Sketch a qualitatively accurate graph of the entropy of a substance

In summary, the graph shows the entropy of a substance plotted against temperature at a fixed pressure. The substance is solid at lower temperatures, liquid at intermediate temperatures, and gas at higher temperatures. The features of the graph can be explained by the equation for internal energy (dU = -P*dV + Q*dS), the relationship between volume and temperature (V = C*T), and the specific heat capacity (C). The solid region corresponds to a lower number of degrees of freedom (f = 3) and the gas region has a higher number of degrees of freedom (f > 5). The liquid and gas phases are qualitatively accurate, but the solid region may be a rough approximation.
  • #1
LCSphysicist
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Homework Statement
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Relevant Equations
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Sketch a qualitatively accurate graph of the entropy of a substance
as a function of temperature, at fixed pressure. Indicate where the
substance is solid, liquid, and gas . Explain each feature of the graph briefly.

What you think about?:

dU = -P*dV + Q*dS (1)
V = C*T => dV = C*dT
Nfk*dT/2 + P*C*dT = Q*dS

Nfk/2 + P*C = Q*(dS/dT)

For a solid, i assumed f = 3, for a liquid f = 5 and for the gas, f > 5 (i think is not necessary say the obvious this is a rough approximation)

1596418979115.png
That would be the result.
 
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  • #2
LCSphysicist said:
View attachment 267215That would be the result.

The liquid and gas phases look about right to me (not 100% sure about the solid region). Usually, I see plots of [itex] T [/itex] vs [itex] s [/itex] for liquids and vapours, so I don't want to comment on that (solid) region as I am not too knowledgeable.
 

1. What is entropy and why is it important in science?

Entropy is a measure of the disorder or randomness of a system. In science, it is important because it helps us understand the behavior and changes of substances and systems. It is a fundamental concept in thermodynamics and is used to predict the direction of chemical reactions and the stability of systems.

2. How do you sketch a qualitatively accurate graph of the entropy of a substance?

To sketch a qualitatively accurate graph of the entropy of a substance, you first need to determine the factors that affect the entropy of the substance. These factors include temperature, pressure, and the physical state of the substance. Then, you can plot these factors on the x-axis and the corresponding entropy values on the y-axis. The resulting graph should show a general trend of increasing or decreasing entropy with changes in these factors.

3. Can the entropy of a substance ever decrease?

Yes, the entropy of a substance can decrease under certain conditions. For example, when a substance undergoes a phase change from a gas to a liquid, its entropy decreases because the molecules become more ordered. However, in most cases, the entropy of a substance tends to increase with changes in temperature, pressure, or physical state.

4. How does the entropy of a substance relate to its molecular structure?

The molecular structure of a substance plays a significant role in determining its entropy. Generally, substances with more complex molecular structures have higher entropy values because they have more ways to arrange themselves. On the other hand, substances with simpler molecular structures have lower entropy values because they have fewer ways to arrange themselves.

5. Can the entropy of a substance be negative?

No, the entropy of a substance cannot be negative. According to the second law of thermodynamics, the entropy of a closed system always increases or remains constant. Therefore, the entropy of a substance can only be zero or a positive value.

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