Class 10 Science Chapter- 4 Carbon and its Compounds Questions and Answers
Q U E S T I O N S Page no: 61
Question: 1. What would be the electron dot structure of carbon dioxide which has the formula CO2 ?
Answer: The electron dot structure for carbon dioxide can be represented as follows:
– Carbon (C) has 4 valence electrons.
– Oxygen (O) has 6 valence electrons.
To achieve a stable configuration, carbon shares its four valence electrons with two oxygen atoms. Each oxygen atom shares two electrons with carbon, resulting in a double bond between carbon and each oxygen atom.
Electron Dot Structure: O=C=O
In this structure:-
1. The “=” sign indicates a double bond.
2. Each oxygen atom has two lone pairs of electrons remaining after forming the bonds with carbon.
Question: 2. 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 molecule of sulphur consisting of eight atoms (S₈) is arranged in a ring formation. Each sulphur atom shares its electrons with two neighboring sulphur atoms to form single bonds.
Electron Dot Structure:
S – S
/ \
S S
| |
S S
\ /
S – S
In this structure:
– Each “S” represents a sulphur atom.
– The lines between the sulphur atoms represent single bonds formed by the sharing of electrons.
This ring structure allows each sulphur atom to achieve a stable electron configuration by sharing electrons with its neighbors.
Summary
1. The electron dot structure for carbon dioxide (CO₂) shows a double bond between carbon and each oxygen atom.
2. The electron dot structure for sulphur (S₈) illustrates a ring formation where each sulphur atom is bonded to two others.
Q U E S T I O N S Page no: 68
Question: 1. How many structural isomers can you draw for pentane?
Answer: Pentane (C₅H₁₂) has three structural isomers. These are:
1. n-Pentane: A straight-chain structure.
2. Isopentane (2-Methylbutane): A branched structure with a methyl group on the second carbon.
3. Neopentane (2,2-Dimethylpropane): A branched structure with two methyl groups on the second carbon.
These isomers differ in the arrangement of carbon atoms, leading to different structural configurations while maintaining the same molecular formula.
Question: 2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer: The two key properties of carbon that contribute to the vast number of carbon compounds are:
1. Tetravalency: Carbon has four valence electrons, allowing it to form four covalent bonds with other atoms. This enables the formation of a diverse range of compounds, including chains and rings.
2. Catenation: Carbon can bond with other carbon atoms to form long chains or complex structures. This property allows for the creation of a wide variety of organic compounds, including straight chains, branched chains, and cyclic structures.
These properties together facilitate the formation of millions of carbon compounds, making carbon a fundamental element in organic chemistry.
Question: 3. What will be the formula and electron dot structure of cyclopentane?
Answer: The molecular formula of cyclopentane is C₅H₁₀.
Electron Dot Structure: In cyclopentane, five carbon atoms are arranged in a ring, with each carbon atom bonded to two hydrogen atoms. The electron dot structure can be represented as follows:
H H
\ /
H – C – H
| |
H – C C – H
| |
H – C – H
/ \
H H
In this structure, each line represents a covalent bond between carbon and hydrogen atoms, and the arrangement shows the cyclic nature of the compound. Each carbon atom shares bonds with two neighboring carbon atoms and two hydrogen atoms, satisfying the tetravalency of carbon.
Page no: 69
Question: 4. Draw the structures for the following compounds.
(i) Ethanoic acid
Answer: Ethanoic acid (CH₃COOH) The structure of ethanoic acid can be represented as follows:
O
||
H₃C—C—OH
Here, the carbon atom is double-bonded to one oxygen atom (the carbonyl group) and single-bonded to a hydroxyl group (-OH).
(ii) Bromopentane*
Answer: Bromopentane (C₅H₁₁Br)
Bromopentane can exist in several structural isomers. One possible structure is:
H H H H
| | | |
H—C—C—C—C—C—Br
| | | |
H H H H
However, structural isomers are indeed possible for bromopentane, as the bromine atom can be attached to different carbon atoms in the chain, leading to variations in structure.
(iii) Butanone
Answer: Butanone (C₄H₈O)
The structure of butanone, which is a ketone, can be represented as:
O
||
H₃C—C—C—CH₃
The carbonyl group (C=O) is located on the second carbon.
(iv) Hexanal. *Are structural isomers possible for bromopentane?
Answer: Hexanal (C₆H₁₂O)
The structure of hexanal can be depicted as:
O
||
H₃C—C—C—C—C—H
Here, the carbonyl group is at the end of the carbon chain, indicating that it is an aldehyde.
Question: 5. How would you name the following compounds?
(i) CH₃—CH₂—Br
Answer: Name: Bromethane
– Explanation: This compound is a two-carbon alkane (ethane) with a bromine atom (Br) substituting one of the hydrogen atoms. The prefix “bromo-” indicates the presence of the bromine substituent.
(ii) H
|
H-C=O
|
H
Answer: Name: Methanal (commonly known as Formaldehyde)
– Explanation: This compound has one carbon atom with a double bond to an oxygen atom (carbonyl group) and is classified as an aldehyde. The suffix “-al” is used for aldehydes, and since there is only one carbon, it is named methanal.
(iii) H H H H
| | | |
H – C – C – C – C – C=C-H
| | | |
H H H H
Answer: Named 5-hexene
1.Identify the Longest Carbon Chain: The longest chain has six carbon atoms (C6).
2. Identify the Type of Compound: The presence of a double bond indicates that this is an alkene.
3. Number the Carbon Chain: Number the carbon atoms from the end nearest to the double bond. Here, the double bond starts at carbon 5.
4. Name the Compound: The base name for a six-carbon alkene is “hexene.” Since the double bond starts at carbon 5, the compound is named 5-hexene
Q U E S T I O N S Page no: 71
Question: 1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Answer: The conversion of ethanol (C₂H₅OH) to ethanoic acid (CH₃COOH) is classified as an oxidation reaction because it involves the addition of oxygen or the removal of hydrogen from the ethanol molecule. In this reaction, ethanol is oxidized to form ethanoic acid, which can be represented as follows: C₂H₅OH+O→CH₃COOH+H₂O.
During this process, the hydroxyl group (-OH) in ethanol is converted into a carboxylic acid group (-COOH) in ethanoic acid. The presence of an oxidizing agent, such as alkaline potassium permanganate, facilitates this transformation by providing the necessary oxygen for the reaction. Thus, the increase in the oxidation state of the carbon atom in the conversion signifies that oxidation has occurred.
Question: 2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer: A mixture of ethyne (C₂H₂) and air is not used for welding due to the following reasons:
– Temperature Control: Ethyne burns with a very high flame temperature when mixed with pure oxygen, which is essential for welding. The flame produced is hotter than that from a mixture of ethyne and air. The high temperature is necessary to melt and fuse metals effectively during the welding process.
– Incomplete Combustion: When ethyne is burned in air, the supply of oxygen is limited, leading to incomplete combustion. This results in the production of carbon soot and less heat, which is not suitable for welding applications. In contrast, using pure oxygen ensures complete combustion, producing a clean flame with maximum heat output.
– Safety and Efficiency: Using pure oxygen reduces the risk of producing harmful by-products and ensures a more efficient welding process. The controlled environment provided by pure oxygen minimizes the chances of accidents and improves the quality of the weld.
– In summary, the use of oxygen instead of air in welding with ethyne allows for higher temperatures and cleaner combustion, making the welding process more effective and efficient.
Q U E S T I O N S Page no: 74
Question: 1. How would you distinguish experimentally between an alcohol and a carboxylic acid?
Answer: To differentiate between an alcohol and a carboxylic acid, you can perform the following experiments:
1. Litmus Test:-
– Procedure: Take a small amount of the substance and dissolve it in water. Use litmus paper to test the solution.
– Observation:
- Alcohols are neutral and do not change the color of litmus paper.
- Carboxylic acids will turn blue litmus paper red, indicating their acidic nature.
2. Reaction with Sodium Bicarbonate:
– Procedure: Add a few drops of sodium bicarbonate (baking soda) to separate samples of the alcohol and the carboxylic acid.
– Observation:
- Carboxylic acids will react with sodium bicarbonate to produce carbon dioxide gas, which can be observed as bubbles.
- Alcohols will not show any reaction with sodium bicarbonate.
These tests effectively highlight the differences in the chemical properties of alcohols and carboxylic acids.
Question: 2. What are oxidising agents?
Answer: Oxidising agents are substances that facilitate the oxidation of other substances by accepting electrons from them. In the process, the oxidising agent itself gets reduced.
– Characteristics of Oxidising Agents:
1. They increase the oxidation state of the substance being oxidised.
2. Common oxidising agents include:
– Potassium permanganate (KMnO₄): Often used in organic reactions to oxidise alcohols to carboxylic acids.
– Potassium dichromate (K₂Cr₂O₇): Another strong oxidiser used in various chemical reactions.
– Oxygen (O₂): The most common oxidising agent, involved in combustion reactions.
Example: In the oxidation of ethanol (an alcohol) to ethanoic acid (a carboxylic acid), potassium permanganate acts as the oxidising agent, accepting electrons and facilitating the conversion.
These definitions and examples clarify the role and significance of oxidising agents in chemical reactions.
Q U E S T I O N S Page no: 76
Question: 1. Would you be able to check if water is hard by using a detergent?
Answer: Yes, you can check if water is hard by using a detergent. When detergent is added to hard water, it does not form a lather easily and may produce a curdy precipitate. This is because the calcium and magnesium ions present in hard water react with the detergent, making it less effective. In contrast, when detergent is added to soft water, it readily forms a lather without any precipitate. Therefore, if you observe that the detergent does not create foam or forms a curdy substance, it indicates the presence of hardness in the water.
Question: 2. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Answer: Agitation is crucial for effective cleaning because it helps to disperse the soap or detergent throughout the fabric and water. When clothes are agitated, the soap molecules can interact more efficiently with the dirt and oil on the fabric. This mechanical action helps to break down and lift away the dirt particles, allowing them to be rinsed away more easily. Without sufficient agitation, the soap may not be able to penetrate the fabric effectively, leading to less effective cleaning. Thus, methods like beating the clothes, scrubbing, or using a washing machine enhance the cleaning process by ensuring that the detergent works optimally.
E X E R C I S E S Page no: 77
Question: 1. Ethane, with the molecular formula C₂ H₆ has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.
Answer: (b) 7 covalent bonds.
Explanation: Ethane (C₂H₆) consists of two carbon atoms, each forming four covalent bonds. Each carbon atom is bonded to three hydrogen atoms and to the other carbon atom, resulting in a total of 7 covalent bonds (3 from one carbon + 3 from the other carbon + 1 bond between the two carbons).
Question: 2. Butanone is a four-carbon compound with the functional group
(a) carboxylic acid.
(b) alcohol.
(c) ketone.
(d) aldehyde.
Answer: (c) ketone.
Explanation: Butanone, also known as methyl ethyl ketone (MEK), contains a carbonyl group (C=O) located within the carbon chain, which is characteristic of ketones.
Question: 3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.
Answer: (b) the fuel is not burning completely.
Explanation: A blackened bottom indicates incomplete combustion of the fuel, which produces soot (carbon particles) instead of a clean flame. This typically happens when there is insufficient oxygen for the fuel to burn completely.
Page no: 78
Question: 4. Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Answer: The covalent bond in methyl chloride (CH₃Cl) is formed through the sharing of electrons between the carbon atom and the chlorine atom. Here’s how it works:
1. Carbon Atom: Carbon has four valence electrons and needs four more to achieve a stable octet configuration. In CH₃Cl, carbon forms four bonds: three with hydrogen atoms and one with chlorine.
2. Hydrogen Atoms: Each hydrogen atom has one valence electron. When carbon shares one electron with each hydrogen atom, it forms three single covalent bonds (C-H).
3. Chlorine Atom: Chlorine has seven valence electrons and needs one more to complete its octet. In CH₃Cl, carbon shares one of its four valence electrons with chlorine, forming a single covalent bond (C-Cl).
The electron dot structure for CH₃Cl can be represented as follows:
H H
| |
H – C – Cl
|
H
In this representation:
- Each line represents a pair of shared electrons (a bond).
- The structure shows that carbon shares its electrons with three hydrogen atoms and one chlorine atom, resulting in a stable molecule.
Question: 5. Draw the electron dot structures for
(a) ethanoic acid.
Answer: (CH₃COOH)
The electron dot structure for ethanoic acid can be represented as follows:
H O
| ||
H – C – C – O – H
|
H
- The first carbon (C) is bonded to three hydrogen atoms and to the second carbon (C).
- The second carbon is double-bonded to one oxygen (O) and single-bonded to another oxygen (O) which is also bonded to a hydrogen (H).
(b) H₂S.
Answer: (b) Hydrogen Sulfide (H₂S)
The electron dot structure for hydrogen sulfide is:
H
|
H – S
– Sulfur (S) has six valence electrons and shares one with each of the two hydrogen atoms, forming two single covalent bonds.
(c) propanone.
Answer: (c) Propanone (C₃H₆O)
The electron dot structure for propanone can be represented as:
O
||
H – C – C – C – H
|
H
– The central carbon (C) is double-bonded to an oxygen (O) and single-bonded to two other carbons.
(d) F₂.
Answer: (d) Fluorine (F₂)
The electron dot structure for fluorine is:
F – F
– Each fluorine atom shares one electron with the other, forming a single covalent bond.
Question: 6. What is an homologous series? Explain with an example.
Answer: A homologous series is a group of organic compounds that share a common functional group and have a similar general formula. The members of a homologous series differ from each other by a constant unit, typically a -CH₂- group. This results in a gradual change in their physical properties, such as boiling and melting points, while their chemical properties remain similar due to the presence of the same functional group.
Example: Consider the alkane series, which is a homologous series of saturated hydrocarbons. The general formula for alkanes is C_nH_{2n+2}, where n is the number of carbon atoms.
- Methane (CH₄): The first member with 1 carbon atom.
- Ethane (C₂H₆): The second member with 2 carbon atoms.
- Propane (C₃H₈): The third member with 3 carbon atoms.
- Butane (C₄H₁₀): The fourth member with 4 carbon atoms.
Each successive compound differs by a -CH₂- unit, and they all exhibit similar chemical behavior due to the presence of single bonds between carbon atoms.
Question: 7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Answer: Ethanol and ethanoic acid can be differentiated based on several physical and chemical properties:
Physical Properties:
1. State:
– Ethanol: A colorless, volatile liquid at room temperature.
– Ethanoic Acid: A colorless liquid that can solidify into a crystalline form (glacial acetic acid) at lower temperatures.
2. Boiling Point:
– Ethanol: Boils at approximately 78°C.
– Ethanoic Acid: Boils at about 118°C, indicating a higher boiling point due to stronger intermolecular forces (hydrogen bonding).
3. Odor:
– Ethanol: Has a characteristic alcoholic smell.
– Ethanoic Acid: Has a pungent, vinegar-like smell.
Chemical Properties:
1. Reactivity with Sodium:
– Ethanol: Reacts with sodium to produce hydrogen gas and sodium ethoxide.
– Ethanoic Acid: Also reacts with sodium but produces sodium acetate and hydrogen gas.
2. Reaction with Carbonates:
– Ethanol: Does not react with carbonates.
– Ethanoic Acid: Reacts with carbonates to produce carbon dioxide, water, and sodium acetate.
3. Oxidation:
– Ethanol: Can be oxidized to form ethanoic acid.
– Ethanoic Acid: Does not undergo oxidation in the same way, as it is already an oxidized form.
Question: 8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Answer: Hydrophilic and Hydrophobic Ends: Soap molecules have two distinct ends — a hydrophilic (water-attracting) ionic end and a hydrophobic (water-repelling) hydrocarbon tail.
1. Interaction with Water: When soap is added to water, the hydrophilic ends interact with water molecules, while the hydrophobic tails avoid water and seek to associate with oil or grease.
2. Formation of Micelles: This dual nature causes soap molecules to arrange themselves into spherical structures called micelles, where the hydrophobic tails point inward, trapping the oil or dirt, while the hydrophilic heads face outward, interacting with water.
3. Cleaning Action: The formation of micelles allows the soap to emulsify oily dirt, making it easier to wash away with water.
4. Solvent Compatibility: Micelles are specifically formed in polar solvents like water. In non-polar solvents such as ethanol, the hydrophobic tails of soap may not behave the same way, leading to different interactions and potentially no micelle formation.
Question: 9. Why are carbon and its compounds used as fuels for most applications?
Answer: High Energy Content: Carbon compounds, particularly hydrocarbons, possess a high energy density, releasing significant energy upon combustion.
1. Combustion Products: When burned, carbon compounds typically produce carbon dioxide and water, which are less harmful compared to other combustion by-products.
2. Availability: Carbon-based fuels like coal, petroleum, and natural gas are abundant and readily available, making them convenient for widespread use.
3. Versatility: Carbon compounds can be used in various forms, such as solid (coal), liquid (petrol, diesel), and gas (natural gas), catering to different energy needs.
4. Ease of Storage and Transport: Carbon fuels can be easily stored and transported, making them practical for both industrial and domestic applications.
5. Established Infrastructure: There is a well-developed infrastructure for the extraction, refining, and distribution of carbon-based fuels, facilitating their use in various sectors.
Question: 10. Explain the formation of scum when hard water is treated with soap.
Answer: Definition of Hard Water: Hard water contains dissolved minerals, primarily calcium and magnesium ions.
1. Soap Composition: Soap is a sodium or potassium salt of long-chain carboxylic acids.
2. Reaction with Hard Water: When soap is added to hard water, it reacts with calcium and magnesium ions to form insoluble salts.
3. Formation of Scum: These insoluble salts precipitate out of the solution, forming a white curdy substance known as scum.
4. Effectiveness of Soap: The formation of scum reduces the effectiveness of soap, as it prevents the soap from lathering properly.
5. Increased Soap Requirement: More soap is needed to achieve the desired cleaning effect due to the presence of scum.
Question: 11. What change will you observe if you test soap with litmus paper (red and blue)?
Answer: 1. Testing with Red Litmus Paper: When soap is tested with red litmus paper, it will turn blue, indicating that soap is basic in nature.
2. Testing with Blue Litmus Paper: There will be no change observed when blue litmus paper is tested with soap, as it remains blue.
3. Conclusion: This behavior confirms that soap has alkaline properties.
Question: 12. What is hydrogenation? What is its industrial application?
Answer: Definition of Hydrogenation: Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) to unsaturated hydrocarbons, typically in the presence of a catalyst.
1. Process: During hydrogenation, double or triple bonds in unsaturated hydrocarbons are converted into single bonds, resulting in saturated hydrocarbons.
2. Catalysts Used: Common catalysts for hydrogenation include nickel, palladium, or platinum.
3. Industrial Application:
4. Food Industry: Hydrogenation is widely used in the food industry to convert liquid vegetable oils into solid or semi-solid fats (e.g., margarine).
5. Production of Fuels: It is also used in the production of fuels and chemicals from unsaturated hydrocarbons.
6. Health Implications: Hydrogenated fats are often considered less healthy due to the formation of trans fats during the process, which can have adverse health effects.
Question: 13. Which of the following hydrocarbons undergo addition reactions: C₂H₆ , C₃ H₈ , C₃ H₆ , C₂ H₂ and CH₄ .
Answer: C₂H₆ (Ethane): Does not undergo addition reactions as it is a saturated hydrocarbon.
- C₃H₈ (Propane): Does not undergo addition reactions as it is also a saturated hydrocarbon.
- C₃H₆ (Propene): Undergoes addition reactions due to the presence of a double bond.
- C₂H₂ (Ethyne): Undergoes addition reactions due to the presence of a triple bond.
- CH₄ (Methane): Does not undergo addition reactions as it is a saturated hydrocarbon.
Conclusion: The hydrocarbons that undergo addition reactions are C₃H₆ (Propene) and C₂H₂ (Ethyne).
Question: 14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Answer: Bromine Water Test:
- Procedure: Add a few drops of bromine water to the hydrocarbon sample.
- Observation:
- If the reddish-brown color of bromine water disappears, the hydrocarbon is unsaturated (contains double or triple bonds).
- If the color remains, the hydrocarbon is saturated (contains only single bonds).
Question: 15. Explain the mechanism of the cleaning action of soaps.
Answer: Structure of Soap:
- Soap molecules consist of a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail.
- Formation of Micelles:
- When soap is added to water, the molecules arrange into structures called micelles.
- The hydrophilic heads face outward towards the water, while the hydrophobic tails face inward, trapping oil or dirt.
- Emulsification:
- The hydrophobic tails interact with oily dirt, allowing the dirt to be surrounded by soap molecules.
- This forms an emulsion, which allows the oily dirt to be suspended in water.
- Washing Action:
- During rinsing, the micelles (containing trapped dirt) are washed away, resulting in clean surfaces or fabrics.
- Effectiveness in Hard Water:
- Soap is less effective in hard water due to the formation of insoluble precipitates (scum) with calcium and magnesium ions.
- Detergents, which do not form scum, are often used in hard water conditions.