Difference between ionic and molar conductivity ? (Electrochemistry)?

In summary, the molar conductivity increases as the concentration decreases, but this is due to the number of ions per unit volume decreasing on dilution.
  • #1
nishantve1
76
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..
 
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  • #2
nishantve1 said:
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 ?

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.
 
  • #3
Borek said:
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.

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?
 
  • #4
nishantve1 said:
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?

And the conductivity of the solution increases, but not linearly with the concentration.
 
  • #5
Borek said:
And the conductivity of the solution increases, but not linearly with the concentration.

But the formula
Molar conductivity = specific conductivity /Concentration

Suggests that molar conductivity should decrease with increase in concentration ..
 
  • #6
nishantve1 said:
Molar conductivity = specific conductivity /Concentration

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

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

No contradiction here - molar conductivity does decrease with the concentration, but slowly. In effect conductivity of the solution goes up with the increasing concentration.
 
  • #7
@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
Thanks
 
  • #8
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]
 
  • #9
Borek said:
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.

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
 
  • #10
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.
 
  • #11
chemisttree said:
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.

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 ?
 
  • #12
nishantve1 said:
@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"

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:

1. What is the difference between ionic and molar conductivity?

Ionic conductivity refers to the ability of an electrolyte solution to conduct electricity due to the movement of charged ions. Molar conductivity, on the other hand, refers to the conductivity of a solution when a fixed amount of the electrolyte is dissolved in a given volume of the solution.

2. How are ionic and molar conductivity related?

Ionic conductivity is directly proportional to molar conductivity. This means that the higher the concentration of ions in a solution, the higher the ionic conductivity and molar conductivity will be.

3. What factors affect the ionic and molar conductivity of a solution?

The ionic and molar conductivity of a solution is affected by factors such as temperature, concentration of the electrolyte, and the nature of the electrolyte itself. Higher temperatures and higher concentrations of electrolytes generally result in higher conductivity.

4. What units are used to measure ionic and molar conductivity?

Ionic conductivity is measured in siemens per meter (S/m), while molar conductivity is measured in siemens per meter squared per mole (S·m²·mol⁻¹).

5. How is the concept of conductivity important in electrochemistry?

Conductivity is an important concept in electrochemistry as it helps in understanding the movement of charged ions in solution, which is crucial in processes such as electrolysis and electroplating. It also plays a role in determining the strength of an electrolyte solution and predicting its behavior in a chemical reaction.

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