Experimental Techniques and Chemical Analysis Structured Questions | Cambridge O Level Chemistry

Chemistry 5070 · Question 1[13 marks]

A mixture of ethanol ( $\ce{CH3CH2OH}$ , bp = 78 °C) and water ( $\ce{H2O}$ , bp = 100 °C) produced by yeast fermentation is purified by fractional distillation.

(a)

Explain how the fractionating column containing glass beads enables efficient separation of ethanol and water, despite both liquids being miscible and having relatively close boiling points.

[3]

(b)

Which substance distils over first as Fraction 1? Explain your answer in terms of boiling points and intermolecular forces.

[3]

(c)

Refer to the temperature versus time graph in the diagram:

[3]

(i)

Explain why the temperature remains constant at 78 °C during the collection of Fraction 1.

[1]

(ii)

Explain why the temperature rises rapidly from 78 °C to 100 °C before stabilizing at a second plateau.

[2]

(d)

Ethanol is a highly flammable liquid. Explain why a Bunsen burner must NOT be used to heat the distillation flask directly, and suggest a safer heating method.

[2]

(e)

If the fermented mixture also contained an insoluble, immiscible liquid flavouring oil of density 0.85 g cm⁻³, describe how this oil layer could be separated prior to distillation.

[2]