Peculiar results obtained (concerns electrolytes)

[PedroB]

Recently, I conducted an experiment to establish the relationship between the temperature of an electrolyte (I used aqueous Calcium Chloride, though the results obtained with other ionic solutions followed the same trend). As I explained in my hypothesis, I expected the conductivity to proportionally increase with temperature (given that the ions would have more KE, and would therefore move more 'freely').

My results, however, proved the exact opposite, showing a direct inverse proportionality between both variables (ie conductivity decreased with T). My colleagues also had similar results. I have not been able to find any info. regarding this (everything I have found so far supports my hypothesis), and am hoping someone can clear this for me. (if it helps, conductivity probes connected to data loggers recorded the conductivity).

Thanks in advance

 

[Drakkith]

Conductivity is the flow of electrons, correct? I believe this is similar to a normal conductor in that the increase in temperature causes the electrons to encounter more resistance than at a lower temperature thanks to the increased movement of the atoms and molecules. I don't know if an electrolyte would be different however.

 

[PedroB]

I had considered this, but the fundamental concepts are quite disparate. Whilst in metals increased temperature causes individual particles to vibrate more (thus interfering with the electrons' paths, which leads to a decrease in conductivity), in electrolytes, there is no 'interfering agent', given that the charge carriers are ions moving through a liquid. (though I am open to any potential theories).

Perhaps the increased movement of the water particles as T increases cancels out (due to interference) the effect that would have otherwise led to increased conductivity (ie the more rapid movement of the ions)?

 

[Drakkith]

Well, correct me if I am wrong, but the ions are simply molecules with a + or - net charge correct? The electrons are still bound to them. What is the difference in the particles in a metal and the particles in an electrolyte that might make a difference for conductivity based on temperature?

 

[PedroB]

In metals, electrons flow through the 'layers' of metal cations when a current is made to flow through them. As temperature increases, these ions vibrate more vigorously, thus interfering with the electron's movement (to use an analogy, it would be similar to a sideways collision with a moving car). Increased T = More vigorous vibrations= More interference = Decreased conductivity.

In electrolytes, the charge carries are much more massive ions, which do not follow an allocated 'path' (the equivalent of the movement through the 'layers in metals), and are much less affected by collisions, in this case of the water particles (which would be more infrequent then the vibrations felt by the electrons in metals) due to their mass. In theory increased T= More rapid movement of the ions = current travels more easily = increased conductivity. The electrons (which are fundamental to ions as they dictate their charge) can be neglected when talking about collisions as they are a minute part (mass wise) of the ion.

 

[Drakkith]

I understand what you are saying, and I just have a few questions.

First, when measuring the conductivity of the electrolyte are you doing essentially the same thing that one would do to measure the conductivity of something like a copper wire?

Second, in an electrolyte why are the ions the charge carriers? (Or what exactly does that mean) Is there a chemical reaction taking place that needs to have the ions move in order to function?

Third, if the ions are the charge carriers, why would increased random motion due to heat cause a decrease in conductivity? Wouldn't the increase in motion cause more resistance to movement in one direction?