QUESTION 119 (5 marks)
A flask contains liquid ethanol and ethanol vapour at equilibrium at constant temperature. Some liquid remains throughout the experiment. The stopper is then removed and vapour disperses into the surrounding air.
(a) Write an equation for the physical equilibrium, including state symbols. [1 marks]
(b) Explain why the vapour concentration was constant before the stopper was removed. [2 marks]
(c) Explain why liquid ethanol is lost after the flask is opened. [2 marks]
Practice marking scheme
Answer
. Initially evaporation and condensation have equal, non-zero rates. Opening allows vapour to escape, so evaporation exceeds condensation.
Working
In the closed flask, particles leave the liquid and return from the vapour continuously. At equilibrium their rates are equal. After opening, vapour disperses and fewer vapour particles return to the liquid; the escaping vapour prevents the original closed-system equilibrium from being maintained.
Marking criteria
- Writes the reversible liquid–gas equation with correct states. [1 mark]
- States that evaporation and condensation continue. [1 mark]
- States that their rates are equal before opening. [1 mark]
- Identifies escape/dispersal of ethanol vapour from the open flask. [1 mark]
- Links reduced return/condensation to a net loss of liquid. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.