Exam Style Question - 6

QuestionEthanol is oxidised by acidified potassium dichromate(VI). The boiling temperatures of the compounds involved are:• ethanol, CH₃CH₂OH: 78 °C• ethanal, CH₃CHO: 21 °C• ethanoic acid, CH₃COOH: 118 °C(a) Explain, in terms of intermolecular forces, why ethanal has the lowest boiling temperature.(b) Explain why ethanal can be collected by distillation but ethanoic acid is made by heating under reflux.(c) Write an equation, using [O], for the oxidation of ethanol to ethanoic acid.Answer(a) Ethanol and ethanoic acid molecules have an H atom bonded to an oxygen atom, so they form hydrogen bonds with each other. Ethanal has no H atom bonded to an oxygen atom, so its molecules cannot hydrogen bond with each other. Ethanal only has London forces and permanent dipole-dipole forces, which are weaker, so less energy is needed to separate the molecules.(b) Ethanal has the lowest boiling temperature, so it boils off first. If the vapour temperature is kept below the boiling temperature of ethanol, 78 °C, ethanal distils off as it forms and escapes further oxidation. Ethanoic acid boils at 118 °C, the highest of the three, so to make it the mixture is heated under reflux. The ethanal and ethanol vapours condense and return to the flask until they are fully oxidised.(c)CH₃CH₂OH + 2[O] → CH₃COOH + H₂OExaminer tip: say which molecules can or cannot form hydrogen bonds with each other. Ethanal can accept hydrogen bonds from water, but not from other ethanal molecules.