CBSE Class 10 Science HOTs Carbon And its Compounds Set 06

Versatile Nature of Carbon

Question. (a) State the reason why carbon can neither form \( C^{4+} \) cations nor \( C^{4-} \) anions, but forms covalent bonds. Also state reasons to explain why covalent compounds
(i) are bad conductors of electricity
(ii) have low melting and boiling points.
(b) Write the structural formula of benzene, \( C_6H_6 \).
Answer: (a) Carbon has 4 valence electrons. Gaining 4 electrons to form \( C^{4-} \) is difficult because a nucleus with 6 protons cannot easily hold 10 electrons. Losing 4 electrons to form \( C^{4+} \) requires a very high amount of energy. Thus, it shares electrons to form covalent bonds.
(i) They are bad conductors because they do not form ions and have no free electrons.
(ii) They have low melting and boiling points because the intermolecular forces between the molecules are weak.
(b) The structural formula of benzene is a hexagonal ring with alternating single and double bonds between carbon atoms, with one hydrogen atom attached to each carbon.

Question. Explain why carbon forms compounds mainly by covalent bond. Explain in brief two main reasons for carbon forming a large number of compounds. Why does carbon form strong bond with most other elements?
Answer: Carbon forms covalent bonds because its tetravalency makes losing or gaining four electrons energy-intensive; sharing is more feasible. Two reasons for the large number of compounds are: (1) Catenation (self-linking) and (2) Tetravalency. Carbon forms strong bonds because of its small atomic size, which allows the nucleus to hold the shared electron pairs strongly.

Question. What are hydrocarbons? Distinguish alkanes from alkenes and each of them from alkynes, giving one example of each. Draw the structure of each compound cited as example to justify your answer.
Answer: Hydrocarbons are compounds composed only of carbon and hydrogen. Alkanes are saturated (single bonds), Alkenes are unsaturated with double bonds, and Alkynes are unsaturated with triple bonds.
Example Alkane: Ethane \( (C_2H_6) \) - structure shows single C-C bond.
Example Alkene: Ethene \( (C_2H_4) \) - structure shows double C=C bond.
Example Alkyne: Ethyne \( (C_2H_2) \) - structure shows triple \( C \equiv C \) bond.

Chemical Properties of Carbon Compounds

Question. Name the process by which unsaturated fats are changed to saturated fats.
Answer: Hydrogenation.

Question. Write the chemical equation to show what happens when methane is treated with chlorine in the presence of sunlight ?
OR
Write one chemical equation to represent the following type of reaction of organic substances : substitution.
Answer: \( CH_4 + Cl_2 \xrightarrow{\text{Sunlight}} CH_3Cl + HCl \)

Question. Write the respective chemical reaction to show what happens when methane is burnt in presence of oxygen?
Answer: \( CH_4 + 2O_2 \to CO_2 + 2H_2O + \text{heat and light} \)

Question. 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: When alkaline \( KMnO_4 \) is added to warm ethanol, it gets oxidized to ethanoic acid. The pink color of \( KMnO_4 \) disappears initially and then persists when added in excess. Alkaline \( KMnO_4 \) acts as an oxidizing agent.

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.
(i) How is 5% solution of \( KMnO_4 \) prepared?
(ii) State the role of alkaline potassium permanganate in this reaction. What happens on adding it in excess?
(iii) Write chemical equation of this reaction.
Answer: (i) It is prepared by dissolving 5 g of potassium permanganate in 100 mL of water.
(ii) It acts as an oxidizing agent. On adding it in excess, the pink color of the solution persists.
(iii) \( CH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 + \text{heat}} CH_3COOH \)

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.
OR
The molecular formula of two carbon compounds are \( C_4H_8 \) and \( C_3H_8 \). Which one of the two 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: \( C_4H_8 \) is most likely to show an addition reaction because it is an unsaturated hydrocarbon (alkene). Saturated hydrocarbons like \( C_5H_{12} \) and \( C_3H_8 \) do not show addition reactions. Addition reaction equation:
\( C_4H_8 + H_2 \xrightarrow{Ni} C_4H_{10} \)

Question. What is an oxidising agent? What happens when an oxidising agent is added to propanol? Explain with the help of a chemical equation.
Answer: An oxidizing agent is a substance that gives oxygen to other substances or removes hydrogen from them. When an oxidizing agent like alkaline \( KMnO_4 \) is added to propanol, it oxidizes to propanoic acid.
\( CH_3CH_2CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 + \text{heat}} CH_3CH_2COOH \)

Question. Draw the electron-dot structure for ethyne. A mixture of ethyne and oxygen is burnt for welding. In your opinion, why cannot we use a mixture of ethyne and air for this purpose?
Answer: Ethyne structure: \( H - C \equiv C - H \). A mixture of ethyne and air cannot be used for welding because air contains only about 21% oxygen. This leads to incomplete combustion of ethyne, resulting in a sooty flame and insufficient temperature for welding. Pure oxygen ensures complete combustion and high temperature.

Question. Write the name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen is possible. State the essential condition for an addition reaction. Stating this condition, write a chemical equation giving the name of the reactant and the product of the reaction.
Answer: Addition reactions are possible in unsaturated hydrocarbons like Alkenes \( (C_nH_{2n}) \) and Alkynes \( (C_nH_{2n-2}) \). The essential condition is the presence of a catalyst like Nickel (Ni) or Palladium (Pd).
Equation: \( C_2H_4 \) (Ethene) \( + H_2 \xrightarrow{Ni} C_2H_6 \) (Ethane).

Question. (i) Write the name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen can take place. Stating the essential condition required for an addition reaction to occur, write the chemical equation giving the name of the reactant and product of such a reaction. How is an addition reaction different from a substitution reaction?
(ii) Write the structure of benzene.
Answer: (i) Alkenes \( (C_nH_{2n}) \) or Alkynes \( (C_nH_{2n-2}) \). Condition: Catalyst (Ni/Pd/Pt). Reaction: Ethene \( + H_2 \xrightarrow{Ni} \) Ethane. Addition reaction adds atoms across a multiple bond, while substitution reaction replaces one atom with another.
(ii) Benzene is a ring of six carbons with alternating single and double bonds.

Question. With the help of a suitable example, explain the process of hydrogenation mentioning the conditions of the reaction and any one change in physical property with the formation of the product.
Answer: Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in presence of a catalyst like Ni. Example: Conversion of liquid vegetable oils (unsaturated) into solid vegetable ghee (saturated). Physical change: Liquid changes to semi-solid/solid.

Some Important Carbon Compounds - Ethanol and Ethanoic acid

Question. Assertion (A) : Esterification is a process in which a sweet smelling substance is produced.
Reason (R) : When esters react with sodium hydroxide, an alcohol and sodium salt of carboxylic acid are obtained.
(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: (b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).

Question. Assertion (A) : Ethanoic acid is also known as glacial acetic acid.
Reason (R) : The melting point of pure ethanoic acid is 290 K and hence it often freezes during winters in cold climates.
(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. A compound ‘X’ on heating with excess conc. sulphuric acid at 443 K gives an unsaturated compound ‘Y’. ‘X’ also reacts with sodium metal to evolve a colourless gas ‘Z’. Identify ‘X’, ‘Y’ and ‘Z’. Write the equation of the chemical reaction of formation of ‘Y’ and also write the role of sulphuric acid in the reaction.
Answer: X is Ethanol \( (C_2H_5OH) \), Y is Ethene \( (C_2H_4) \), and Z is Hydrogen gas \( (H_2) \).
Equation: \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} C_2H_4 + H_2O \).
Role of Conc. \( H_2SO_4 \): It acts as a dehydrating agent.

Question. Write the chemical equations to show what happens when
(i) an ester reacts with a base?
(ii) ethanol reacts with ethanoic acid in the presence of sulphuric acid?
Answer: (i) \( CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \) (Saponification)
(ii) \( CH_3COOH + C_2H_5OH \xrightarrow{\text{Acid}} CH_3COOC_2H_5 + H_2O \) (Esterification)

Question. Write the respective chemical equations to show what happens when
(i) ethanol is heated with concentrated sulphuric acid at 443 K ?
(ii) ethanol reacts with ethanoic acid in the presence of an acid acting as a catalyst?
Answer: (i) \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} C_2H_4 + H_2O \)
(ii) \( C_2H_5OH + CH_3COOH \xrightarrow{H^+} CH_3COOC_2H_5 + H_2O \)

Question. Complete the following chemical equations :
(i) \( CH_3COOC_2H_5 + NaOH \to \)
(ii) \( CH_3COOH + NaOH \to \)
(iii) \( C_2H_5OH + CH_3COOH \xrightarrow{Conc. H_2SO_4} \)
Answer: (i) \( CH_3COONa + C_2H_5OH \)
(ii) \( CH_3COONa + H_2O \)
(iii) \( CH_3COOC_2H_5 + H_2O \)

Question. Complete the following chemical equations:
(i) \( C_2H_5OH + O_2 \to \)
(ii) \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} \)
(iii) \( CH_3COOH + NaHCO_3 \to \)
Answer: (i) \( CO_2 + H_2O + \text{Energy} \)
(ii) \( C_2H_4 + H_2O \)
(iii) \( CH_3COONa + H_2O + CO_2 \)

Question. Write the structural formula of ethanol. What happens when it is heated with excess of conc. \( H_2SO_4 \) at 443 K? Write the chemical equation for the reaction stating the role of conc. \( H_2SO_4 \) in this reaction.
Answer: Ethanol: \( CH_3-CH_2-OH \). When heated with conc. \( H_2SO_4 \), it undergoes dehydration to form ethene.
Equation: \( CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} CH_2=CH_2 + H_2O \).
Role: Dehydrating agent.

Question. What happens when (write chemical equation in each case)
(a) ethanol is burnt in air?
(b) ethanol is heated with excess conc. \( H_2SO_4 \) at 443 K?
(c) a piece of sodium is dropped into ethanol?
Answer: (a) \( C_2H_5OH + 3O_2 \to 2CO_2 + 3H_2O + \text{heat} \)
(b) \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} C_2H_4 + H_2O \)
(c) \( 2C_2H_5OH + 2Na \to 2C_2H_5ONa + H_2 \)

Question. Distinguish between esterification and saponification reaction with the help of the chemical equations for each. State one use of each (i) esters, and (ii) saponification process.
Answer: Esterification is the reaction between alcohol and acid to form ester: \( CH_3COOH + C_2H_5OH \to CH_3COOC_2H_5 + H_2O \). Saponification is the alkaline hydrolysis of an ester: \( CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \).
Use of Esters: In perfumes/flavoring agents. Use of Saponification: Preparation of soaps.

Question. When ethanol reacts with ethanoic acid in the presence of conc. \( H_2SO_4 \), a substance with fruity smell is produced. Answer the following :
(i) State the class of compounds to which the fruity smelling compounds belong. Write the chemical equation for the reaction and write the chemical name of the product formed.
(ii) State the role of conc. \( H_2SO_4 \) in this reaction.
Answer: (i) Esters. Equation: \( CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O \). Product: Ethyl ethanoate.
(ii) Catalyst and dehydrating agent.

Question. Name the compound formed when ethanol is heated in excess of conc. sulphuric acid at 443 K. Also write the chemical equation of the reaction stating the role of conc. sulphuric acid in it. What would happen if hydrogen is added to the product of this reaction in the presence of catalyst such as palladium or nickel?
Answer: Ethene is formed. Equation: \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4} C_2H_4 + H_2O \). Role: Dehydrating agent. If hydrogen is added to ethene (Ni/Pd catalyst), it converts to Ethane \( (C_2H_6) \).

Question. Write chemical equation of the reaction of ethanoic acid with the following :
(a) Sodium; (b) Sodium hydroxide; (c) Ethanol
Write the name of one main product of each reaction.
Answer: (a) \( 2CH_3COOH + 2Na \to 2CH_3COONa + H_2 \). Product: Sodium ethanoate.
(b) \( CH_3COOH + NaOH \to CH_3COONa + H_2O \). Product: Sodium ethanoate.
(c) \( CH_3COOH + C_2H_5OH \xrightarrow{H^+} CH_3COOC_2H_5 + H_2O \). Product: Ethyl ethanoate.

Question. On dropping a small piece of sodium in a test tube containing carbon compound ‘X’ with molecular formula \( C_2H_6O \), a brisk effervescence is observed and a gas ‘Y’ is produced. On bringing a burning splinter at the mouth of the test tube the gas evolved burns with a pop sound. Identify ‘X’ and ‘Y’. Also write the chemical equation for the reaction. Write the name and structure of the product formed, when you heat ‘X’ with excess conc. sulphuric acid.
Answer: X is Ethanol \( (C_2H_5OH) \) and Y is Hydrogen gas \( (H_2) \).
Equation: \( 2C_2H_5OH + 2Na \to 2C_2H_5ONa + H_2 \).
Heating X with conc. \( H_2SO_4 \) gives Ethene \( (CH_2=CH_2) \).

Question. Write three different chemical reactions showing the conversion of ethanoic acid to sodium ethanoate. Write balanced chemical equation in each case. Write the name of the reactants and the products other than ethanoic acid and sodium ethanoate in each case.
Answer: 1. With NaOH: \( CH_3COOH + NaOH \to CH_3COONa + H_2O \) (Reactant: Sodium hydroxide; Product: Water).
2. With \( Na_2CO_3 \): \( 2CH_3COOH + Na_2CO_3 \to 2CH_3COONa + H_2O + CO_2 \) (Reactant: Sodium carbonate; Products: Water, Carbon dioxide).
3. With \( NaHCO_3 \): \( CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2 \) (Reactant: Sodium bicarbonate; Products: Water, Carbon dioxide).

Question. Write the name and molecular formula of an organic compound having its name suffixed with ‘ol’ and having two carbon atoms in its molecule. Write balanced chemical equation to indicate what happens when this compound is heated with excess conc. \( H_2SO_4 \) and the name of main product formed. Also state the role of conc. \( H_2SO_4 \) in the reaction.
Answer: Ethanol, \( C_2H_5OH \). When heated with excess conc. \( H_2SO_4 \), Ethene is formed.
Equation: \( C_2H_5OH \xrightarrow{\text{Conc. } H_2SO_4, 443K} C_2H_4 + H_2O \). Product: Ethene. Role: 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 \( (H_2) \).
Equation 1: \( C_2H_5OH \xrightarrow{\text{Acidified } K_2Cr_2O_7} CH_3COOH \).
Equation 2: \( 2CH_3COOH + 2Na \to 2CH_3COONa + H_2 \).

Question. List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed.
Answer: 1. Litmus Test: Carboxylic acid turns blue litmus red; alcohol has no effect.
2. Sodium Bicarbonate Test: Carboxylic acid gives brisk effervescence of \( CO_2 \) with \( NaHCO_3 \); alcohol does not react.

Question. What are esters? How are they prepared? List two uses of esters.
Answer: Esters are sweet-smelling organic compounds. They are prepared by the reaction of a carboxylic acid and an alcohol in the presence of an acid catalyst. Uses: In perfumes and as flavoring agents in food.

Question. A carboxylic acid (molecular formula \( C_2H_4O_2 \)) reacts with an alcohol in the presence of an acid catalyst to form a compound ‘X’. The alcohol on oxidation with alkaline \( KMnO_4 \) followed by acidification gives the same carboxylic acid \( C_2H_4O_2 \). Write the name and structure of (i) carboxylic acid, (ii) alcohol and (iii) the compound ‘X’.
Answer: (i) Carboxylic acid: Ethanoic acid \( (CH_3COOH) \). (ii) Alcohol: Ethanol \( (C_2H_5OH) \). (iii) Compound X: Ethyl ethanoate \( (CH_3COOC_2H_5) \).

Question. Shristi heated ethanol with a compound A in presence of a few drops of concentrated sulphuric acid and observed a sweet smelling compound B is formed. When B is treated with sodium hydroxide it gives back ethanol and a compound C.
(a) Identify A and C.
(b) Give one use each of compounds A and B.
(c) Write the chemical reactions involved and name the reactions.
OR
(a) What is the role of concentrated sulphuric acid when it is heated with ethanol at 443 K? Give the reaction involved.
(b) Reshu by mistake forgot to label the two test tubes containing ethanol and ethanoic acid. Suggest an experiment to identify the substances correctly? Illustrate the reactions with the help of chemical equations.
Answer: (a) A is Ethanoic acid (\( CH_3COOH \)) and C is Sodium Ethanoate (\( CH_3COONa \)).
(b) Use of A: Ethanoic acid is used as a preservative in food items like pickles. Use of B: Esters (compound B) are used in making perfumes and flavoring agents.
(c) Reaction 1: \( CH_3COOH + C_2H_5OH \xrightarrow{Conc. H_2SO_4} CH_3COOC_2H_5 + H_2O \) (Esterification reaction)
Reaction 2: \( CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \) (Saponification reaction)
OR
(a) Concentrated sulphuric acid acts as a dehydrating agent as it removes water from ethanol. Reaction: \( C_2H_5OH \xrightarrow{Conc. H_2SO_4, 443 K} CH_2=CH_2 + H_2O \)
(b) We can use sodium bicarbonate (\( NaHCO_3 \)) or sodium carbonate (\( Na_2CO_3 \)) to distinguish between them. Ethanoic acid will react with sodium bicarbonate to give brisk effervescence of carbon dioxide gas, while ethanol will not react.
\( CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2 \)

Question. Read the passage given below and answer the following questions :
A hydrocarbon (P) has the molecular formula \( C_{10}H_{22} \). A hydrocarbon (Q) has two carbon atoms less than (P) and belong to the same homologous series. A hydrocarbon (R) has two carbon atoms more than (P) and belong to the same homologous series.

Question. Out of alkanes, alkenes and alkynes, which homologous series do the compound (P), (Q) and (R) belong? Also write the molecular formula of (R).
Answer: Compounds (P), (Q) and (R) are alkanes. Molecular formula of (R) is \( C_{12}H_{26} \) as it has two carbon atoms more than (P).

Question. Write the IUPAC name of the third member of homologous series of (P)?
Answer: Compound (P), (Q) and (R) are alkanes. So, the third member of the alkanes homologous series is propane, having the molecular formula \( C_3H_8 \).

Question. What is the general formula of compounds P, Q and R and classify them as saturated and unsaturated compounds?
Answer: The general formula of alkanes is \( C_nH_{2n+2} \), and all of these are classified as saturated hydrocarbons.