer: (b) They work well with hard water.

Question. What are micelles? Why does it form when soap is added to water? Will a micelle be formed in other solvents such as ethanol also? State briefly how the formation of micelles help to clean the clothes having oily spots.
Answer: Micelles: When molecular ions in soaps and detergents aggregate, they form micelles. It is because a large number of molecular ions of soaps get aggregated and form a colloidal solution. Soap has a hydrophobic tail (hydrocarbon) which dissolves in the hydrocarbon part (the oily spot) and a hydrophilic part which dissolves in water. Ethanol is a non-polar solvent, therefore micelles are not formed because the hydrocarbon part gets attracted towards ethanol and the ionic end will not dissolve in alcohol. The formation of micelles helps clean oily spots because the hydrophobic tails of soap molecules attach to the oil, trapping it in the center of the micelle, while the hydrophilic heads remain in the water, allowing the entire cluster to be washed away.

PRACTICE QUESTIONS

Question. (a) With the help of diagram show the formation of micelles, when soap is applied to oily dirt. (b) Take two test tubes ‘X’ and ‘Y’ with 10 mL of hard water in each. In test tube ‘X’ add few drops of soap solution and in test tube ‘Y’ add a few drops of detergent solution. Shake both the test tubes for the same period. (1) In which test tube the formation of foam will be more? Why? (2) In which test tube curdy solid will be formed? Why?
Answer: (b) (1) In test tube ‘Y’, the formation of foam will be more. This is because detergents work effectively in hard water and do not form insoluble precipitates with calcium and magnesium ions. (2) In test tube ‘X’, a curdy solid (scum) will be formed. This is because soap reacts with \( Ca^{2+} \) and \( Mg^{2+} \) ions present in hard water to form insoluble salts.

Question. (a) What are soaps? Explain the mechanism of cleaning action of soaps with the help of labelled diagram. (b) Detergents are better than soaps. Justify.
Answer: (a) Soaps are sodium or potassium salts of long-chain fatty acids. The cleaning mechanism involves the hydrophobic tails of soap molecules attaching to grease while the hydrophilic heads stay in water, forming micelles that lift the dirt off the surface. (b) Detergents are better than soaps because they work effectively in hard water, whereas soaps form an insoluble precipitate (scum) with the calcium and magnesium ions present in hard water.

INTEGRATED (MIXED) QUESTIONS

Question. (a) Write three difference between soaps and detergents. (b) Give two uses of ethanoic acid. (c) Name the products formed when ethanol reacts with sodium metal.
Answer: (a) 1. Soaps are salts of fatty acids; detergents are salts of sulphonic acids. 2. Soaps do not work in hard water; detergents work in hard water. 3. Soaps are biodegradable; some detergents are not. (b) 1. Used as vinegar in food preservation. 2. Used in the manufacture of esters. (c) Sodium ethoxide and hydrogen gas.

Question. (a) What are isomers? Write the structures of two compounds having molecular formula \( C_3H_6O \) and give their names. (b) What are soaps? How are they chemically different from detergents? Why do soaps do not work effectively in hard water?
Answer: (a) Isomers are compounds with the same molecular formula but different structural arrangements. For \( C_3H_6O \): Propanal (\( CH_3-CH_2-CHO \)) and Propanone (\( CH_3-CO-CH_3 \)). (b) Soaps are sodium/potassium salts of fatty acids, while detergents are sodium/potassium salts of long-chain alkyl hydrogen sulphates or alkyl benzene sulphonic acids. Soaps do not work in hard water because they form insoluble curdy precipitates (scum) with calcium and magnesium ions.

Question. Write chemical equations for the following: (a) Combustion of methane (b) Oxidation of alcohol (c) Hydrogenation of ethene (d) Esterification (e) Saponification reaction.
Answer: (a) \( CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + Heat + Light \)
(b) \( CH_3CH_2OH \xrightarrow{Alkaline \ KMnO_4 + Heat} CH_3COOH \)
(c) \( CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3 \)
(d) \( CH_3COOH + C_2H_5OH \xrightarrow{Conc. H_2SO_4} CH_3COOC_2H_5 + H_2O \)
(e) \( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \)