State the uses of ethanol as: (a) a solvent (b) a fuel

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Organic Chemistry – Alcohols

Alcohols at a glance (Cambridge IGCSE 0620)
  • General formula: CₙH₂ₙ₊₁OH (n = 1–4 for the syllabus).
  • Classification by the carbon bearing the –OH group
    • Primary (1°) alcohol: –CH₂–OH (e.g., methanol, ethanol, propan‑1‑ol, butan‑1‑ol)
    • Secondary (2°) alcohol: –CH(OH)– (e.g., propan‑2‑ol, butan‑2‑ol)
    • Tertiary (3°) alcohol: –C(OH)(CH₃)₂ (e.g., 2‑methyl‑2‑propanol)
  • Key syllabus requirements for ethanol
    • Name and draw the structure.
    • State that it is a primary alcohol.
    • Describe the two main industrial routes (fermentation and acid‑catalysed hydration).
    • Write the balanced combustion equation.
    • Explain its uses as a solvent and as a fuel.

Ethanol (C₂H₅OH)

Ethanol, also called ethyl alcohol, is the simplest primary alcohol. It is a colourless, volatile liquid.

Structural formula CH₃‑CH₂‑OH
Structural formula of ethanol showing the hydroxyl (‑OH) functional group.
Dot‑and‑cross diagram of ethanol
Dot‑and‑cross (Lewis) diagram of ethanol.

1. Production of Ethanol

Method Raw material Reaction Key features
Fermentation Sugars (glucose, sucrose, starch hydrolysates) C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
  • Biological route – yeast acts as catalyst.
  • Low temperature, low energy input.
  • By‑product CO₂ is used in the beverage industry.
  • Typical yield ≈ 90 % of theoretical.
Acid‑catalysed hydration of ethene Ethene (C₂H₄) + water C₂H₄ + H₂O → C₂H₅OH (H₂SO₄, 300 °C, 70 atm)
  • Petrochemical route – high temperature & pressure.
  • No carbon‑based by‑product (heat is the only waste).
  • Yield ≈ 95 % of theoretical.

2. Physical Properties

Property Value Reason
Molar mass 46.07 g mol⁻¹
Density (20 °C) 0.789 g cm⁻³ Less dense than water; only one –OH group balances the carbon chain.
Boiling point 78.4 °C Strong H‑bonding between molecules raises the boiling point above that of comparable hydrocarbons.
Melting point –114.1 °C Relatively weak intermolecular forces in the solid state; H‑bonds break easily on heating.
Solubility in water Fully miscible –OH group forms hydrogen bonds with water molecules.
Flammability Highly flammable; flash point ≈ 13 °C Combustible C–H bonds and high vapour pressure at room temperature.

3. Characteristic Chemical Reactions

  • Combustion (redox)

    C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O ΔH ≈ ‑1367 kJ mol⁻¹

  • Oxidation (controlled)
    • To acetaldehyde (ethanal): C₂H₅OH + [O] → CH₃CHO + H₂O
    • Further oxidation to ethanoic acid: CH₃CHO + [O] → CH₃COOH
  • Esterification with acetic acid

    C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O (ethyl acetate)

    Acid catalyst (H₂SO₄) required; removal of water drives the equilibrium to the right.

  • Dehydration (acid‑catalysed) – preparation of ethene

    C₂H₅OH → C₂H₄ + H₂O (concentrated H₂SO₄, 170 °C)

4. Sample Quantitative Problem (AO2)

Problem: If 46 g of ethanol are completely burnt, how many grams of carbon dioxide are produced? (Molar masses: C = 12.01, O = 16.00, H = 1.01 g mol⁻¹)

  1. Calculate moles of ethanol: \[ n_{\text{C₂H₅OH}}=\frac{46\ \text{g}}{46.07\ \text{g mol}^{-1}}=1.00\ \text{mol} \]
  2. From the balanced combustion equation, 1 mol ethanol → 2 mol CO₂.
  3. Moles of CO₂ produced: 2.00 mol.
  4. Mass of CO₂: \[ m_{\text{CO₂}}=2.00\ \text{mol}\times(12.01+2\times16.00)\ \text{g mol}^{-1}=88.0\ \text{g} \]

Answer: 88 g of CO₂ are formed.

5. Uses of Ethanol

(a) As a Solvent
  • Polar protic solvent – dissolves many ionic compounds and a wide range of organic molecules.
  • Extraction of plant pigments, alkaloids, essential oils and caffeine.
  • Reaction medium for esterifications, condensations and nucleophilic substitutions (e.g., SN1, SN2).
  • Preparation of standard solutions for UV‑Vis and IR spectroscopy (low absorbance above 200 nm).
  • Cleaning laboratory glassware – removes residues that water cannot dissolve and evaporates quickly.
  • Formulation of pharmaceuticals, cosmetics and personal‑care products where a volatile, low‑toxicity solvent is required.
(b) As a Fuel
  • Pure ethanol (E100) – burns cleanly to CO₂ and H₂O; energy content ≈ 30 MJ kg⁻¹ (≈ 90 % of gasoline).
  • Gasoline‑ethanol blends – e.g., E10 (10 % ethanol) and E85 (85 % ethanol) lower emissions of CO, unburnt hydrocarbons and particulates.
  • Used in flexible‑fuel vehicles (FFVs) that can operate on a range of ethanol‑gasoline mixtures.
  • Renewable – produced from biomass (sugarcane, corn, wheat). The CO₂ released on combustion is largely re‑absorbed by the next crop, giving a lower net carbon footprint when produced sustainably.

6. Environmental & Health Considerations

  • Safety: Highly flammable; vapour can travel to an ignition source. Use in a well‑ventilated area, keep away from open flames, store in sealed containers.
  • Toxicity: Low acute toxicity (LD₅₀ ≈ 7 g kg⁻¹ in rats) but causes CNS depression at high concentrations; prolonged exposure may irritate eyes and skin.
  • Environmental impact:
    • Bio‑ethanol reduces dependence on fossil fuels and can lower greenhouse‑gas emissions if the feedstock is sustainably managed.
    • Spills are readily biodegradable, but large releases can temporarily deplete dissolved‑oxygen levels in water bodies.

7. Links to Other Syllabus Areas (Cross‑Reference)

  • Redox – combustion and oxidation of ethanol illustrate electron transfer.
  • Energy – compare energy released per gram of ethanol, gasoline and diesel (useful for AO2 calculations).
  • Organic series – compare physical properties of methanol, ethanol, propan‑1‑ol and butan‑1‑ol (boiling point, miscibility).
  • Environmental chemistry – discuss carbon‑budget of bio‑fuels versus fossil fuels.
  • Acids & bases – ethanol is a very weak acid (pKₐ ≈ 16) and a very weak base (proton acceptor).

Key Points to Remember

  1. Ethanol is a primary alcohol (‑CH₂CH₃‑OH) and is fully miscible with water because of hydrogen bonding.
  2. Industrial production: (i) fermentation of sugars (biological route) and (ii) acid‑catalysed hydration of ethene (petrochemical route).
  3. Important reactions: combustion, oxidation to acetaldehyde/ethanoic acid, esterification with acids, and dehydration to ethene.
  4. As a solvent it dissolves a wide range of substances, evaporates quickly and is ideal for recrystallisation, extractions and cleaning.
  5. As a fuel it burns cleanly, can be blended with gasoline, and is renewable when derived from biomass.
  6. Safety: highly flammable and mildly toxic; handle in a fume cupboard, store in tightly sealed containers away from heat.

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