Ethanol (C₂H₅OH) and Ethanoic Acid (CH₃COOH) – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain why ethanol is completely miscible with water and dries quickly when used as a sanitizer. Support your answer with everyday examples.
Answer:
Ethanol is completely miscible with water because its –OH group can form hydrogen bonds with water molecules. This strong attraction allows ethanol and water to mix in any proportion without forming layers. Ethanol also has a low boiling point (≈78°C) and is highly volatile, so it evaporates quickly at room temperature. During evaporation, it absorbs latent heat from your skin, causing a cooling effect. That is why sanitizers feel cold and dry rapidly. Everyday examples include: spirit lamps burning cleanly due to ethanol’s volatility, perfumes using ethanol as a solvent because it carries fragrance and evaporates fast, and open beakers of ethanol in the lab drying faster than water. These properties make ethanol ideal for sanitizers, perfumes, and quick-drying solvents.
Q2. Describe four key chemical reactions of ethanol and write their balanced equations with observations.
Answer:
Ethanol shows several important reactions:
- With sodium (Na): It forms sodium ethoxide and liberates hydrogen gas. Bubbles are seen.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑
- Combustion: In oxygen, ethanol burns with a blue flame to give CO₂ and H₂O, releasing energy.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Energy
- Dehydration: Hot conc. H₂SO₄ (dehydrating agent) converts ethanol to ethene (C₂H₄) and water; the gas decolorizes bromine water.
C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, Δ)
- Oxidation: Acidified K₂Cr₂O₇/KMnO₄ oxidizes ethanol to ethanoic acid. Dichromate turns orange to green.
C₂H₅OH + [O] → CH₃COOH + H₂O
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Q3. Ethanoic acid is called a weak acid. Explain this term and discuss its reactions with metals, carbonates/bicarbonates, and bases with equations.
Answer:
A weak acid like ethanoic acid partially ionizes in water, producing limited H⁺ ions, so its pH is around 3–4. The ionization is reversible:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Key reactions include:
- With active metals (Na, Zn): It gives hydrogen gas and acetate salt.
2CH₃COOH + 2Na → 2CH₃COONa + H₂↑
- With carbonates/bicarbonates: It produces CO₂, water, and acetate—seen as fizzing.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂↑ + H₂O
CH₃COOH + NaHCO₃ → CH₃COONa + CO₂↑ + H₂O
- With bases (neutralization): It forms salt and water.
CH₃COOH + NaOH → CH₃COONa + H₂O
These reactions confirm its acidic nature, explain the vinegar–baking soda fizz, and show how acids can be neutralized in labs and everyday applications.
Q4. What is esterification? Explain how ethyl ethanoate is prepared from ethanol and ethanoic acid, including conditions, observations, and uses.
Answer:
Esterification is the reaction between a carboxylic acid and an alcohol to form an ester and water, typically in the presence of conc. H₂SO₄ as a catalyst and dehydrating agent. For ethyl ethanoate:
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄, warm)
The ester formed, ethyl ethanoate, has a sweet, fruity smell (like pineapple). In the lab, mixing ethanol and ethanoic acid with a few drops of conc. H₂SO₄ and warming gently releases this pleasant fragrance. The reaction is reversible and can be driven forward by removing the water or using excess reactant. Esters are widely used in flavourings, perfumes, and solvents. This experiment illustrates concepts of functional groups, catalysis, and equilibrium in organic reactions.
Q5. Compare ethanol and ethanoic acid based on their properties and simple tests that distinguish them. Provide reasons for each difference.
Answer:
- Nature: Ethanol is an alcohol, while ethanoic acid is a carboxylic acid; this structural difference controls their behavior.
- Odor: Ethanol has a pleasant/fruity smell; ethanoic acid is pungent/vinegar-like due to the –COOH group.
- pH: Ethanol is neutral (~7); ethanoic acid is acidic (pH 3–4) because it releases H⁺.
- Litmus: Ethanol shows no effect; ethanoic acid turns blue litmus red.
- With NaOH: Ethanol shows no reaction; ethanoic acid undergoes neutralization to form sodium acetate and water.
CH₃COOH + NaOH → CH₃COONa + H₂O
- With Na metal: Both give H₂ gas, but ethanol forms sodium ethoxide (C₂H₅ONa) and ethanoic acid forms sodium acetate (CH₃COONa).
These tests exploit differences in acidity, functional groups, and reactivity.
High Complexity (Analytical & Scenario-Based)
Q6. In a lab, you receive two colorless liquids: ethanol and ethanoic acid. Design a sequence of tests to identify each, predict observations, and justify your choices.
Answer:
- Odor check (carefully waft): One smells pleasant (ethanol), the other pungent/vinegary (ethanoic acid).
- Blue litmus test: Ethanol shows no change; ethanoic acid turns it red due to H⁺ release.
- Sodium bicarbonate (NaHCO₃): Only ethanoic acid gives effervescence (CO₂).
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
- Sodium metal (small piece, safety first): Both release H₂ gas; identify salts—ethanol makes C₂H₅ONa, acid makes CH₃COONa.
- Neutralization with NaOH: Only ethanoic acid forms sodium acetate; ethanol shows no reaction.
- Ester test (optional): Mix each with the other plus conc. H₂SO₄, warm: production of fruity smell confirms both reagents are correct.
This sequence uses differences in acidity, gas evolution, and salt formation to distinguish the two.
Q7. Evaluate the benefits and limitations of using ethanol as a fuel, especially as ethanol-blended petrol (E10). Use chemical reasoning to support your points.
Answer:
- Benefits:
- Ethanol burns cleanly to CO₂ and H₂O, giving a blue flame and fewer particulates.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Energy
- As an oxygenated fuel, it promotes more complete combustion, potentially reducing CO and unburnt hydrocarbons.
- E10 (10% ethanol) can reduce reliance on pure petrol and lower net emissions when ethanol is bio-derived.
- Limitations:
- Lower calorific value than petrol means slightly lower mileage.
- Ethanol is hygroscopic and miscible with water, which can cause phase separation and corrosion in fuel systems not designed for it.
- High ethanol blends may require engine modifications.
Overall, ethanol is a cleaner-burning, partially renewable fuel, but blending levels must consider engine compatibility and energy content.
Q8. Alcoholic drinks sometimes turn sour on standing. Explain this change using the oxidation of ethanol, reagents involved in the lab, and observable changes.
Answer:
When exposed to air and certain microbes, ethanol undergoes oxidation to ethanoic acid, making the liquid sour. In the lab, this chemical change is modeled using acidified oxidizing agents:
C₂H₅OH + [O] → CH₃COOH + H₂O
Common oxidants include acidified K₂Cr₂O₇ (orange to green as Cr⁶⁺ → Cr³⁺) and KMnO₄ (purple to colorless/brown). The process explains why wines or fermented beverages can sour on prolonged exposure, forming acetic acid (vinegar). Factors that speed this change include oxygen, warmth, and bacterial action. In a controlled lab, students observe a color change in the oxidizing solution, detect a pungent smell, and can confirm acidity with litmus/pH. Thus, everyday souring is a real-world example of alcohol → carboxylic acid oxidation.
Q9. You are asked to verify if an unknown liquid is acetic acid using sodium bicarbonate and a balloon setup. Describe the procedure, safety, expected results, and the chemical basis.
Answer:
- Procedure: Add ~2 mL of the unknown liquid to a test tube. Fit a balloon on the mouth. Use a funnel to add a pinch of NaHCO₃ under the balloon. Let the solid fall in and react.
- Safety: Wear goggles, avoid skin contact, and do not inhale vapors. Use small quantities to control effervescence.
- Observation: If the liquid is acetic acid, rapid effervescence occurs and the balloon inflates with CO₂.
- Chemistry:
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
The evolution of CO₂ confirms an acid–bicarbonate reaction, characteristic of carboxylic acids. As a control, repeat with distilled water (no gas) and with a known vinegar sample (gas forms). This setup demonstrates gas identification by effect and a classic qualitative analysis test.
Q10. A student heats ethanol with concentrated H₂SO₄ and obtains a gas that decolorizes bromine water. Explain the reaction pathway, name the gas, write the equation, and discuss the role of the acid and precautions.
Answer:
Heating ethanol with conc. H₂SO₄ at about 170°C causes dehydration to an alkene. The gas formed is ethene (C₂H₄), which decolorizes bromine water due to addition across the C=C bond.
C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, Δ)
Role of conc. H₂SO₄: It acts as a dehydrating agent (removes H₂O) and catalyst for the elimination reaction, converting the alcohol to an alkene. Precautions include using a dry apparatus, controlling temperature to avoid charring, venting the gas safely, and preventing backflow into hot acid. The test with bromine water (loss of brown color) confirms unsaturation. This experiment links functional group transformations (alcohol → alkene) and charact...
