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I always had the misconception that when people say ions conduct electricity, what they actually mean is a connection is made between the cathode and the anode by the ions and electrons can flow from the cathode to the anode through this connection.

However, I recently learned that ions conduct electricity because chemical reactions are happening around the electrodes.

Taking the example of the electrolysis of sodium chloride water solution. At the cathode, $Na^+$ captures electron ($Na^+ + e^- \rightarrow Na$); at the anode, $Cl^-$ releases electron ($2Cl^- \rightarrow Cl_2 + 2e^-$). And this is because the battery takes electrons from the anode and feeds electrons to the cathode, and positively charged anode attracts anions and negatively charged cathode attracts cations. That is to say electrons do not flow from cathode to anode but are given to and taken from the ions.

I still feel confused after knowing this explanation because it seems to me that the anode and the cathode do not need to be connected by aqueous electrolytes to "conduct electricity". Suppose the cathode and the anode are placed into different solutions in different containers, anode still attracts anions and cathode still attracts cations. So that there should still be electricity flowing through the circuit.

Some people say that a "closed circuit" is required for ions to conduct electricity. If that is the case then what am I missing here?

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  • $\begingroup$ Forget ions for a moment; chemistry is complicated. It is said that electrons conduct electricity in metals. What if I connect two pieces of wire to the positive and negative electrodes, but without a connection between the pieces? Will the electrons still flow from the negative electrode to the attached piece of wire? Why not? $\endgroup$ – Ivan Neretin Dec 19 '20 at 19:30
  • $\begingroup$ @IvanNeretin Electrodes are mostly made of metal so attaching wires do not change anything. Electrons will flow from the negative electrode to the wire, until the voltage between the two wires is the same as the voltage of the battery, just like how electrons can flow to the cathode. The reason why ions are different from metals in this case is that ions can give and take electrons, therefore creating non-stopping current until electrolytes are depleted (or not?). $\endgroup$ – Haitian Ling Dec 19 '20 at 19:57
  • $\begingroup$ Well, same thing here: the electrons will flow from the negative electrode to the solution, until the voltage between the two solutions is the same as the voltage of the battery. Ions may react all they want; what they can't do is to make the charge disappear. $\endgroup$ – Ivan Neretin Dec 19 '20 at 20:19
  • $\begingroup$ The question risen on the title Do ions still "conduct electricity" if a physical connection made by aqueous electrolytes between the anode and the cathode is nonexistent? does not make sense to me. If there is no electrolyte between the electrodes, where are the ions then and why to call them the anode and cathode ?? $\endgroup$ – Poutnik Dec 19 '20 at 21:51
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    $\begingroup$ @Poutnik Yes, the title can be misleading, but the emphasis is on "a physical connection" instead of "electrolytes". Ionized gases are essentially plasma and have free electrons, so they conduct electricity the same way metals do. And the question I'm having is not around conductivity either. $\endgroup$ – Haitian Ling Dec 19 '20 at 23:06
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If a cathode (negative pole) is placed alone in an ionic solution (without any anode in the solution), it will attract cations from the solution, and discharge maybe one or two or one thousand of these cations. This will last maybe 1 nanosecond, and then it will stop. The reason is that this number of atoms is extremely small, so small that the neutral atoms which are deposited on the cathode cannot be seen or weighed. And then the cathodic discharge will stop, because the whole solution has lost positive charges (cations), but has not lost any negative charges (anions). As a consequence, the whole solution becomes negatively charged. This negative charge increases with the time and prevents the cathode from bringing new electrons necessary for discharging further cations. The electrolysis will stop. The electrode is said to be "polarized".

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    $\begingroup$ Thank you @IvanNeretin and Maurice , I see what I'm overlooking here. So what happens is that the solution can become charged to create an electric field that cancels out the electrodes' ability to attract ions, therefore preventing chemical reactions, unless the solution is able to remain neutrally charged. I can now see how the analogy between metal and electrolytes applies here. When connected to a battery, metal is essentially also creating an electric field to cancel out the battery's ability to attract and repel electrons. $\endgroup$ – Haitian Ling Dec 19 '20 at 23:03
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I want to further expound upon your newly developed misconception as written in your comments although Maurice has already explained it well.

I see what I'm overlooking here. So what happens is that the solution can become charged to create an electric field that cancels out the electrodes' ability to attract ions, therefore preventing chemical reactions, unless the solution is able to remain neutrally charged.

No the bulk solution has to remain neutral all the time. There can be no charge build up in the electrolytic cell. Everything takes place at the interface of the dipping electrode and the solution next to it.

By definition, whenever electricity will pass through an electrically conducting solution, chemical reactions will take place. "Something" has to oxidize at the anode and "something" has to reduce at the cathode. That something could be the solvent, neutral molecules, ions or anything in the solution. Something has to lose electrons and something has to gain electrons.

I think you are looking for the right words, because it seems your confusion arises from the semantics. Ions carry electric charge, and rate of movement of charges constitutes current. Therefore, we say that nature of current in a solution is ionic current.

Suppose the cathode and the anode are placed into different solutions in different containers, anode still attracts anions and cathode still attracts cations. So that there should still be electricity flowing through the circuit.

Yes, do you recall a capacitor which are two disconnected plates of metal. When it is connected to a battery, quickly a small amount of current flow. It it is exactly like this scenario. It may be called as capacitive current.

Now remember that capacitors in a circuit do not allow direct current to pass through it. Similarly two separated electrodes sitting in two beakers will not allow direct current to pass in the external circuit.

Some people say that a "closed circuit" is required for ions to conduct electricity. If that is the case then what am I missing here?

Now this is a different situation. When you want the direct current to flow, then yes, the circuit has to be closed. Think of a battery as a recycling pump. Imagine a pump in your house, which fills up a water tank placed on the roof of your house, and this tank is connected to your faucets, and all that water you use is pumped back into the water tank. Dirty analogy but good for a battery.

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  • $\begingroup$ Thank you for going out of your way to help me better understand. I found your answer very informative. You said Maurice explained it well but I was mostly rephrasing Maurice's answer by saying that "the solution can become charged". What Maurice said is, "As a consequence, the whole solution becomes negatively charged". You pointed out that "There can be no charge build up in the electrolytic cell". Is this a contradiction or I'm missing something here? $\endgroup$ – Haitian Ling Dec 20 '20 at 4:40
  • $\begingroup$ The solution can never be charged by dipping an electrode and there can be no charge build up! It always remains electrically neutral all the time. Maurice may have written this incorrectly. $\endgroup$ – M. Farooq Dec 20 '20 at 4:59
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    $\begingroup$ @ M. Farooq. You are right. The solution never gets charged. I should have said that this "charged solution" is only a hypothesis stated to show that no chemistry happens around a cathode dipped alone in a solution, $\endgroup$ – Maurice Dec 20 '20 at 13:41
  • $\begingroup$ @ Maurice, There might be a small capacitive current but no redox process. $\endgroup$ – M. Farooq Dec 20 '20 at 17:20

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