Liquid chloroform (CHCl3) is placed in a closed container and allowed to equilibrate with chloroform vapor. With the temperature held constant, additional chloroform is introduced into the system, and the vapor-liquid equilibrium is reestablished. Following this, the chloroform vapor pressure will:
a) be higher
b) be lower
c) be the same if both liquid and vapor phases are present
d) be lower if the heat of vaporization is positive
e) be higher if the heat of vaporization is positive
The correct answer is C but I don't quite get it why so? Could you please explain? Thank you
Yes, the answer is C. Any liquid placed in a closed system will come to equilibrium with its vapor at a particular temperature. The vapor pressure then is say x. Adding more liquid will NOT make the liquid evaporate more which would, of course, increase the vapor. The vapor pressure is a function of the temperature. Those molecules with sufficient energy will be able to jump out of the liquid and that depends upon T. Increasing T will give them more energy and increase the vapor pressure but that's the only way to increase p. You may say "but increasing volume will give it more room to evaporate" and that is very true, however, more molecules will enter the vapor phase but when the vapor pressure reaches equilibrium it will be the SAME it was at the smaller volume
Will the Le Chatelier's principle work here? Because pure liquid has no effect on the equilibrium position, it will not move towards products; thus, chloroform vapor will not form.
To understand why the answer is C, let's break down the process and the concept of vapor-liquid equilibrium.
Vapor-liquid equilibrium occurs when the rate of vaporization from the liquid phase equals the rate of condensation from the vapor phase. At this point, the system is at equilibrium, meaning there is no net change in the amount of liquid or vapor. The pressure exerted by the vapor is called the vapor pressure, and it is determined by the temperature.
In the given scenario, chloroform in a closed container is already at equilibrium. When additional chloroform is introduced into the system, it will cause some of it to vaporize to establish a new equilibrium. However, since the temperature is held constant, there will be no change in the vapor pressure.
Option C states that the vapor pressure will remain the same if both liquid and vapor phases are present. This is correct because the concept of vapor-liquid equilibrium means that the system is already in balance. The introduction of more chloroform simply maintains the equilibrium, without disturbing the pressure.
Options A, B, D, and E are incorrect:
- Option A is incorrect because the vapor pressure does not increase when more chloroform is added and the system reaches a new equilibrium.
- Option B is incorrect because the vapor pressure does not decrease when more chloroform is added.
- Option D is incorrect because the statement about the heat of vaporization (which is the energy required to convert a substance from the liquid phase to the vapor phase) does not affect the vapor pressure in this scenario.
- Option E is incorrect for the same reason as option D. The heat of vaporization's sign does not affect the vapor pressure in this case.
Therefore, the correct answer is C, as the vapor pressure remains the same when both liquid and vapor phases are present at equilibrium.