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Can a reaction be exothermic due to increased kinetic energy?

  1. Nov 29, 2013 #1
    1. The problem statement, all variables and given/known data

    Examkrackers Lecture question - MCAT Prep book

    116. A student holds a beaker of pure liquid A in one hand and pure liquid B in the other. Liquid A has a higher boiling point than liquid B. When the student pours a small amount of liquid B into liquid A, the temperature of the solution increases. Which of the following statements is true?

    Tempting answer choices:

    B. The boiling point of the solutions is higher than either pure liquid A or B.
    C. The freezing point of the solution is higher than either pure liquid A or B.

    2. Relevant equations

    None were supplied.

    3. The attempt at a solution

    I can argue for either answer B or answer C.

    C: By mixing the solutions together, any lattice which would form upon freezing must have been disrupted, producing a higher melting point. Is this thinking correct? Why or why not?

    B: Maybe not because isn't it possible that mixing two solutions would increase the kinetic energy of the particles? If it is possible that kinetic energy is responsible for the increase in temperature, then, B is not necessarily correct. Could that be the case? Why or why not?

    (The obvious other possibility is that stronger bonds formed releasing energy and increasing the boiling point.)

    Thanks for your help!
  2. jcsd
  3. Nov 29, 2013 #2

    Simon Bridge

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    Exothermic reactions lead to increased kinetic energy... you can increase kinetic energy to start an exothermic reaction though, so it is not clear how you mean the cause and effect here?

    Note: just because no equations were supplied does not mean that no equations are relevant.

    Can you think of two liquids that are like the example?
    Alcohol and water for example?
  4. Nov 29, 2013 #3


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    If you're disrupting interactions in the solid state, making them fall apart easier, wouldn't the solid melt at a lower temperature and not a higher temperature?

    In general, how do the melting points and boiling points of solutions compare to the melting and boiling points of pure substances?
  5. Nov 30, 2013 #4
    Good job! You just answered that part of the question.
  6. Nov 30, 2013 #5

    Simon Bridge

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    The idea is for you to answer it ... ygggdrasil helped you answer that part of the question ;)
  7. Nov 30, 2013 #6
    I'm not entirely sure where you are finding ambiguity.

    To reiterate the sequence:
    1. Solution A and Solution B are mixed. THEN
    2. Hypothetically the molecules in the mixture gain kinetic energy, say by repelling each other or something. THEN
    3. Because of the increased kinetic energy described in step 2, heat is evolved

    The reason I prefer NOT to include the information in your last 2 paragraphs is that that would change the question from a general one (about any solution using any mechanism) to a specific one. I would like to keep it general because that's the way the textbook authors seemed to intend it.
  8. Nov 30, 2013 #7
    I can't see why considering that I have the answer key right in front of me (hence, it's an "MCAT" question).
  9. Nov 30, 2013 #8


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    Liquids are held together by the intermolecular forces between molecules in the liquid. Once the temperature rises enough to easily break those intermolecular bonds, the liquid can begin to boil. Liquids containing stronger intermolecular bonds will boil at a higher temperature than liquids containing weaker intermolecular bonds.

    When you mix liquids A and B, you are breaking apart A-A and B-B interactions to form A-B interactions. The question tells you that mixing liquids A & B is an exothermic process. What does that tell you about the A-B intermolecular bonds in comparison to the A-A and B-B intermolecular bonds? What might this suggest about the boiling point of the mixture in comparison to the boiling point of the pure liquids?

    (if you're still having trouble with the question, read through the following page: http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html)
  10. Nov 30, 2013 #9
    I already addressed that remedial point in the 2nd to last line of the topic.

    To rephrase my question, is there any other reason a reaction could be exothermic? (In addition, to what was already mentioned about bonds forming being stronger than bonds broken.)
    Last edited: Nov 30, 2013
  11. Nov 30, 2013 #10

    Simon Bridge

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    Cause and effect - increased kinetic energy is pretty much what "exothermic" means. The question seems to be asking if one can be caused by the other.

    Do you understand the difference between heat and kinetic energy here?

    Wouldn't you rather prefer to be able to work these sorts of problems for yourself?

    We shouldn't be doing these problems for you - but we should be guiding you towards the kind of thinking that gets you to good answers by yourself. The kind of thinking you are exhibiting, "I'm not going to consider specific stuff if the question is general", is unhelpful and makes it easy to make mistakes. After all, how will you check your answers (my answers, the book's answers)?

    The specific example is by way of a reality check.
    Do your answers work for a specific mixture that you are familiar with?
    Can you find a mixture you know about which your answers do work for?

    You are training to solve problems that nobody knows the answer to - you need to learn to check you own answers.

    I think the answer you are looking for is "no" - but qualified.

    A reaction (of any kind - you wanted "general") is "exothermic" if it releases potential energy as heat. There are lots of ways of storing energy, ergo, lots of forms of potential energy. In chemistry, the energy is primarily thought of as stored within bonds of some sort. In this model, not all bonds are chemical bonds. You can get heat from reactions other than those which change chemical bonds. Which is why the "no" is qualified. In the specific case of a chemical reaction, the energy released comes from rearranging the chemical bonds - so, by definition, exothermic chemical reactions get their heat from rearranging chemical bonds.
    Last edited: Nov 30, 2013
  12. Nov 30, 2013 #11


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    Ah, I think I understand your question now. Assuming that the small amount of liquid B is at the same temperature as liquid A and that no chemical reaction occurs when liquid A and liquid B are mixed, then the increase in temperature of solution indicates that A-B intermolecular interactions are stronger than A-A and B-B intermolecular interactions. That increased kinetic energy of the solution has to come somewhere, so it must come from lowering the chemical potential energy of the molecules through stronger intermolecular interactions.
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