CBSE Class 10 Science Carbon And Its Compounds Worksheet Set H

Question. Why are certain compounds 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.
Answer: Compounds consisting of carbon and hydrogen are known as hydrocarbons.
(a) Saturated hydrocarbons : Alkanes \( (C_nH_{2n+2}) \) are the compounds of carbon which have single bond.
Structure of Methane (first member):
    H
    |
H—C—H
    |
    H
(b) Unsaturated hydrocarbons : The compounds of carbon having double bonds are alkene \( (C_nH_{2n}) \) and those having triple bonds are alkyne \( (C_nH_{2n-2}) \).
The reaction which converts unsaturated hydrocarbons to saturated hydrocarbons i.e., alkenes to alkane is known as hydrogenation reaction. It is used to obtain ghee from oil.
\( R_2C = CR_2 + H_2 \xrightarrow{Ni/Pd} R_2CH — CHR_2 \)

Question. Soaps and detergents are both types of salts. State the difference between the two. Write the mechanism of the cleansing action of soaps. Why do soaps not form lather (foam) with hard water? Mention any two problems that arise due to the use of detergents instead of soaps.
Answer:
Differences:
1. Soaps are sodium salts of long chain carboxylic acids, while Detergents are sodium salts of long chain benzene sulphonic acids.
2. The ionic group in soap is \( COO^- \), \( Na^+ \), while in detergents it is \( SO_3^- \), \( Na^+ \).
3. Soaps are not useful when water is hard, but Detergent can be used for washing purposes even when water is hard.
4. Soaps are biodegradable, while some detergents are non-biodegradable.
5. Soaps have relatively weak cleansing action, while Detergents have strong cleansing action.
Mechanism of cleansing action: The dirt present on clothes is organic matter and insoluble in water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelles form and trap the dirt at the centre of the cluster. These micelles remain suspended in the water and are rinsed away.
Soaps and hard water: Hard water contains salts of calcium and magnesium. When soap is added, these ions displace sodium or potassium ions in soap, forming an insoluble substance called scum.
Problems with detergents: (a) They are non-biodegradable and cause environmental pollution. (b) Their entry into the food chain leads to bio-accumulation and health issues.

Question. Complete the following chemical equations:
(a) \( C_2H_5OH + O_2 \longrightarrow \)
(b) \( C_2H_5OH \xrightarrow{Conc. H_2SO_4, 443 K} \)
(c) \( CH_3COOH + NaHCO_3 \longrightarrow \)
Answer:
(a) \( C_2H_5OH + 3O_2 \xrightarrow{\Delta} 2CO_2 + 3H_2O + \text{Energy} \)
(b) \( C_2H_5OH \xrightarrow{Conc. H_2SO_4, 443 K} CH_2 = CH_2 + H_2O \)
(c) \( CH_3COOH + NaHCO_3 \longrightarrow CH_3COONa + H_2O + CO_2 \uparrow \)

Question. (a) Explain the formation of the oxygen molecule. (b) Give Lewis dot structure of methane.
Answer: (a) Oxygen (atomic number 8) has 6 electrons in its outermost shell. It shares two electrons with another atom of oxygen to make a molecule of oxygen \( (O_2) \). By doing so, both atoms get 8 electrons in their outermost shell, forming a double bond \( (O=O) \).
(b) Lewis dot structure of methane \( (CH_4) \) shows a central Carbon atom with 4 valence electrons shared with one electron from each of the 4 Hydrogen atoms to form four single covalent bonds.

Question. (a) What is a homologous series? Explain with an example. (b) State two characteristics of a homologous series. (c) The molecular formula of an organic compound is \( C_{18}H_{36} \). Name its homologous series. (d) Select the hydrocarbons which belong to the same homologous series. Give the name of each series: \( CH_4, C_2H_2, C_2H_4, C_2H_6, C_4H_{10}, C_3H_4, C_3H_6 \). (e) What is meant by ‘heteroatom’? Give examples. Write the names and formulae of two organic compounds containing different heteroatoms.
Answer: (a) A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by a \( CH_2 \) group. Example: Alkanes (\( CH_4, C_2H_6, C_3H_8 \)).
(b) (i) All members have the same general formula. (ii) Any two adjacent homologues differ by 1 carbon and 2 hydrogen atoms.
(c) Alkene, \( C_nH_{2n} \).
(d) Alkanes: \( CH_4, C_2H_6, C_4H_{10} \); Alkenes: \( C_2H_4, C_3H_6 \); Alkynes: \( C_2H_2, C_3H_4 \).
(e) In an organic compound, any atom other than carbon and hydrogen is called a heteroatom. Examples: Chlorine (Cl), Bromine (Br), Oxygen (O). Compounds: Chloromethane (\( CH_3Cl \)) and Methanol (\( CH_3OH \)).

Question. Explain the given reactions with the examples: (a) Hydrogenation reaction (b) Oxidation reaction (c) Substitution reaction (d) Saponification reaction (e) Combustion reaction.
Answer: (a) Hydrogenation: Addition of hydrogen to unsaturated hydrocarbons in the presence of a catalyst like Nickel. \( R_2C=CR_2 + H_2 \xrightarrow{Ni} R_2CH-CHR_2 \).
(b) Oxidation: Ethanol is oxidized to ethanoic acid using alkaline \( KMnO_4 \) and heat. \( CH_3CH_2OH \xrightarrow{\text{Alk. } KMnO_4 + \text{Heat}} CH_3COOH \).
(c) Substitution: Hydrogen in a hydrocarbon is replaced by another atom like Chlorine in sunlight. \( CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl \).
(d) Saponification: Reaction of an ester with \( NaOH \) to form soap and alcohol. \( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \).
(e) Combustion: Release of heat and light when burning carbon compounds. \( CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat and light} \).

Question. (a) Write a chemical test to distinguish between saturated and unsaturated hydrocarbons. (b) Name the products formed when ethane burns in air. Write chemical equation for the reaction showing various types of energies liberated. (c) Write the reaction between methane and chlorine in the presence of sunlight. Why is this reaction considered a substitution reaction?
Answer: (a) Bromine water test: Unsaturated hydrocarbons decolourise red-brown bromine water, while saturated hydrocarbons do not.
(b) Products are Carbon dioxide \( (CO_2) \) and Water \( (H_2O) \). Equation: \( 2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O + \text{Heat} + \text{Light} \).
(c) \( CH_4 + Cl_2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl \). It is a substitution reaction because a hydrogen atom is replaced by a chlorine atom.

Question. What are esters? How are esters prepared? Write the chemical equation for the reaction involved. What happens when an ester reacts with sodium hydroxide? Write the chemical equation for the reaction and also state the name and use of this reaction.
Answer: Esters are sweet-smelling organic compounds with the general formula \( RCOOR' \).
Preparation: They are prepared by the reaction between carboxylic acids and alcohols in the presence of an acid. \( CH_3CH_2OH + CH_3COOH \xrightarrow{\text{Acid}} CH_3COOCH_2CH_3 + H_2O \).
Reaction with NaOH: \( CH_3COOCH_2CH_3 \xrightarrow{NaOH} CH_3CH_2OH + CH_3COOH \). This is called a saponification reaction and is used in the preparation of soaps.

Question. (a) You have three unlabelled test tubes containing ethanol, ethanoic acid and soap solution. Explain the method you would use to identify the compounds in different test tubes by chemical tests using litmus paper and sodium metal. (b) Give the reason of formation of scum when soaps are used with hard water.
Answer: (a)
- Ethanol: No change on blue or red litmus; reacts with Sodium to release \( H_2 \) gas.
- Ethanoic acid: Turns blue litmus red; no change on red litmus; reacts with Sodium to release \( H_2 \) gas.
- Soap solution: No change on blue litmus; turns red litmus blue; reacts with Sodium to release \( H_2 \) gas.
(b) Hard water contains calcium and magnesium ions which react with soap to form an insoluble precipitate called scum.

Question. What are micelles? Why does it form when soap is added to water? Will a micelle form in other solvents such as ethanol also? State briefly how the formation of micelles help to clean the clothes having oily spots.
Answer: Micelles are clusters formed by soap molecules in water where the hydrophobic tails point inwards and hydrophilic heads point outwards. They do not form in ethanol because the soap's hydrocarbon chain dissolves in ethanol. Micelles clean oily spots by trapping the oil in the centre of the cluster, creating an emulsion that is washed away with water.

Assertion and Reasoning Based Questions

Question. Assertion : Carbon and its compounds are used as fuels.
Reason : They give lot of heat and light when burnt in air.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.
Answer: (a)

Question. Assertion : Detergents are better cleansing agent than soaps.
Reason : It is because they work with soft water.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.
Answer: (c)

Question. Assertion : General formula of alkanes is \( C_nH_{2n + 2} \).
Reason : It is because they are saturated compounds.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.
Answer: (b)

Question. Assertion : Hard water contains magnesium and phosphate salts.
Reason : It is because they are easily available in water.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.
Answer: (d)

Question. Assertion : Ethane is the homologous of methane.
Reason : It is because it differs by \( CH_3 \) group.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.
Answer: (b)

Creating Based Questions

Question. (a) Covalent bonds formed as a result of sharing of electron pairs between two atoms are strong in nature. But the covalently bonded molecules have low melting and boiling points compared to the ionic molecules. What is the reason behind such behaviour? (b) Why are the unsaturated carbon compounds more reactive than saturated carbon compounds?
Answer: (a) Covalent bonds are strong within the molecule, but the intermolecular forces (forces between molecules) are weak, leading to low melting and boiling points.
(b) Unsaturated carbon compounds contain double or triple bonds which are less stable and have free electrons for bonding, making them more reactive than single-bonded saturated compounds.

Question. (a) Which property of ethanol enables its use in preparation of certain types of medicines? (b) A shiny soft metal is put in a test tube containing ethanol, a reaction takes place within the test tube with evolution of bubbles. The gas is collected and exposed to a burning candle. The gas burns with a pop sound. (i) Name the shiny soft metal. (ii) Name the gas evolved as a result of chemical reaction. (iii) Write the equation for chemical reaction.
Answer: (a) Ethanol is a very efficient solvent.
(b) (i) Sodium. (ii) Hydrogen gas. (iii) \( 2Na(s) + 2C_2H_5OH(l) \longrightarrow 2C_2H_5ONa + H_2(g) \).