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Resistance versus temperature in weak acids, strong acids, and water

  1. Feb 7, 2010 #1
    The question below is a rather theoretical one and does not concern any actual calculations. So I have decided to abandon the traditional format.

    In the past few weeks, I have designed and carried out a lab in which I would test the resistance of a strong acid (0.5 M HCl), a weak acid (0.5 M vinegar) and water at varying temperatures (10 - 50 deg Celcius to avoid error caused by evaporation). The same volume of solutions put into the same type of beaker (i.e. geometric proportions of the solutions are conserved) with resistance measured across the same two points in the solution every time were used as controls.

    I had assumed that the resistances would go down as a function of temperature because:
    1) The Keq/dissociation constant of the acid equilibriums increase, thus producing more H3O+ ions and conjugate base ions.
    2) The increased temperature increases the kinetic energy of the solutions and the ions therein, thus increasing conductivity and decreasing resistance.
    3) Water tends to auto-ionize more at greater temperatures (this relates to point 1 and is also shown to be true in the experiment as the trials show lower resistances versus higher temperatures)

    The lab turned out to be a success and a general trend of high temperature / lower resistance was shown. At the onset, this trend looks to be a logarithmic one. However, when I plot conductance (1/R) versus temperature (celcius), the trend becomes linear. In other words, temperature is directly proportional to (1/R).

    My question is: Why does this happen?

    The lab data is contained in an attachment below.
     

    Attached Files:

  2. jcsd
  3. Feb 8, 2010 #2

    chemisttree

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    Review the Nernst Equation, eg... E = Eo - (RT/nF)lnQ
     
  4. Feb 8, 2010 #3
    Interesting equation. However, the sample of acids I am testing are not really electrochemical cells in any way. They are just simple solutions of vinegar and HCl at 0.5 M.
     
  5. Feb 10, 2010 #4

    chemisttree

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    Did you notice the temperature term in the equation? Notice how it's first order?
     
  6. Feb 11, 2010 #5
    Yes.. but i was under the equation that the Nernst equation only applied to working chemical cells
     
  7. Feb 11, 2010 #6

    chemisttree

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    Last edited by a moderator: Apr 24, 2017
  8. Feb 11, 2010 #7
    Ok. But reading the contents of the link you've sent, I see that the final equation derived relates molar conductivity or conductivity to the equation:
    http://tannerm.com/images/difcond12.gif
    However, as you can see, the conductivity is proportional to T^-1 where my experiment seemed to indicate that conductivity is proportional to T
     
  9. Feb 12, 2010 #8

    chemisttree

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    My bad. I thought you had plotted resistance vs temp. Take another look at your data. It isn't linear at all. It looks like a typical y = a(1/x) graph.
     
    Last edited: Feb 12, 2010
  10. Feb 13, 2010 #9
    I did plot resistance versus temperature. That was the graph that looked like the 1.x graph. Take a look at the third graph from the left. That is a conductance versus temperature graph and it seems to be linear.
     
  11. Feb 14, 2010 #10

    chemisttree

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    Are you asking if you plot conductance vs temperature you should get something nonlinear if you already know that the resistance vs temperature looks like it varies inversely with temperature?
     
  12. Feb 14, 2010 #11
    No i did plot conductance vs temperature. According to the site you linked, the conductivity should be inversely proportional. However, i found in my experiment that it was proportional to temperature.
     
  13. Oct 23, 2011 #12
    What is the surface area of the electrodes your using?
     
  14. Oct 23, 2011 #13

    Borek

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    Ah, there is nothing like to post in an abandoned, year and half old thread.

    Locked.
     
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