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B Thermodynamics questions

  1. Nov 11, 2016 #1
    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 dont 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?
  2. jcsd
  3. Nov 11, 2016 #2


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    Staff: Mentor

    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.

    Phase transitions are emergent phenomena. If you consider a single pair of molecules, you will never see any phase transition.

    It is not chemical bonds that are broken, but the potential energy due to molecule-molecule interactions that becomes markedly different.

    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.

    Mental pictures are still useful. You just have to have the right ones, and be careful that they are only imperfect pictures.

    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.

    Do you mean if it is negligible?
  4. Nov 11, 2016 #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.
  5. Nov 11, 2016 #4


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    Staff: Mentor

    Don't hesitate to say it if my answers are not understandable.

    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.

    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.

    Hope the confusion is cleared up by what I wrote above.

    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

    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
  6. Nov 11, 2016 #5

    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 example

    About 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 table

    About 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 dont 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
  7. Feb 14, 2017 #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,

  8. Feb 14, 2017 #7
    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.
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