Understanding Thermodynamics: Exploring Key Concepts and Common Misconceptions

In summary: In solutions, My books says that if the mass of the salt is not negligible then you just add the mass of the salt and water together and use the specific heat of water. Is that because the ratio of the mass of the salt to the mass of the solution times the specific heat for the dissolved salt is negligible?Yes, this is the general rule.
  • #1
Biker
416
52
I am going to learn an introductory to thermodynamics in my chemistry book(Senior year) so I thought I could ask some questions here about it to confirm my understanding:

1) Temperature: I have seen a lot of discussions here about what temperature really is. I came up with a summary (Thanks to Dr.Claude):
Temperature initially was what a thermometer reads. It is a scale which we use to represent the system tendency to give out heat until it reaches equilibrium. But now because of the kinetic theory, Temperature relates to the average of kinetic energy of the substance. So we say that the temperature now represents the average of kinetic energy of the molecules.

2) Is it wrong to starting thinking about pair of molecules and how they interact during phase changes?
I mean the temperature stays the same during phase changes because the heat goes into breaking the chemical bonds instead of increasing the kinetic energy. The question that comes to my mind is all these forces interactions, molecules moving around, some of them gaining energy and other losing (Kinetic) and you are telling me that the kinetic energy stays constant. At this stage should I just work stick to the math and stop trying to imagine all these physics interactions?

3) Does specific heat change with temperature? I have seen also threads about that here but most of them were a bit complicated, talking about quantum mechanics and all. So can someone explain it in kind of a simple way or a bit advanced and I will try to keep up? Just enough to satisfy my desire to know it.. (Just a disease, I am not required to search all of that. We are just required to understand enthlapy and hess' law but the problem they oversimplify things which I don't like)

4) In solutions, My books says that if the mass of the salt is not negligible then you just add the mass of the salt and water together and use the specific heat of water. Is that because the ratio of the mass of the salt to the mass of the solution times the specific heat for the dissolved salt is negligible?
 
Science news on Phys.org
  • #2
Biker said:
1) Temperature: I have seen a lot of discussions here about what temperature really is. I came up with a summary (Thanks to Dr.Claude):
Temperature initially was what a thermometer reads. It is a scale which we use to represent the system tendency to give out heat until it reaches equilibrium. But now because of the kinetic theory, Temperature relates to the average of kinetic energy of the substance. So we say that the temperature now represents the average of kinetic energy of the molecules.
Temperature is not the average kinetic energy of the molecules. The kinetic energy of the molecules is proportional to the temperature, but temperature is related to the entire energy content of the system (look up equipartition theorem, for instance). The contemporary definition of temperature comes from statistical physics:
$$
\frac{1}{T} =\left( \frac{\partial S}{\partial U} \right)_{V,N}
$$
i.e., it relates the change in entropy with the change in energy.

Biker said:
2) Is it wrong to starting thinking about pair of molecules and how they interact during phase changes?
Phase transitions are emergent phenomena. If you consider a single pair of molecules, you will never see any phase transition.

Biker said:
I mean the temperature stays the same during phase changes because the heat goes into breaking the chemical bonds instead of increasing the kinetic energy.
It is not chemical bonds that are broken, but the potential energy due to molecule-molecule interactions that becomes markedly different.

Biker said:
The question that comes to my mind is all these forces interactions, molecules moving around, some of them gaining energy and other losing (Kinetic) and you are telling me that the kinetic energy stays constant.
Again, temperature is not kinetic energy. This is especially important here, because when two phases coexist, the average kinetic energy in both phases (say liquid and gas) is very different, although, since T is constant during the transition, the average KE in each phase stays the same. Consider going from a liquid to a gas: as molecules go from l to g, the interaction potential energy decreases and the kinetic energy increases a lot.

Biker said:
At this stage should I just work stick to the math and stop trying to imagine all these physics interactions?
Mental pictures are still useful. You just have to have the right ones, and be careful that they are only imperfect pictures.
Biker said:
3) Does specific heat change with temperature?
Yes (but not necessarily by much). This is again due to molecule-molecule interactions(*).

(*) A good way to figure this out is to compare with an ideal gas. If a behavior is not present in an ideal gas, then interactions most probably have something to do with it.

Biker said:
4) In solutions, My books says that if the mass of the salt is not negligible then you just add the mass of the salt and water together and use the specific heat of water. Is that because the ratio of the mass of the salt to the mass of the solution times the specific heat for the dissolved salt is negligible?
Do you mean if it is negligible?
 
  • #3
I will mark your replies from 1 to 7:
Btw, I said I am in Highschool senior year. However, I will still try to look for the concepts you gave me.
1) That why I said:
Temperature relates to the average of kinetic energy of the substance. So we say that the temperature now represents the average of kinetic energy of the molecules.

Because you strongly argued about this in previous threads

2) Thank your for your answer.

3) Sorry yes, wrong word. The interactions between molecules becomes weaker.

4) "interaction potential energy decreases" You mean increases? The forces attracting them decreases but the potential increases.
When water changes from liquid to gas, The system gains heat which is stored as potential energy due to molecules moving far a part. When the water completely becomes gas, Its kinetic energy starts to increase which relate to temperature increasing.

Bit confused here: "Again, temperature is not kinetic energy... since T is constant during the transition, the average KE in each phase stays the same.""
You mean temperature relates to average but not the sum of kinetic energy.. right? I meant the average stays constant during the changing phase regardless of all these interactions.

6)Can you explain how does it relate to molecule-molecule interactions? What I have seen online was about vibration, rotation and transnational movement of molecules and how it affects specific heat capacity.

7) No, It is not negligible what I think the book means is like typical grams of solutions. I referred to the ratio between the mass of the salt and the solution as negligible.
 
  • #4
Biker said:
I will mark your replies from 1 to 7:
Btw, I said I am in Highschool senior year. However, I will still try to look for the concepts you gave me.
Don't hesitate to say it if my answers are not understandable.

Biker said:
1) That why I said:
Temperature relates to the average of kinetic energy of the substance. So we say that the temperature now represents the average of kinetic energy of the molecules.

Because you strongly argued about this in previous threads
There is a relation between kinetic energy and temperature. Yes, when the temperature is higher, so is the kinetic energy, and you can figure out the temperature of a gas by measuring the kinetic energy of the molecules that compose it. But, we have to be careful not to say that kinetic energy is temperature. Going back to the case of the phase transition, consider the extreme example of the sublimation of CO2: you have two phases, solid and gas, at the same temperature, but the average kinetic energy of a given molecule is much different depending on which phase it is in.

Biker said:
4) "interaction potential energy decreases" You mean increases? The forces attracting them decreases but the potential increases.
When water changes from liquid to gas, The system gains heat which is stored as potential energy due to molecules moving far a part. When the water completely becomes gas, Its kinetic energy starts to increase which relate to temperature increasing.
Correct: I meant that the potential energy increases. My main point is again that the kenetic energy can be different at the same temperature, see above.

Biker said:
Bit confused here: "Again, temperature is not kinetic energy... since T is constant during the transition, the average KE in each phase stays the same.""
You mean temperature relates to average but not the sum of kinetic energy.. right? I meant the average stays constant during the changing phase regardless of all these interactions.
Hope the confusion is cleared up by what I wrote above.

Biker said:
6)Can you explain how does it relate to molecule-molecule interactions? What I have seen online was about vibration, rotation and transnational movement of molecules and how it affects specific heat capacity.
There are two things to consider here: the heat capacities for different molecules and the heat capacity as a function of temperature. It is the first that is very much affected by vibrations and rotations. To increase the temperature of a molecule, you need to give it energy not only to increase its kinetic energy (translation), but also to increase its vibration and its rotation (again, this relates to the equiparition theorem). The variation of the heat capacity with temperature is mostly related to molecular interactions, but that variation is relatively small, see for instance http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html

Biker said:
7) No, It is not negligible what I think the book means is like typical grams of solutions. I referred to the ratio between the mass of the salt and the solution as negligible.
If the mass of the salt is not negligible, than indeed you need to add it to the mass of the solvent to get the mass of the solution. But the mass has to be small for it not to change the heat capacity of the solution compared to that of the solvent, see for example http://www.engineeringtoolbox.com/sodium-chloride-water-d_1187.html
 
  • Like
Likes Biker
  • #5
Awesome!,

So to summarize what you have told me,
Having same temperature doesn't mean that all these molecules have the same kinetic energy, It depends on what states it is in. However the average kinetic energy of all molecules relates to temperature which is constant in phase change for exampleAbout heat capacity, So they were talking about why different molecules have different heat capacities not the variation of heat capacity as a function of temperature? The change is indeed small, That is why at least we approximate it to be constant. Thanks for the tableAbout salt mass, Again thanks for the chart. Apparently they give us solutions with mass concentration above 10 (In NaCl case the change of heat capacity was a bit high) they don't take into account the heat capacity changes. But at least now, I know how it changes.

Thank you so much for clearing this out for me :D
 
  • #6
I have a question I hope someone may be able to answer.

I am currently working out an adiabatic process that gives me an initial pressure,volume, and temperature. Along with a final pressure and given that the cylinder/piston assembly does not conduct any heat. Another given is that the Cp=3.5R

I have done the work to solve for my gamma and the final volume and temperature for the system. With that I was able to calculate work done by the system which is also the change in internal energy.

My question is how do I find ΔH?

I know that ΔH=ΔU+ΔPV, but I just don't think my way of thinking is correct as I would assume that I would substitute for ΔU=Q-W where I know Q will equal zero.

leading me to an equation of ===> ΔH=dQ-dW+ΔPV leading me to an answer of 0.

Is this a correct assumption? I look forward to any and all productive feed back.

Best Regards,

Dylan
 
  • #7
HethensEnd25 said:
I have a question I hope someone may be able to answer.

I am currently working out an adiabatic process that gives me an initial pressure,volume, and temperature. Along with a final pressure and given that the cylinder/piston assembly does not conduct any heat. Another given is that the Cp=3.5R

I have done the work to solve for my gamma and the final volume and temperature for the system. With that I was able to calculate work done by the system which is also the change in internal energy.

My question is how do I find ΔH?

I know that ΔH=ΔU+ΔPV, but I just don't think my way of thinking is correct as I would assume that I would substitute for ΔU=Q-W where I know Q will equal zero.

leading me to an equation of ===> ΔH=dQ-dW+ΔPV leading me to an answer of 0.

Is this a correct assumption? I look forward to any and all productive feed back.

Best Regards,

Dylan
Please submit this as a separate thread in the Homework forums, probably under Biology, Chemistry, and Earth Homework. I am closing the present thread since Biker seems to have all his questions answered.
 

1. What is thermodynamics?

Thermodynamics is the branch of physics that deals with the relationships between heat, energy, and work. It studies how energy is transferred between systems and how it affects their properties and behavior.

2. What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern the behavior of energy in thermodynamic systems. They include the first law, which states that energy cannot be created or destroyed, only transferred or converted, and the second law, which states that the total entropy of an isolated system will always increase over time.

3. How is thermodynamics used in everyday life?

Thermodynamics is used in many everyday applications, such as heating and cooling systems, refrigerators, car engines, and power plants. It also plays a crucial role in understanding weather patterns, chemical reactions, and biological processes.

4. What is the difference between heat and temperature in thermodynamics?

Heat and temperature are related but distinct concepts in thermodynamics. Temperature is a measure of the average kinetic energy of particles in a system, while heat is the transfer of thermal energy from one system to another due to a difference in temperature.

5. Can the laws of thermodynamics be violated?

No, the laws of thermodynamics are fundamental principles that have been extensively tested and have been shown to hold true in all physical systems. Any apparent violations can be explained by a lack of understanding or incomplete information about the system in question.

Similar threads

Replies
2
Views
389
Replies
32
Views
1K
Replies
5
Views
999
  • Thermodynamics
Replies
3
Views
747
Replies
3
Views
958
Replies
16
Views
939
Replies
15
Views
1K
  • Thermodynamics
Replies
3
Views
952
  • Thermodynamics
Replies
14
Views
1K
  • Thermodynamics
Replies
2
Views
907
Back
Top