Difference between ionic and molar conductivity ? (Electrochemistry)?

  1. What I know is Molar conductivity is The conductivity of the elctrolyte divided by its concentration . Can someone please explain the physical significance of this ? Also ionic conductivity depends on concentration, if I dilute the electrolyte the ionic conductivity decreases but Molar conductivity is inversely proportional to the concentration, if I increase the concentration molar conductivity decreases, what is happening here ? Aren't the molar and ionic conductivity related to each other ? Some one please get me outta here..
  2. jcsd
  3. Borek

    Staff: Mentor

    In a way it is not different from - say - specific heat of the substance. It ties the conductivity of the solution with the concentration, just like heat capacity of an object is tied to teh object mass by the specific heat of the substance.

    Problem is, it works nicely only for diluted solutions, when ions travel independently. The more concentrated solution is, the more the ions interfere, they bump on each other, they attract each other creating neutral ions pairs (or even larger conglomerates), so the solution generally behaves as if the number of ions was lower than it is.
  4. The more concentrated the solution is the more number of ions are present in the solution so the conductivity should increase, is this reasoning wrong?
  5. Borek

    Staff: Mentor

    And the conductivity of the solution increases, but not linearly with the concentration.
  6. But the formula
    Molar conductivity = specific conductivity /Concentration

    Suggests that molar conductivity should decrease with increase in concentration ..
  7. Borek

    Staff: Mentor

    Not specific conductivity (which suggests something constant an characteristic for a ion or substance), but measured conductivity of thr solution.

    No contradiction here - molar conductivity does decrease with the concentration, but slowly. In effect conductivity of the solution goes up with the increasing concentration.
  8. @Borek
    I am a little confused here, Heres what my textbook says
    "Conductivity always decreases with decrease in concentration of both strong and weak electrolytes.This can be explained by the fact that the number of ions per unit volume that carry the current decreases on dilution"

    Now coming to the Molar conductivity part (I am still not sure what is the use of this term)

    The textbook says : "Molar conductivity of a solution at a given concentration is the conductance of volume V of a solution containing one mole of the electrolyte kept between two electrodes,( ^m = kV)
    ^m = molar conductivity
    k = specific conductivity
    V = volume between two electrodes

    Molar conductivity increases with decrease in concentration, This is because the total volume V of the solution containing one mole of the electrolyte increases on dilution"

    How does this equation ( ^m = kV) makes sense ?
    also if you could please explain what the underlined term actually means I will be really appreciative
  9. Borek

    Staff: Mentor

    This is a funny definition of the molar conductivity (although perfectly correct). In general nobody is going to test it this way, as it is not possible to change volume of the cell, all measurements are done using a cell with known parameters (sorry, I don't remember English name) - size of the electrodes, distance of the electrodes, they are just combined into one number that is specific for the cell. What you do is you measure the conductivity of the solution of known concentration.

    The equation. As you wrote in your very first post, molar conductivity is the conductivity divided by the molar concentration:

    [tex]\Lambda_m = \frac {\kappa}{c} = \frac {\kappa V} {n}[/tex]

    but n=1 (that's where this funny definition becomes handy), so you end with

    [tex]\Lambda_m = {\kappa V}[/tex]
  10. That's called the Cell constant.
    Cell constant = l / A
    l is the distance between the electrodes and A is the area of the electrodes .

    I am still confused here, how come the conductivity decreases on decreasing concentration but molar conductivity increases ? Looking forward to a qualitative explanation . Sorry for being so dumb and a pain in butt, but I really need to get this thing cleared
    Thanks again
  11. chemisttree

    chemisttree 3,717
    Science Advisor
    Homework Helper
    Gold Member

    That definition for molar conductivity should have used the more exact term 'activity' instead. Read up on the term 'activity' vs concentration and all will be revealed.
  12. So,Activity is the effective concentration - i.e. that portion of the ions which are free to take part in a given reaction - It becomes proportionately less as the concentration increases.
    So that means if I increase concentration number of ions taking part in electrical conductivity decreases so the conductivity decreases .
    That makes sense to me .
    But why use activity for molar conductance and Concentration for conductance of an electrolyte as a whole ?
  13. Im not sure about this definition: Conductivity decrease with concentration decrease.
    With strong electrolytes , mineral acids , alkalies and salts with a concentration increase there is more ionic interference that reduces velocity. As stated earlier.
    With weak electrolytes, organic acids and bases with concentration increase there is a decrease in degree of ionization .
    Both of these conductivity vs concentration curves have the same general shape.
    All the expressions like this one by Kohlrausch : Ac = A0-kc1/2 show increase of conductivity with concentration decrease
    And if you use activity in molar conductivity you get : Ac = electrolytic conductivity/concentration x activity coefficient
    Last edited: Dec 7, 2012
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