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Solvation heat pump

  1. Sep 5, 2010 #1

    Lately, i have been attempting to develop a better understanding of thermodynamics, but i have encountered a few problems.

    I considered that endothermic solvation, (e.g. dissolving KNO3 in water) is essentially the same as an absorption heat pump. This idea was based on the simple fact, that dissolving KNO3 (Potassium Nitrate) in water, absorbs heat energy; evaporating the water heating it up, returns the potassium nitrate to a solid form, where it then can be used once again to absorb heat energy.

    Before coming to a conclusion and incorporating a consideration into my understanding i always make sure they are congruent with the laws and principles.

    In this case with a thought experiment I tested the congruency of how i would consider a solvation heat pump to behave and the second law of thermodynamics.

    Thought Experiment

    Solid KNO3 is dissolved in water until the solution is saturated. As this solvation is endothermic, the solution is colder than lab that this is conducted in, but after a duration of time it reaches equilibrium with the lab, which has not changed temperature during this process.

    This solution is poured into compartment A of a container that is sealed and perfectly insulated. Heat is introduced to this container with the use of a Carnot heat pump, that extracts heat from the lab and heats the container above the temperature of the lab.

    As the temperature increase inside this container, the solution begins to evaporate. Compartment A is mainly taken up by the solution and the water vapour is forced to move through a small tube at the top of compartment A to compartment B, which is significantly larger than compartment A.

    The container is ceases to be heated once all the water has been evaporated. As the water is a gas it it is evenly diffused throughout the container. The far majority of the water resides in compartment B due to the size difference between the compartments, but all the potassium nitrate remains in compartment A in solid form.

    The heat gradient between the container and the lab is then utilised by a Carnot heat engine. As he temperature falls, the water within each compartment condensing as liquid. The container returns to the temperature of the lab with liquid water residing in both compartments with the far majority residing in compartment B. The the far majority of the KNO3 remains in compartment A as a solid, with the rest dissolved in the water in compartment A.


    In this thought experiment, the potassium nitrate solution absorbed heat energy from the lab, which is then expelled when it returns to a solid. The majority of the heat energy is not reabsorbed when the water re-condenses, as most of the water condenses in a different compartment.

    As this energy is not reabsorbed, it is utilised by the Carnot heat engine and a portion of this heat energy is converted to a higher quality form of energy.

    I would conclude that such a conclusion is a violation if the second law of thermodynamics.

    This as with my other post “the altitude hypothesis” have a very similar problem as my undeveloped understanding suggests that ‘ heat energy can be converted to potential energy allowing a heat gradient to be formed and utilised, reducing entropy.’

    If anyone out there can explain the flaw in this reasoning, it would be greatly appreciated.
  2. jcsd
  3. Sep 6, 2010 #2
    Attached is a diagram of the container before it is heated by the heat pump.

    The saturated KNO3 solution resides in the smaller compartment A. As can be seen there is a small tube that connects compartment A to compartment B, as well as a plug in compartment A that allows one to pour in solution and then seal the container. The features marked P and E stand for heat pump and heat engine. Anything black can be considered a perfect insulator, when the heat pump and engine are active, they cease to insulate and allow heat transfer to occur between the lab and the container.

    Restating the issue, breaking the inter molecular bonding between the ions and water molecules, then bonding the ions to each other (forming solid KNO3) and the water molecules to each other, converts potential energy to heat energy. To regenerate the solution one just needs to mix the water and KNO3 together where it will absorb heat energy from the environment as solvation occurs. When breaking the bonds of the solution you require heat energy, but one will find that the output of heat energy from doing so is greater than the input. Hypothetically a Carnot heat pump could deliver this required input and with a Carnot heat engine recycling this energy input and utilising the heat energy that has come from separating the water from the KNO3; resulting in a greater high quality energy output then input and a reduction in entropy.

    Attached Files:

  4. Sep 21, 2010 #3
    Does anyone want to comment, even if they do not resolve the issue?
  5. Sep 22, 2010 #4


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    Staff: Mentor

    You are saying that it takes heat to break the bonds, and that when those bonds break they also release more heat energy? Are you sure that's correct?

    Edit: After some thought i think i see the problem. When you initially heat container A, you have to input work to transfer the heat from the room to the container. Once the water is evaporated, the heat pump is reversed to a heat engine to produce work using the difference in temperature. However, you wouldn't get nearly as much work out of this heat engine as you had input using the heat pump. The KNO3 solution doesnt matter here. It would absorb energy from the container to dissolve, and would release the same amount of energy as the water evaporated, balancing that out. You simply cannot get more work out of the Carnot engine as it took to introduce the heat into the container in the first place.
    Last edited: Sep 22, 2010
  6. Sep 22, 2010 #5
    KNO3 when dissolved will absorb heat energy. When KNO3 solution is evaporated it will release heat energy. This I am sure is correct.

    From my understanding of a carnot heat pumps and engines. The amount of energy required to create a heat gradient is the same as can be utilised, under conditions where nothing changes except the temperature of the system. This is one of those systems where something does change.
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