Equilibrium in a pure substance

In summary: The pressure is always constant at the top of the water due to the weights and surface area. Vapor pressure is not relevant in this case since there is no vapor.
  • #1
mech-eng
828
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upload_2017-4-24_11-0-24.png


The book says when the water is heated, the pressure stays constant. But I do not understand pressure for a liquid-water system. What is the pressure for a liquid here? the pressure on the surface, botton or sides?

Source: Fundamentals of Thermodynamics by Sonntag/Borgnakke/Van Wylen.

Thank you.
 
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  • #2
The pressure at the top surface of the water is determined by the weights and the area. The pressure at other points depends on the surface pressure AND the density and depth at that point.
 
  • #3
In this case the pressure everywhere stays constant because as the water expands the density reduces. I believe these two things cancel out.

The pressure is primarily determined by the weights and area.
 
  • #4
I especially ask about vapor-pressure curve. On my copy of book, I cannot find it, I can its picture later. In that curve perpendicular axis is pressure and horizontal axis is temperature and there is parabolic curve with positive slope called vapor-pressure curve again. I cannot understand what it represents. Can it be related to pic a, at the top, only compressed water case?

Thank you.
 
  • #5
This is the diagram.

upload_2017-4-24_17-30-47.jpeg


It seems strange to me because for picture a at the top, compressed liquid case, there is no vapor so there should be no vapor pressure but there is a temperature value. Then should the graph start from somewhere on temperature axis, at which vapor pressure is zero, then going horizantal to saturation temperature then making the curve. But I am confused that I cannot surely determine if the pressure should only belong to vapor?

Thank you.
 
  • #6
mech-eng said:
It seems strange to me because for picture a at the top, compressed liquid case, there is no vapor so there should be no vapor pressure but there is a temperature value.
Vapor pressure refers to the pressure you would get in the gas phase in a closed system with room for the gas. But it may not be the pressure of the liquid. If you have a glass of water (between 0 and 100 °C), then the water is at 1 atm pressure, whatever the value of the vapor pressure.

The process in the OP is at constant pressure, so it corresponds to a horizontal line on the graph above. Point (a) is to the left of the vapor-pressure curve, point (b) is on the curve, and point (c) is to the right of the curve.
 
  • #7
DrClaude said:
The process in the OP is at constant pressure, so it corresponds to a horizontal line on the graph above. Point (a) is to the left of the vapor-pressure curve, point (b) is on the curve, and point (c) is to the right of the curve

I.e pressure is constant while temperature is always rising? And I do not understand why point c should be to the right of the curve instead of being on it?

Thank you.
 
  • #8
mech-eng said:
I.e pressure is constant while temperature is always rising? And I do not understand why point c should be to the right of the curve instead of being on it?
If the external temperature is exactly equal to the phase transition temperature, then nothing will happen when the liquid reached that temperature. To be converted into a gas, additional energy is required, which is usually achieved by having an external temperature above the phase transition temperature. As a result, when the phase transition is completed, the vapor will most often be heated.

In theory, figs. 2.1 (a)-(c) could all be exactly at the transition temperature, but in practice this is never the case.
 
  • #9
DrClaude said:
If the external temperature is exactly equal to the phase transition temperature, then nothing will happen when the liquid reached that temperature. To be converted into a gas, additional energy is required, which is usually achieved by having an external temperature above the phase transition temperature. As a result, when the phase transition is completed, the vapor will most often be heated.

In theory, figs. 2.1 (a)-(c) could all be exactly at the transition temperature, but in practice this is never the case.

I understood from your last post. It is the curve denoting the saturation temperatures corresponding to saturation pressures. I thought it was about vapor pressures, saturated or superheated, corresponding to temperatures. But it is not.I think the authors names it incorrectly. It is name is confusing.

Thank you.
 
  • #10
mech-eng said:
I think the authors names it incorrectly. It is name is confusing.
I don't think there is anything named incorrectly. Look up the definitions for the different names and see if you can try and sort them out.
 
  • #11
This diagram from another source, Çengel/Boles, is quite different from the belonging to Sonntag/Borgnakke.

upload_2017-4-24_21-38-46.png


Thank you.
 
  • #12
mech-eng said:
This diagram from another source, Çengel/Boles, is quite different from the belonging to Sonntag/Borgnakke.
I don't see any significant difference (especially keeping in mind that the first one is schematic).
 

Question 1: What is equilibrium in a pure substance?

Equilibrium in a pure substance refers to a state where the rate of the forward reaction is equal to the rate of the reverse reaction, resulting in no net change in the concentration of the substance.

Question 2: How is equilibrium achieved in a pure substance?

Equilibrium is achieved in a pure substance when the rate of the forward reaction is equal to the rate of the reverse reaction. This can be achieved by altering the temperature, pressure, or concentration of the reactants and products.

Question 3: What factors affect equilibrium in a pure substance?

The factors that affect equilibrium in a pure substance include temperature, pressure, concentration, and the presence of a catalyst. These factors can alter the rates of the forward and reverse reactions, ultimately affecting the equilibrium state of the substance.

Question 4: How does Le Chatelier's principle apply to equilibrium in a pure substance?

Le Chatelier's principle states that when a stress is applied to a system in equilibrium, the system will shift in a way that minimizes the stress. In the case of a pure substance, this means that if the concentration, temperature, or pressure is changed, the equilibrium will shift to reduce the effect of the change.

Question 5: Can equilibrium be disturbed in a pure substance?

Yes, equilibrium in a pure substance can be disturbed by changing the conditions of the reaction. For example, increasing the concentration of one of the reactants will shift the equilibrium to the right, favoring the formation of products. Similarly, decreasing the temperature will shift the equilibrium to the left, favoring the formation of reactants.

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