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4.5Organic Chemistry

Alcohols — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 11 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Alcohols: Ethanol Production, Properties and Reactions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the structure and uses of alcohols and compare the manufacture of ethanol by fermentation and by hydration of ethene.Alcohols have the functional group –OH (hydroxyl) and the …

  • Combustion
  • Dehydration
  • Reaction with sodium
  • Worked example 1
  • Worked example 2
  • Worked example 3

Section 1 of 8The alcohol homologous series

Alcohols have the functional group –OH (hydroxyl) and the general formula CnH2n+1OH.

-ol
Names end in -ol: methanol CH3OH, ethanol C2H5OH, propan-1-ol C3H7OH, butan-1-ol C4H9OH.
The shorter alcohols are colourless liquids that mix with water because the –OH group can interact with water molecules; solubility falls as the carbon chain gets longer.
Boiling points increase with chain length and are higher than those of alkanes of similar size because of attractions between the –OH groups.
Propanol has two isomers, propan-1-ol and propan-2-ol, depending on the position of the –OH.

Section 2 of 8Reactions of ethanol

Combustion
C2H5OH + 3O2 → 2CO2 + 3H2O. Ethanol burns with a clean blue flame and is used as a biofuel and in spirit burners.
Oxidation
Oxidation heated with acidified potassium dichromate(VI) or by microbes in air, ethanol is oxidised to ethanoic acid (CH3COOH). The orange dichromate turns green. This is why wine goes sour (vinegar).
Dehydration
Dehydration passing ethanol vapour over hot aluminium oxide removes water and forms ethene: C2H5OH → C2H4 + H2O.
Reaction with sodium
Reaction with sodium gives hydrogen and sodium ethoxide, slower than the reaction of sodium with water.

Section 3 of 8Making ethanol: fermentation versus hydration

1Fermentation
Fermentation glucose from sugar cane or crops is mixed with yeast, kept at about 30 °C without oxygen (anaerobic) in a warm vessel.
  • C6H12O6 → 2C2H5OH + 2CO2.
  • The yeast enzymes act as catalysts.
  • Fractional distillation concentrates the alcohol.
2Hydration of ethene
Hydration of ethene ethene from cracking reacts with steam at 300 °C and 60 atm over a phosphoric acid catalyst: C2H4 + H2O → C2H5OH.
FactorFermentationHydration
Raw materialRenewable (plants)Non-renewable (crude oil)
RateSlowFast
Process typeBatchContinuous
EnergyLow temperatureHigh temperature and pressure
ProductImpure, dilutePure

Section 4 of 8Uses of ethanol

Ethanol is a solvent (perfumes, paints), a fuel (mixed with petrol as gasohol), a feedstock for chemicals and an ingredient of alcoholic drinks. Biofuels are close to carbon neutral since the crop absorbs the carbon dioxide released, but growing crops competes with food production and uses land and energy.

Section 5 of 8Worked questions: yield and distillation

Worked yield question

  • 90 g of glucose (Mr 180) ferments to ethanol.
  • Moles of glucose = 0.50, so 1.0 mol of ethanol forms, a mass of 46 g, and 0.50 mol (22 g) of carbon dioxide.
  • If 33.5 g of ethanol is isolated the percentage yield = 33.5 ÷ 46 × 100 = 73%.

Distillationthe fermentation mixture contains about 10% ethanol.

Worked questions: yield and distillation
Fractional distillation separates ethanol (boiling point 78 °C) from water (100 °C) because the ethanol vaporises first and condenses in the column; a thermometer at the top of the column reading 78 °C shows that pure ethanol is being collected.
The apparatus uses a round-bottomed flask, fractionating column, thermometer, condenser with cold water entering at the bottom and leaving at the top, and a receiving flask.
Heat with an electric heater or water bath because ethanol is flammable.

Comparing alcohols

  • methanol is toxic and used as a solvent and fuel;
    • ethanol is less toxic;
    • the heat released per mole on burning rises with chain length (methanol about −726 kJ/mol, ethanol −1367, propanol −2021, butanol −2676).

Worked questions: yield and distillation
In a calorimetry experiment the same trend is seen, with experimental values lower owing to heat loss.
A small molecule such as ethanol also dissolves many organic substances, making it useful in perfumes and medicines.

Section 6 of 8Worked examples

  1. Write the equation for fermentation. [2]2 marks

  2. State two conditions needed for fermentation. [2]2 marks

  3. Compare the hydration of ethene with fermentation. [3]3 marks

  4. What colour change shows ethanol being oxidised by acidified dichromate(VI)? [1]1 mark

Section 7 of 8Key terms

TermMeaning
AlcoholCompound with the –OH group
FermentationYeast converts glucose to ethanol and CO2
HydrationAddition of water to an alkene
AnaerobicWithout oxygen
BiofuelFuel made from biological material
OxidationConversion to ethanoic acid by oxidising agents

Section 8 of 8Exam technique

1
Using oxygen in fermentation.
2
Saying hydration happens with liquid water.
3
Forgetting that dilute ethanol needs distillation.
4
Writing the formula of ethanol as C2H4OH.

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