Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermodyanmics question.

  1. Oct 13, 2005 #1
    My chemistry teacher recently made something that must be one of the most stupid statements ever made:
    She claimed that this was a demonstration of the 2nd law of thermodynamics
    "We can usethe analogy of dissolving sugar in coffee as an example of increasing entropy, in this case - disorder. No matter how you stirred, the sugar would not separate from the coffee again because that would be bringing order out of chaos."

    Isn't that ridiculous? Crystallisation is an exothermic reaction and could anyone be so dumb as to think that an exothermic reaction could proceed when you add mechanical energy to it.

    It just shows hercomplete lack of understanding of thermodynamics in her insistence of using a false 'analogy' involving the spontaneity of an exothermic process depending on the addition of mechanical energy.

    But she refuses to admit she was wrong!
  2. jcsd
  3. Oct 13, 2005 #2
    Your teacher's statement is perfectly correct. Whether you stir or not,crystallization will not take place. That is assuming that the temperature does not change and there is no evaporation. It is a good example of increasing entropy.
  4. Oct 14, 2005 #3


    User Avatar
    Science Advisor
    Homework Helper

    spontaneity really has no direct relation to enthalpy, it's determined by the change in the total entropy of the system. dG=dH-TdS, pertains to a constant temperature, constant pressure process, all the variables refer to the system, but it's derived from when for a spontaneous process

    dSsys>dSsurr, dSsys>-dq/T, for constnat pressure

    dSsys>dH/T, TdSsys-dH>0, we assign the function G=H+TS

    dG=dH+TdS, dG>0 for constant temperature, pressure process

    )dStotal>0 for a spontaneous process, dSsys-dSsurr>0)
  5. Oct 16, 2005 #4

    When she added stirring she invalidated the whole example. Because she is trying to make an EXOTHERMIC reaction dependent on adding mechanical energy.

    Crystallization is exothermic. Stirring is adding energy. Exothermic reactions can't proceed if you add energy (unless you include activation energy which doesn't count).

    She tries to pretend that stirring is adding disorder to the system, that if the two solutions were at the same temperature, cool enough for crystals to form, the stirred solution would not crystallise because it was having disorder added to it.

    In her example where she said no matter "how much you stirred" the implication is that this exothermic process depends on the addition of mechanical energy.

    Which is just totally ignorant.
  6. Oct 17, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    So why is it that when you stir a hot solution, the solution cools faster. You're increasing the collisional frequency between molecules, the energy is dissipated by stirring the solution.

    I think you're making a lot of assumptions here. Just because a reaction is exothermic doesn't require that the process be spontaneous, in fact, it has no direct bearing on whether a reaction will proceed in many cases. Total change in entropy of the system and surroundings determines spontaneity, and when your teacher was referring to the situation of stirring, she probably was referring to increasing the entropy of the system and perhaps additional chances for crystallization (of course, the chances are incredibly small).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook