Consider the fraction #1 collected in the experiment you have performed. What would happen if it were reheated?

Question

BlackTom AI · Accepted Answer

Raoult’s law and vapor–liquid equilibrium govern what happens on reheating a distillate: the partial vapor pressure of each component equals its liquid mole fraction times its pure-component vapor pressure (Pi = xi·Pi°), and the vapor mole fraction is yi = (xi·Pi°)/Ptotal. For an ethanol–water mix, ethanol has the higher pure vapor pressure (lower boiling point), so the vapor is preferentially enriched in ethanol compared with the liquid. Step-by-step reasoning: 1) Let xE be the mole fraction of ethanol in fraction #1 (the collected liquid). When reheated, the vapor composition yE above that liquid is yE = (xE·PE°) / (xE·PE° + xW·PW°). Because PE° > PW°, for any 0 < xE < 1 (except pure ethanol), yE > xE — the vapor is richer in ethanol than the liquid. 2) Thus the immediate, observable result of reheating fraction #1 is vapour with a higher percent ethanol than the liquid you reheated. Collecting and condensing that vapor yields a more ethanol-rich distillate. 3) Repeating this process (fractional distillation) further enriches ethanol in successive condensates, approaching but not exceeding the ethanol–water azeotrope composition (~95.6% ethanol by volume). The azeotrope is a constant-boiling mixture that prevents obtaining pure ethanol by simple distillation alone. Why the correct option (d) is correct: it directly describes the immediate thermodynamic outcome — reheating produces vapor with a higher % ethanol because ethanol is the more volatile component. Why the other options are incorrect: - a (boil at a higher temperature): reheating does not necessarily raise the boiling point; boiling point depends on composition, and the vapor will be relatively lower-boiling (ethanol-enriched). - b (repeating will increase % ethanol to 95.6% but not higher): this statement about the azeotrope is true in context of many repetitions, but it does not address the immediate effect of reheating fraction #1; it is not the best direct answer here. - c (pure ethanol could be obtained): false by simple distillation because of the azeotrope; pure ethanol cannot be obtained without special methods (e.g., drying with molecular sieves or azeotropic distillation). Conclusion: Reheating fraction #1 will produce vapour with higher % ethanol (option d), consistent with Raoult’s law and vapor–liquid equilibrium.

Consider the fraction #1 collected in the experiment you have performed. What would happen if it were reheated?单项选择题

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