Une certaine quantité d'un liquide se trouve, en équilibre...

Une certaine quantité d'un liquide se trouve, en équilibre avec sa vapeur à une température T, dans une enceinte dont on peut modifier le volume et la température. 1) Que se passe t-il si : a) On augmente le volume. b) On diminue le volume. c) On abaisse la température. d) On élève la température. e) On introduit de la vapeur, à la même température. f) On introduit du liquide, à la même température g) On introduit du diazote, à la même température. 2) Pourrait-on prévoir les évolutions en utilisant le principe de Le Chatelier ?

Une certaine quantité d'un liquide se trouve, en équilibre avec sa vapeur à une température T, dans une enceinte dont on peut modifier le volume et la température.
1) Que se passe t-il si :
a) On augmente le volume.
b) On diminue le volume.
c) On abaisse la température.
d) On élève la température.
e) On introduit de la vapeur, à la même température.
f) On introduit du liquide, à la même température
g) On introduit du diazote, à la même température.
2) Pourrait-on prévoir les évolutions en utilisant le principe de Le Chatelier ?

Key Concepts

Effect of Volume (Pressure) Changes on Equilibrium

Modifying the volume of the system affects the pressure exerted by the vapor. An increase in volume generally leads to a decrease in pressure, while a decrease in volume results in a pressure increase. The system responds by shifting the equilibrium position to counteract these changes, often resulting in phase changes between liquid and vapor.

Phase Equilibrium

This concept deals with the coexistence of two phases (such as liquid and vapor) in a stable state where the rate of evaporation equals the rate of condensation. Understanding phase equilibrium is crucial since it explains how changes in external conditions can shift the balance between phases.

Vapor Pressure and Temperature Relationship

Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid at a given temperature. It is highly temperature-dependent; as temperature increases, vapor pressure increases, and as temperature decreases, vapor pressure decreases, directly affecting the phase distribution.

Addition of Components to the System

Introducing additional substances, whether it be more vapor, liquid, or an inert gas, can disturb the equilibrium through changes in concentration or partial pressures. The way the system responds depends on whether the added component is active in the equilibrium or acts as an inert species that only changes total pressure.

Le Chatelier's Principle

This principle predicts how a system at equilibrium will respond to an external change such as variations in temperature, pressure, or concentration. It is used to qualitatively determine the direction of the shift in equilibrium when the system is subjected to disturbances, making it a fundamental tool in understanding adjustments in phase equilibria.

Transcript

00:01 For this question, we are going to consider the three isothermal processes.

00:04 So one, subscript 1 represents initial, subscript 2 represents final.

00:09 So p1 v1 equals p2 v2, and where p1 is pressure, or p is pressure, v is volume.

00:16 And v1 over v2 is equal to temperature initial divided by temperature final, t1 over t2, which is equal to p1 over p2.

00:23 So for part a here, we're going to do isothermal processes.

00:30 And for the first one, we're going to do where the final state, p2, is 10 % more than the initial state.

00:38 So it's equal to 1 .10 p1.

00:41 Well, if that's true, then v2, which is equal to p1 v1 over p2.

01:01 And replacing p2 with 1 .10p1, we find that v2 is equal to 0 .91v1.

01:07 So we can say to achieve this process, what's something that we can do? let's type it out.

01:23 We can say to achieve one, we can place the piston in a water bath to maintain constant temperature, place some weight on the piston or some force to compress the volume.

01:54 So the volume two, the final say volume is 0 .91 times volume one.

02:00 So it's compressed.

02:01 Okay, so now let's do the next one.

02:05 So the final state volume we're told here, let's do v3 now is equal to 0 .8v2.

02:23 So the final temperature here, t3, which is equal to v3 over v2, t2, based upon the initial equations that we had.

02:43 So now replace 0 .8v2 for v3.

02:47 So you see that t3 is equal to 0 .8t2.

02:58 So what can we do to achieve this? we can say to achieve slowly pull the water, the temperature t3.

03:23 So the frictionless has to be frictionless so we don't lose heat or get heat from the friction.

03:34 It's freely until b3 equals 0 .8 b2.

03:44 Okay.

03:45 And the next situation that we're going to consider here now is where t4, so that the next part is equal to 1 .15 t3.

04:15 So it stands to reason then that p4, which is equal to t4 over t3, p3, is equal to 1 .15p3.

04:52 Okay, now let's describe what's happening to achieve this.

04:58 We are increasing the mass of the piston, or on the piston, until the required pressure is obtained.

05:10 So now for part b, we're supposed to represent all the processes in p versus t, v versus t, and p versus v versus v for the graphs...

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