Alcohols Exam Questions

(a) The hydroxyl group, -OH [1]

(b) CnH2n+1OH [1]

(c) Ethanol, C2H5OH [1]

Examiner note: Do not confuse the hydroxyl group (-OH) in alcohols with the hydroxide ion (OH-). The -OH in alcohols is covalently bonded to a carbon atom, not ionic. The molecular formula can also be written as C2H6O, but C2H5OH is preferred because it shows the functional group.

• Glucose (from sugar cane, corn, or other plant material) is dissolved in water [1]
• Yeast is added to the glucose solution [1]
• The mixture is kept at 25-37 degrees C in anaerobic conditions (absence of air/oxygen) [1]
• The yeast produces enzymes that convert glucose to ethanol and carbon dioxide: C6H12O6 → 2C2H5OH + 2CO2 [1]

Examiner note: Anaerobic means without oxygen — if oxygen is present, the ethanol is further oxidised to ethanoic acid (vinegar). The temperature must not exceed approximately 37 degrees C or the yeast enzymes denature and the reaction stops. The mixture is then purified by fractional distillation to separate the ethanol from water.

Examiner note: This is one of the most commonly tested comparison questions in organic chemistry. Each row typically earns 1 mark per method. The key trade-off is that fermentation uses renewable resources but is slow and produces impure ethanol, while hydration is fast and continuous but relies on non-renewable crude oil. Give at least three clear points of comparison for full marks.

Equation: C2H5OH + 3O2 → 2CO2 + 3H2O [1]

Advantages (any two):

• Ethanol is a renewable fuel (can be made from crops by fermentation) [1]
• Burns cleanly / produces less soot than petrol / CO2 released is offset by CO2 absorbed by crops during growth (carbon neutral) [1]

Examiner note: Ethanol is not truly carbon neutral because energy (often from fossil fuels) is used to grow, harvest, and process the crops, and to distil the ethanol. However, for IGCSE purposes, the argument that crops absorb CO2 during photosynthesis is accepted as making ethanol approximately carbon neutral.

Any three from:

• As a solvent (in perfumes, varnishes, aftershaves) [1]
• As a fuel (in cars, spirit lamps, camping stoves) [1]
• In alcoholic drinks (beer, wine, spirits) [1]
• As a feedstock to produce other chemicals (ethanoic acid, esters) [1]

Examiner note: Ethanol dissolves many organic substances that water cannot, making it useful as a solvent. It evaporates quickly, which is why it is used in perfumes and aftershaves. These are factual recall questions worth easy marks.

(a) Acidified potassium dichromate(VI) / acidified K2Cr2O7 [1]

(b) The solution changes colour from orange to green [1]

(c) C2H5OH + 2[O] → CH3COOH + H2O [1]

(d) A carboxylic acid [1]

Examiner note: The orange-to-green colour change is due to the reduction of dichromate(VI) ions (Cr2O7 2-, orange) to chromium(III) ions (Cr3+, green). This is the same reagent used to test for alcohols. The oxidation first produces an aldehyde (ethanal), which is further oxidised to ethanoic acid, but at IGCSE the single-step equation is sufficient.

(a) The sodium fizzes/effervesces / bubbles of gas are produced / the sodium dissolves [1]

(b) Hydrogen [1]

(c) Hold a burning splint near the mouth of the test tube — hydrogen burns with a squeaky pop [1]

Examiner note: The reaction is: 2C2H5OH + 2Na → 2C2H5ONa + H2. Ethanol reacts more slowly with sodium than water does. The product, sodium ethoxide (C2H5ONa), dissolves in the excess ethanol. This reaction demonstrates that the -OH group in ethanol can react, but less vigorously than the -OH in water.

(a) Removal/elimination of water from a molecule [1]

(b) Heat with excess concentrated sulfuric acid (or pass ethanol vapour over hot aluminium oxide) [1]

(c) C2H5OH → C2H4 + H2O [1]

(d) Concentrated sulfuric acid (acts as a dehydrating agent/acid catalyst) or aluminium oxide (a heterogeneous catalyst) [1]

Examiner note: Dehydration is the reverse of hydration. Hydration adds water to ethene to form ethanol; dehydration removes water from ethanol to form ethene. This is also an elimination reaction. The product, ethene, can be tested with bromine water (it decolourises it).