Energetics

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Question 23
Medium

The molar enthalpy of vaporisation (ΔHvap\Delta H_{\text{vap}}ΔHvap​) of a liquid is the enthalpy change when one mole of liquid is converted to vapour at the boiling point of the liquid.

A student does an experiment to determine ΔHvap\Delta H_{\text{vap}}ΔHvap​_for ethanol (C2H5OH\text{C}_2\text{H}_5\text{OH}C2​H5​OH).

The student:

  • places a beaker on a balance
  • pours some ethanol into the beaker
  • uses a 1.8 kW1.8\text{ kW}1.8 kW heater to raise the temperature of the ethanol to its boiling point (78 ∘C78\text{ }^{\circ}\text{C}78 ∘C)
  • records the mass of the beaker and hot ethanol
  • uses the 1.8 kW1.8\text{ kW}1.8 kW heater to boil the ethanol for 150 s150\text{ s}150 s
  • records the mass of the beaker and remaining ethanol.

The loss in mass is 315 g315\text{ g}315 g.

Calculate ΔHvap\Delta H_{\text{vap}}ΔHvap​_for ethanol.

(Molar mass of ethanol = 46.0 g mol−146.0\text{ g mol}^{-1}46.0 g mol−1)

[1 kW=1 kJ s−1][1\text{ kW} = 1\text{ kJ s}^{-1}][1 kW=1 kJ s−1]

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Energetics Questions

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