CBSE Class 10 Science Carbon and its compounds Assignment Set 10

This is Assignment Set 10 for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds, with marks for each question and a step-by-step answer with a Teacher's Note. It focuses on the chemical properties of carbon compounds, including reactions of ethanol and ethanoic acid, esterification, saponification, addition and substitution reactions, and the cleansing action of soaps and detergents.

Very Short and Short Answer Questions

Question 1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction? [2 marks]
Answer:
In this conversion, oxygen is added to ethanol (the –CH₂OH group changes to –COOH). Addition of oxygen is oxidation, so the change is an oxidation reaction.
CH₃CH₂OH → CH₃COOH (with alkaline KMnO₄ or acidified K₂Cr₂O₇ as the oxidising agent, with heating)
In simple words: Ethanol gains oxygen to become ethanoic acid, which is oxidation.

Teacher's Note:
1. The reagent that supplies the oxygen is called an oxidising agent.
2. Oxidation can also be seen as loss of hydrogen.

Question 2. Hard water do not produce foam with soap easily. Why? [2 marks]
Answer:
Hard water contains calcium and magnesium salts. The Ca²⁺ and Mg²⁺ ions react with the soap (a sodium salt of a fatty acid) and change it into insoluble calcium and magnesium salts, which separate as a curdy white precipitate called scum.
The soap is used up in forming scum, so no lather forms until all the hardness-causing ions have reacted.
In simple words: Soap is wasted by the Ca and Mg ions of hard water, which turn it into scum.

Teacher's Note:
1. Foam appears only after the ions are used up.
2. Detergents do not form scum.

Question 3. Why does micelle formation takes place when soap is added to water? Will a micelle is formed in other solvents such as ethanol also? [3 marks]
Answer:
Step 1: A soap molecule has an ionic head that dissolves in water and a long hydrocarbon tail that does not. In water the tails group together in the inside, away from water, and the ionic heads stay on the outside in contact with water. This cluster is a micelle. Oily dirt collects in the centre of the micelle, and so soap can clean.
Step 2: In ethanol, the hydrocarbon tail of soap dissolves, because ethanol is an organic solvent. The tails therefore need not hide from the solvent, so micelles are not formed in ethanol.
In simple words: Tails avoid water and cluster together, forming micelles; in ethanol the tails dissolve, so no micelles form.

Teacher's Note:
1. Hydrophobic means water-hating, hydrophilic means water-loving.
2. Like dissolves like: hydrocarbons dissolve in organic solvents.

Question 4. List two chemical properties on the basis of which ethanol and ethanoic acid may be differentiated and explain how. [3 marks]
Answer:
1. Reaction with sodium hydrogencarbonate: Ethanoic acid reacts with NaHCO₃ to give brisk effervescence of CO₂ gas (which turns lime water milky). Ethanol gives no reaction.
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
2. Action on litmus: Ethanoic acid turns blue litmus red because of its carboxylic acid group. Ethanol is neutral and has no effect on litmus.
In simple words: Only ethanoic acid fizzes with sodium hydrogencarbonate and turns blue litmus red.

Teacher's Note:
1. Both liquids are colourless, so a chemical test is needed.
2. Ethanol reacts with sodium too, so that test does not separate them.

Question 5. Unsaturated hydrocarbons contain multiple bonds between two carbon atoms and these compounds show addition reactions. Out of saturated and unsaturated carbon compounds, which compounds are more reactive? Write a test to distinguish ethane from ethene. [2 marks]
Answer:
More reactive: Unsaturated compounds, because their double or triple bonds are easily broken to add atoms.
Test: Add bromine water (reddish-brown) to both gases. Ethene decolourises it, because it adds bromine: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br. Ethane does not change the colour of bromine water.
In simple words: Bromine water loses its colour with ethene (unsaturated) but not with ethane.

Teacher's Note:
1. Alkaline KMnO₄ can also be used; it is decolourised by ethene.
2. Burning gives a sooty flame with ethene.

Question 6. What happens when ethanol reacts with (i) sodium, (ii) potassium permanganate solution. [2 marks]
Answer:
(i) Ethanol reacts with sodium metal to give sodium ethoxide and hydrogen gas: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂↑
(ii) With alkaline potassium permanganate (on warming) ethanol is oxidised to ethanoic acid, and the purple colour of the permanganate fades: CH₃CH₂OH → CH₃COOH (alkaline KMnO₄, heat)
In simple words: Sodium gives hydrogen gas and sodium ethoxide; alkaline KMnO₄ oxidises ethanol to ethanoic acid.

Teacher's Note:
1. The pop sound test confirms hydrogen.
2. KMnO₄ acts as an oxidising agent, not as a reactant that remains in the product.

Question 7. Answer the following questions: (i) What name is given to the reaction which take place when ethanoic acid reacts with ethanol in the presence of conc. sulphuric acid? Name the products obtained in this reaction. (ii) The structural formula of an ester is HCOOCH₂CH₂CH₃ write the formula of the acid and the alcohol from which it is made? [3 marks]
Answer:
(i) The reaction is esterification. The products are ethyl ethanoate (an ester with a fruity smell) and water: CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O (conc. H₂SO₄)
(ii) The ester HCOOCH₂CH₂CH₃ is made from methanoic acid, HCOOH, and propan-1-ol, CH₃CH₂CH₂OH. The part before the –O– comes from the acid (HCOO–) and the part after it comes from the alcohol (–CH₂CH₂CH₃).
In simple words: An acid and an alcohol with conc. H₂SO₄ give a sweet-smelling ester and water.

Teacher's Note:
1. The ester is named alkyl (from the alcohol) alkanoate (from the acid).
2. Conc. H₂SO₄ acts as a catalyst and removes water.

Question 8. List any two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed? [3 marks]
Answer:
1. Litmus test: Place a drop of each liquid on blue litmus paper. The carboxylic acid turns it red; the alcohol has no effect.
2. Sodium hydrogencarbonate test: Add a pinch of NaHCO₃ to each liquid. The carboxylic acid gives brisk effervescence of colourless CO₂ gas (which turns lime water milky); the alcohol shows no effervescence.
In simple words: Litmus turns red and baking soda fizzes only with the carboxylic acid.

Teacher's Note:
1. Pass the gas through lime water to confirm CO₂.
2. The acid group –COOH releases H⁺ ions.

Question 9. An organic compound 'P' is a constituent of wine. 'P' on reacting with acidified K₂Cr₂O₇ forms another compound 'Q'. When a piece of sodium is added to 'Q' a gas 'R' evolves which burns with pop sound. Identify P, Q, R and write the balanced chemical equation of the reactions involved. [3 marks]
Answer:
P is ethanol (the alcohol in wine). Acidified K₂Cr₂O₇ oxidises it to Q, ethanoic acid. Sodium reacts with ethanoic acid to liberate R, hydrogen gas, which burns with a pop sound.
CH₃CH₂OH → CH₃COOH (acidified K₂Cr₂O₇, heat)
2CH₃COOH + 2Na → 2CH₃COONa + H₂↑
In simple words: Wine alcohol is oxidised to the acid, and the acid releases hydrogen with sodium.

Teacher's Note:
1. The pop sound identifies hydrogen.
2. Both ethanol and ethanoic acid react with sodium.

Question 10. Complete the following chemical equations: (i) CH₃COOC₂H₅ + NaOH → ______ (ii) CH₃COOH + NaOH → ______ (iii) C₂H₅OH + CH₃COOH → ______ [3 marks]
Answer:
(i) CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (saponification: sodium ethanoate and ethanol)
(ii) CH₃COOH + NaOH → CH₃COONa + H₂O (neutralisation: sodium ethanoate and water)
(iii) C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (esterification, in the presence of conc. H₂SO₄: ethyl ethanoate and water)
In simple words: An ester with alkali gives a salt and alcohol; an acid with alkali gives salt and water; an acid and an alcohol give an ester.

Teacher's Note:
1. Make sure each equation is balanced.
2. The same functional group changes appear again and again in board papers.

Question 11. What is an oxidising agent? What happens when an oxidising agent is added to Propanol? Explain with the help of a chemical equation. [3 marks]
Answer:
An oxidising agent is a substance that gives oxygen to another substance (or removes hydrogen from it), and is itself reduced. Examples: alkaline potassium permanganate and acidified potassium dichromate.
When an oxidising agent is added to propanol it is oxidised to propanoic acid:
CH₃CH₂CH₂OH → CH₃CH₂COOH (alkaline KMnO₄ or acidified K₂Cr₂O₇, heat)
In simple words: An oxidising agent adds oxygen; it turns propanol into propanoic acid.

Teacher's Note:
1. The number of carbon atoms does not change.
2. The purple colour of KMnO₄ disappears as it reacts.

Question 12. With the help of an example, explain the process of hydrogenation. Mention the essential condition for the reaction and state the change in physical property with the formation of the product. [3 marks]
Answer:
Hydrogenation is the addition of hydrogen to an unsaturated hydrocarbon to form a saturated hydrocarbon. Example: ethene + hydrogen gives ethane: CH₂=CH₂ + H₂ → CH₃–CH₃
Essential condition: a catalyst such as nickel, palladium or platinum (and heat).
Change in physical property: a liquid vegetable oil changes into a solid fat (vanaspati ghee), so the melting point and the density increase.
In simple words: Hydrogen is added over a nickel catalyst, turning liquid unsaturated oil into solid saturated fat.

Teacher's Note:
1. Hydrogenation is used in the industry to make vegetable ghee.
2. Only unsaturated compounds can be hydrogenated.

Question 13. What is the difference between the molecules of soaps and detergents, chemically? Explain the cleansing action of soaps. [5 marks]
Answer:
Difference: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids), RCOO⁻Na⁺. Detergents are generally sodium salts of sulphonic acids, or ammonium salts, of long-chain compounds (for example, alkylbenzene sulphonates).
Cleansing action of soap: A soap molecule has two ends: an ionic head (hydrophilic, water loving) and a long hydrocarbon tail (hydrophobic, water repelling, oil loving).
In water the soap molecules form clusters called micelles, with the hydrocarbon tails in the interior and the ionic heads on the surface facing the water.
The oily dirt gets collected in the centre of the micelle, since the tails dissolve in oil. When the cloth is agitated, the dirt is pulled out of the fibres and held within micelles. Because the heads are on the outside, the micelles stay suspended in water and are rinsed away with the dirt.
In simple words: Soap is a carboxylate salt, detergent a sulphonate salt; soap micelles trap oily dirt and wash it away.

Teacher's Note:
1. Draw a labelled micelle if the question asks for a diagram.
2. Both soaps and detergents work by forming micelles.

Question 14. Write the balanced chemical equation for the following: (i) Methane is burned in sufficient air. (ii) Ethanol is treated with sodium. (iii) Ethanoic acid is reacted with sodium hydroxide. (iv) Ethanoic acid is treated with sodium carbonate. (v) Ethanol is mixed with ethanoic acid in the presence of an acid. [5 marks]
Answer:
(i) CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
(ii) 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
(iii) CH₃COOH + NaOH → CH₃COONa + H₂O
(iv) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
(v) CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O (conc. H₂SO₄ as catalyst)
In simple words: Write each reactant and product and balance the atoms of C, H, O and Na.

Teacher's Note:
1. Balance the oxygen atoms last.
2. Methane needs two molecules of oxygen, not five.

Question 15. Answer the following questions: (i) Give a chemical test to distinguish between saturated and unsaturated hydrocarbon. (ii) Name the products formed when ethane burns in air. Write the balanced chemical equation for the reaction showing type of energies liberated. (iii) Why is reaction between methane and chlorine in the presence of sunlight considered a substitution reaction. [3 marks]
Answer:
(i) Bromine water test: Pass the hydrocarbon through reddish-brown bromine water. An unsaturated hydrocarbon decolourises it; a saturated one does not.
(ii) Ethane burns in air to give carbon dioxide and water, with the release of heat and light energy: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + heat + light
(iii) In sunlight, chlorine replaces a hydrogen atom of methane: CH₄ + Cl₂ → CH₃Cl + HCl. A hydrogen atom is substituted by a chlorine atom, so it is a substitution reaction.
In simple words: Bromine water tests for unsaturation; ethane burns to CO₂ and water; chlorine replaces hydrogen in methane.

Teacher's Note:
1. Check that the ethane equation is balanced: 4 C, 12 H and 14 O on both sides.
2. Substitution gives two products.

Long Answer and Case Based Questions

Question 16. Explain the given reactions with the examples : (i) Hydrogenation reaction (ii) Oxidation reaction (iii) Substitution reaction (iv) Saponification reaction (v) Combustion reaction [5 marks]
Answer:
(i) Hydrogenation: unsaturated hydrocarbons add hydrogen in the presence of nickel to give saturated hydrocarbons: CH₂=CH₂ + H₂ → CH₃CH₃
(ii) Oxidation: ethanol is oxidised to ethanoic acid by alkaline KMnO₄ on heating: CH₃CH₂OH → CH₃COOH
(iii) Substitution: in sunlight, chlorine replaces hydrogen of a saturated hydrocarbon: CH₄ + Cl₂ → CH₃Cl + HCl
(iv) Saponification: an ester reacts with an alkali to give the salt of the acid and the alcohol: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (the same reaction of fats with alkali is used to make soap)
(v) Combustion: carbon compounds burn in oxygen to give carbon dioxide and water with heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
In simple words: Each reaction type is shown by one example equation.

Teacher's Note:
1. Name the reaction type and give the equation.
2. Substitution and addition are told apart by the number of products.

Question 17. What happens when 5% alkaline potassium permanganate solution is added drop by drop to warm propyl alcohol (propanol) taken in a test tube? Explain with the help of a chemical equation. [2 marks]
Answer:
When alkaline KMnO₄ solution is added drop by drop to warm propanol, the purple colour of the permanganate disappears because propanol is oxidised to propanoic acid. The oxidising agent supplies the oxygen.
CH₃CH₂CH₂OH → CH₃CH₂COOH (alkaline KMnO₄, heat)
When all the alcohol is used up, the purple colour no longer disappears and stays.
In simple words: The purple KMnO₄ is decolourised while propanol is oxidised to propanoic acid.

Teacher's Note:
1. The colour change tells you that oxidation is going on.
2. Same behaviour as ethanol.

Question 18. Srishti heated ethanol with a compound P in presence of a few drops of concentrated sulphuric acid and observed a sweet smelling compound Q is formed. When Q is treated with sodium hydroxide it gives back ethanol and a compound R. (A) Identify P and R. (B) Give one use each of compounds P and Q. (C) Write the chemical reactions involved and name the reactions. [3 marks]
Answer:
(A) A sweet smelling compound formed from ethanol and an acid is an ester. So P is ethanoic acid (CH₃COOH) and Q is ethyl ethanoate. Treating the ester with NaOH gives back ethanol and the sodium salt of the acid, so R is sodium ethanoate (CH₃COONa).
(B) Use of P: as vinegar to preserve pickles. Use of Q: in perfumes and as a flavouring agent.
(C) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄) is esterification.
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH is saponification (alkaline hydrolysis of an ester).
In simple words: Ethanoic acid and ethanol make an ester; the ester with NaOH gives ethanol and sodium ethanoate.

Teacher's Note:
1. Q is the ester, so use the same label as in the question.
2. Saponification is the reverse of esterification under alkaline conditions.

Question 19. A saturated organic compound 'A' belongs to the homologous series of alcohols. On heating 'A' with concentrated sulphuric acid at 443 K, it forms an unsaturated compound 'B' with molecular mass 28 u. The compound 'B' on addition of one mole of hydrogen in the presence of Nickel, changes to a saturated hydrocarbon 'C'. (A) Identify A, B and C. (B) Write the chemical equations showing the conversion of A into B. (C) What happens when compound C undergoes combustion? (D) State one industrial application of hydrogenation reaction. (E) Name the products formed when compound A reacts with sodium. [5 marks]
Answer:
(A) The unsaturated hydrocarbon with a molecular mass of 28 u is ethene, C₂H₄ (2 × 12 + 4). So B = ethene. It is formed by dehydration of an alcohol, so A = ethanol. Adding H₂ gives C = ethane.
(B) CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, 443 K)
(C) Ethane burns in air to form carbon dioxide and water, releasing heat and light: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + heat + light
(D) Hydrogenation is used to convert vegetable oils (unsaturated) into vegetable ghee (saturated fats).
(E) Ethanol reacts with sodium to give sodium ethoxide and hydrogen gas: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
In simple words: Work back from the mass 28 u to ethene, then ethanol and ethane.

Teacher's Note:
1. Molecular mass 28 u fits C₂H₄.
2. Conc. H₂SO₄ removes water from ethanol.

Teacher's Note:
1. Look first at the hint that names the denaturants: it points to ethanol.
2. Next homologue means one more CH₂ unit.