This MCQ module is based on: Chemical Reactions of Carbon Compounds, Ethanol, Ethanoic Acid and Soaps
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Chemical Reactions of Carbon Compounds, Ethanol, Ethanoic Acid and Soaps
🎓 Class 10
Science
CBSE
Theory
Ch 4 — Carbon and its Compounds
⏱ ~26 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_10" science_domain="chemistry" difficulty="intermediate"]
4.3 Chemical Properties of Carbon Compounds
Although carbon compounds are numerous, they show four basic types of reactions that you need to understand for Class 10: combustion, oxidation, addition, and substitution.
4.3.1 Combustion — Burning in Oxygen
All carbon compounds burn in the presence of oxygen, producing carbon dioxide, water, and a large amount of heat and light. This is why hydrocarbon fuels like LPG, petrol, diesel, kerosene and natural gas are used around the world.
\(\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{heat + light}\)
\(\text{C}_2\text{H}_5\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(g) + \text{heat}\)
Flame colour clue:
- Saturated hydrocarbons — burn with a clean, blue flame in sufficient air (complete combustion).
- Unsaturated hydrocarbons — burn with a yellow, sooty flame because their higher carbon content cannot combust fully.
- When the air supply is limited, even saturated compounds burn incompletely, giving toxic carbon monoxide (CO) and black smoke. That is why LPG burners are kept clean and mustn't burn yellow!
4.3.2 Oxidation
Combustion is the most dramatic kind of oxidation, but mild oxidising agents can also oxidise carbon compounds step by step. An important example: an alcohol is converted into a carboxylic acid by oxidising agents such as alkaline KMnO4 (potassium permanganate) or acidified K2Cr2O7 (potassium dichromate).
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\text{heat}]{\text{alk. KMnO}_4 \text{ or acidified K}_2\text{Cr}_2\text{O}_7} \text{CH}_3\text{COOH}\)
Oxidising agents supply oxygen to the alcohol, adding two oxygen atoms and removing two hydrogens to form the –COOH group. The product smells like vinegar — that's ethanoic acid!
4.3.3 Addition Reactions — Hydrogenation
Unsaturated compounds (with C=C or C≡C) add hydrogen across the multiple bond in the presence of a catalyst (nickel or platinum), becoming saturated.
\(\text{R–CH=CH–R'} + \text{H}_2 \xrightarrow{\text{Ni}} \text{R–CH}_2\text{–CH}_2\text{–R'}\)
\(\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{–CH}_3\)
Industrial use — hydrogenation of oils: Vegetable oils (rich in C=C) are treated with H2 in the presence of nickel catalyst to produce vanaspati ghee — a solid, saturated fat. Saturated fats, however, raise bad cholesterol, so unsaturated vegetable oils are healthier for consumption!
4.3.4 Substitution Reactions
Saturated hydrocarbons (alkanes) are relatively unreactive. In the presence of sunlight (hν), they undergo substitution — a hydrogen is replaced, one at a time, by another atom (e.g., chlorine).
\(\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{sunlight}} \text{CH}_3\text{Cl} + \text{HCl}\)
Further substitution can replace more hydrogens, giving CH2Cl2, CHCl3 (chloroform), and CCl4.
4.4 Some Important Carbon Compounds
4.4.1 Ethanol (C₂H₅OH)
Ethanol is a colourless, volatile liquid with a characteristic smell and burning taste. It is completely miscible with water and has a boiling point of 78 °C. It is the alcohol present in alcoholic beverages. It is widely used as a solvent (in tinctures of iodine, cough syrups), as an antiseptic, and as a fuel component (ethanol-blended petrol, E10).
Key reactions of ethanol
(a) Reaction with sodium metal: Ethanol behaves like a weak acid — it releases hydrogen gas on reaction with reactive metals such as sodium, producing sodium ethoxide.
\(2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2(g)\uparrow\)
(b) Dehydration — making ethene: When ethanol is heated at 443 K with excess concentrated sulphuric acid (a dehydrating agent), it loses a water molecule and forms ethene.
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[443\text{ K}]{\text{conc. H}_2\text{SO}_4} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}\)
Harmful effects of alcohol: Consumption of ethanol in moderate amounts produces a feeling of intoxication and impairs judgment (reaction time, coordination) — a major cause of road accidents. Long-term drinking damages the liver (cirrhosis), leads to addiction, and slows the nervous system.
Denatured alcohol: Ethanol is often used industrially for making medicines, cosmetics, perfumes, etc. To prevent its misuse as a drink (and avoid heavy excise duty on drinking alcohol), small amounts of poisonous substances such as methanol, pyridine, or copper sulphate are added. Such alcohol is called denatured alcohol.
4.4.2 Ethanoic Acid (CH₃COOH)
Ethanoic acid — commonly called acetic acid — is familiar to everyone as the sour-tasting component of vinegar. Commercial vinegar is a 5–8% aqueous solution of ethanoic acid and is used widely as a food preservative. Pure ethanoic acid freezes just below room temperature (melting point 16.6 °C); in cold winter weather the frozen form has an ice-like appearance, which is why it is called glacial acetic acid.
Key reactions of ethanoic acid
(a) With a base (NaOH) — neutralisation:
\(\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}\)
Sodium acetate (CH3COONa) is the salt formed.
(b) With a carbonate/bicarbonate — evolves CO₂ (effervescence):
\(2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{CO}_2(g)\uparrow + \text{H}_2\text{O}\)
\(\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{CO}_2(g)\uparrow + \text{H}_2\text{O}\)
(c) Esterification — sweet-smelling products: Acids react with alcohols in the presence of a few drops of concentrated H2SO4 to give sweet-smelling compounds called esters.
\(\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightleftharpoons[]{\text{conc. H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}\)
The ester CH3COOC2H5 is ethyl ethanoate (ethyl acetate) — it smells fruity and is used in perfumes and as a flavouring.
(d) Saponification — the reverse: An ester, when heated with NaOH, breaks back into the sodium salt of the acid and the alcohol. This reaction is used to make soaps.
\(\text{CH}_3\text{COOC}_2\text{H}_5 + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{C}_2\text{H}_5\text{OH}\)
Activity 4.3 — Make a Fruity Ester in the LabL3 Apply
Predict: What will you observe when a small amount of ethanol is mixed with ethanoic acid and a drop of concentrated H₂SO₄, then warmed gently?
- Take 1 mL ethanol (absolute alcohol) and 1 mL glacial ethanoic acid in a test tube.
- Add a few drops of concentrated H2SO4 carefully.
- Warm the tube in a water bath (not an open flame) for about 5 minutes.
- Pour the contents into a beaker of water and smell the liquid carefully (waft the vapour towards your nose).
A sweet, fruity smell is observed — this is the ester ethyl ethanoate (CH3COOC2H5). The concentrated H2SO4 acts as a dehydrating agent and catalyst. Esters like this are used in perfumes and artificial fruit essences.
4.4.3 Soaps and Detergents
A soap is the sodium (or potassium) salt of a long-chain fatty acid — for example, sodium stearate, C17H35COONa. Each soap molecule has two ends with very different personalities.
Cleansing action — formation of a micelle
- When a soapy solution meets a greasy cloth, the long hydrocarbon tails bury themselves into the oil/grease, while the ionic heads stay pointing out into the water.
- As more soap molecules attach, they trap a droplet of oil/grease inside a little ball — the micelle.
- Rinsing with water washes away the micelles — and the dirt trapped inside them — leaving the cloth clean.
Hard water problem — scum
Hard water contains dissolved Ca2+ and Mg2+ ions (usually as bicarbonates, sulphates, chlorides). When soap is added to hard water, these ions react with the soap to form an insoluble curdy white mass called scum — for example, calcium stearate.
\(2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow + 2\text{Na}^+\)
Scum sticks to clothes and is wasteful — a lot of soap must be used before proper lather is formed in hard water.
Detergents — the synthetic alternative
Detergents are synthetic cleansers, typically sodium salts of long-chain benzene sulphonic acids or sulphates. Their calcium and magnesium salts are soluble in water, so they do NOT form scum — making them effective even in hard water. Most shampoos, washing powders and liquid cleaners are detergents.
Concern: some older detergents had branched chains that are not biodegradable — they persisted in rivers and caused foam/pollution. Modern detergents are mostly biodegradable (linear chains).
Worked Examples
Example 1. Why does a candle flame appear yellow, but the flame of a gas stove appear blue?
Solution: A candle (paraffin wax, long-chain hydrocarbons) undergoes incomplete combustion because the air supply at the wick is limited — unburned carbon particles glow, giving a yellow, sooty flame. A gas stove supplies LPG (propane/butane) along with a controlled excess of air, producing complete combustion and a clean blue flame.
Example 2. Convert ethanol to ethanoic acid. Write the balanced equation.
Solution: Oxidation with alkaline KMnO4 (or acidified K2Cr2O7) on heating:
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\Delta]{\text{alk. KMnO}_4} \text{CH}_3\text{COOH}\)
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\Delta]{\text{alk. KMnO}_4} \text{CH}_3\text{COOH}\)
Example 3. Write the product when ethanoic acid reacts with (i) NaOH (ii) Na₂CO₃ (iii) ethanol (with conc. H₂SO₄).
Solution:
(i) CH3COOH + NaOH → CH3COONa + H2O (sodium acetate)
(ii) 2CH3COOH + Na2CO3 → 2CH3COONa + CO2↑ + H2O (effervescence)
(iii) CH3COOH + C2H5OH →(conc. H2SO4) CH3COOC2H5 + H2O (ester — fruity smell)
(ii) 2CH3COOH + Na2CO3 → 2CH3COONa + CO2↑ + H2O (effervescence)
(iii) CH3COOH + C2H5OH →(conc. H2SO4) CH3COOC2H5 + H2O (ester — fruity smell)
Example 4. Why is the conversion of vegetable oil to vanaspati called hydrogenation? What kind of reaction is it?
Solution: Vegetable oils contain many C=C double bonds (unsaturated). Passing H2 gas over the oil in the presence of nickel catalyst adds H atoms across the double bonds, converting them to single C–C bonds. This is an addition reaction; since H2 is added, it is specifically called hydrogenation. The saturated product is solid at room temperature — vanaspati.
Example 5. Distinguish between soaps and detergents (any three points).
Solution:
| Soap | Detergent |
|---|---|
| Sodium salt of long-chain fatty acid (–COONa) | Sodium salt of long-chain benzene sulphonic acid (–SO3Na) |
| Forms scum in hard water — wastes soap | Does not form scum; works in hard water |
| Biodegradable — eco-friendly | Some are non-biodegradable (pollute water) |
Example 6. Explain why a mixture of ethanoic acid and ethanol in the presence of conc. H₂SO₄ gives a sweet smell.
Solution: The conc. H2SO4 catalyses an esterification reaction between the –COOH of the acid and the –OH of the alcohol, producing the ester ethyl ethanoate (CH3COOC2H5) and water. Esters are known for their characteristic sweet/fruity smells.
Interactive: Reaction Predictor L3 Apply
Pick a carbon compound and a reagent — predict the main product!
Choose compound + reagent, then click Predict.
Competency-Based Questions
Pooja notices that when her mother uses the same amount of soap in the village well-water (hard water) and the filtered tap water (soft water), far less lather forms in the well-water, and a grey curdy layer appears on her clothes.
Q1. L1 Remember Which ions in hard water react with soap to form scum?
Answer: B. Ca²⁺ and Mg²⁺ ions precipitate soap molecules as insoluble calcium/magnesium stearate — the curdy scum.
Q2. L2 Understand Explain the difference between combustion of a saturated and an unsaturated hydrocarbon in terms of flame colour and reason. (2 marks)
Answer: Saturated hydrocarbons burn with a clean blue flame in excess air (complete combustion → CO2 + H2O). Unsaturated hydrocarbons have higher carbon content and burn with a yellow, sooty flame because the oxygen is insufficient to burn all the carbon — unburned carbon particles glow yellow and form soot.
Q3. L3 Apply Draw and label a diagram showing the arrangement of soap molecules around an oil droplet in water. (3 marks)
Answer: The diagram is a micelle — a sphere with the oil droplet at the centre. Each soap molecule points its long hydrocarbon tail inward into the oil, while the ionic head (–COO⁻Na⁺) points outward into the water. The water can then carry the entire micelle (with oil trapped inside) away when rinsed.
Q4. L4 Analyse Why does ethanoic acid give brisk effervescence with sodium carbonate but ethanol does not? (3 marks)
Answer: Ethanoic acid is a weak acid (–COOH) and reacts with carbonates to release CO2: 2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O. Ethanol is not acidic enough to react with carbonates — it does not release H⁺ ions easily. Hence no effervescence. This property is used as a test to distinguish acids from alcohols.
Q5. L5 Evaluate Ravi claims that "detergents are better than soaps because they clean in hard water." Evaluate this claim, including any disadvantage of detergents. (3 marks)
Answer: Ravi is partly correct. Detergents DO clean better in hard water because their Ca²⁺/Mg²⁺ salts are soluble — no scum forms, so they lather well. However, older synthetic detergents (branched chains) are non-biodegradable and persist in rivers and lakes, causing foam and harming aquatic life. Soaps, being salts of natural fatty acids, are biodegradable and more eco-friendly. A balanced view: use biodegradable detergents for hard water and prefer soap where water is soft.
Assertion-Reason Questions
Assertion (A): Hydrogenation of vegetable oils produces vanaspati.
Reason (R): Addition of hydrogen across C=C bonds (in the presence of a Ni catalyst) converts unsaturated oil into saturated solid fat.
Answer: A. Both true; the saturation of C=C bonds by H2 raises the melting point, turning liquid oil into solid vanaspati ghee.
Assertion (A): Esterification is used to make perfumes.
Reason (R): Esters have a pleasant, fruity smell.
Answer: A. Both true and R correctly explains A — the fruity smell of esters is the very reason they are used in perfumes and flavourings.
Assertion (A): Soaps clean well in hard water.
Reason (R): Soaps are sodium salts of fatty acids.
Answer: D. A is false — soaps do NOT clean well in hard water because they form scum with Ca²⁺/Mg²⁺ ions. R is true — soaps are indeed sodium salts of long-chain fatty acids.
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