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Topic: Why do we say that ionic solutions are conductors of electricity?  (Read 4130 times)

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Offline confusedstud

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From what I've learned the reason why an ionic solution can conduct electricity is because of its electrochemical properties. For example the conduction can occur as an electrochemical cell or electrolytic cell.

But if were to connect wires to our battery which are connected to an electrolyte and connected to a light bulb why would the light bulb shine? In this case the electrolyte would act as an electrolytic cell. But what if the components of the electrolyte tend not to undergo any oxidation or reduction reaction? For instance NaSO4 electrolyte. In such cases wouldn't the electrolyte not shine?

Offline Enthalpy

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #1 on: July 18, 2015, 05:06:35 AM »
You could write the formula of this electrolyte better. As a solid it's not completely correct, but more importantly for the conduction, what happens when it's dissolved in water?

Then, as the battery creates an electric field in the electrolyte, is there some movement? And where could oxidation and reduction happen, due to what?

Offline confusedstud

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #2 on: July 18, 2015, 05:21:45 AM »
You could write the formula of this electrolyte better. As a solid it's not completely correct, but more importantly for the conduction, what happens when it's dissolved in water?

Then, as the battery creates an electric field in the electrolyte, is there some movement? And where could oxidation and reduction happen, due to what?

My bad the formula should have been Na2SO4 instead. When it dissolved in water it forms ions.

The battery creates an electric field such that the positive ions flow to the negative end of the wire and negative ions flow to the positive end of the wire. So oxidation should occur at the positive end of the wire where electrons from the ions are lost. However I don't think the SO4 2- ions will undergo this oxidation but instead the OH- will. And likewise for the negative end of the wire, I don't think the Na+ ions can undergo reduction to form sodium metal. Instead the H+ ions will.

So the ions that actually allow the electrical current to flow is based on the water's ions and not the sodium sulfate ions. So I don't see why the ionic compound we added here would increase the electrical conductivity at all.

Is there a reason that increases the solution's conductivity when the salt is added?

Offline Enthalpy

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #3 on: July 18, 2015, 09:57:45 AM »
Dissolved acids, bases, salts increase the number of ions hence the conductivity.

Pure water (something very uncommon) would have about 10-7 H+ and OH- moles per L, which is very little. In a microeletronics lab, we had resins to catch the ions from water and make it as pure as possible; a typical resistance would have been 10Mohm, and we were far from 10-7 mol/L.

An easily ionized compound brings many more ions which drop the resistance a lot, say to 100ohm or 10ohm.

Far from the electrodes, these added ions conduct the current hence are useful. It's only at the electrodes that other reactions happen and other compounds may leave the liquid, for instance H2 and O2 - or more complicated ones, which may also stay in the water.

Offline confusedstud

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #4 on: July 19, 2015, 06:16:16 AM »
Dissolved acids, bases, salts increase the number of ions hence the conductivity.

Pure water (something very uncommon) would have about 10-7 H+ and OH- moles per L, which is very little. In a microeletronics lab, we had resins to catch the ions from water and make it as pure as possible; a typical resistance would have been 10Mohm, and we were far from 10-7 mol/L.

An easily ionized compound brings many more ions which drop the resistance a lot, say to 100ohm or 10ohm.

Far from the electrodes, these added ions conduct the current hence are useful. It's only at the electrodes that other reactions happen and other compounds may leave the liquid, for instance H2 and O2 - or more complicated ones, which may also stay in the water.

Oh so the electrodes the reactions occur that are not related to the Na2SO4?

But how would the ions conduct the electricity then? I thought that the conduction process was due to those reactions at the electrodes.

Offline Borek

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #5 on: July 19, 2015, 06:46:15 AM »
Don't confuse current in the bulk of the solution (due to the movement of the charged ions) with the current through the phase boundary (requiring electrode reaction).
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Offline Enthalpy

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #6 on: July 20, 2015, 04:04:37 AM »
In the bulk, the electric field moves ions, and the moving charge is a current.
In the wires, the electric field moves electrons.
At the contact between the conductor and the electrolyte, the ions and the electrodes exchange electrons.

In a first approximation, you can imagine that the dissolved ions get to the electrodes with which they exchange electrons, and that if the species created there are unstable in water (Na for instance) they react to release something else (H2 for instance).

Though, I have no hope that the detailed process is so simple. The reaction must happen while the species still stick at the electrode, through bizarre intermediates, and this must have consequences at the cell voltage and more. For instance, I expect no dissolved atomic hydrogen; it must stay adsorbed at the electrode until meeting an other hydrogen atom to form a hydrogen molecule.

Offline confusedstud

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #7 on: July 29, 2015, 05:15:00 AM »
Don't confuse current in the bulk of the solution (due to the movement of the charged ions) with the current through the phase boundary (requiring electrode reaction).

Hmm but I thought we only needed the current through the phase boundary to consider the solution to be considered an 'electrical conductor'?

I would agree that the current in the bulk of solution would increase with the concentration of the ions similar to metals that has a higher valency would usually be better conductors. But again, isn't that property irrelevant?

Offline Enthalpy

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #8 on: July 30, 2015, 05:14:29 AM »
Charges won't accumulate indefinitely - in fact, they accumulate very little. If they pass from the conductor to the solution, they must go further through the solution.

Offline Irlanur

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #9 on: July 30, 2015, 11:38:14 AM »
Conductivity and the "active" generation of a current are two quite different things.

Offline confusedstud

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Re: Why do we say that ionic solutions are conductors of electricity?
« Reply #10 on: July 31, 2015, 02:31:31 AM »
Charges won't accumulate indefinitely - in fact, they accumulate very little. If they pass from the conductor to the solution, they must go further through the solution.

What do you mean by going further through the solution? For example in a standard copper-zinc electrochemical cell Zn2+ will form and excess NO3- will remain. So the electrical conductivity of the solution represents the NO3- moving to the Zn2+ solution?

If so how would having a higher salt concentration help that conduction process?

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