Conducting current in electrolytes

Question

I keep trying to figure out how current is conducted through an electrolyte but all I can find are incomplete answers. They say the ions conduct, but the specifics are poorly explained or absent.

I understand that if you, for example, put sodium chloride in water it will disassociate into positive sodium ions and negative chlorine ions, and that an electric field would cause the ions to move, allowing a current to flow. But what happens once the charges get to the electrodes? Obviously, they can't just stay there in elemental form stably.

I would expect the sodium, once given an electron, to be looking for a way to give that electron back. Would it cause the water to split into hydrogen gas and hydroxide ions? On the chlorine side, would the chlorine atoms simply turn into chlorine gas.

If this is all correct, it seems one would end up with sodium and hydroxide ions. Does it still conduct then? If so, how so? And don't you have a solution of sodium hydroxide? It seems this process necessitates that the water be split.

Is there any salt or other compound which would act similarly but not result in electrolysis (just conducting)?

Answer 1

An approach using a classical model of electron flow

You can think of an electron flowing through a medium as analogous to a pinball bouncing around a pinball machine. The electric (potential) energy difference driving the displacement of the electron is analogous to the gravitational energy difference in the pinball machine. Now consider two pinball machines with differing numbers of obstacles to a falling pinball. The time it takes for a pinball to reach the bottom in each machine would be different by virtue of these obstacles (on average, taking more time with more obstacles). Similarly, the flow of electrons, I, in the presence of an electric potential, V, is higher for media with less resistance, R , to electron flow. We can use this model (Drude see below) to understand Ohm's law: $$I = \frac{V}{R}$$ The observation that electrons flow more readily through electrolyte solutions than non-electrolyte solutions indicates that solutions with charge carriers reduce the resistance of the media. A better understanding of why this is true at a molecular level will require a more in depth look. Notice, though, that we now have answers to your good questions!

"What happens once the charges get to the electrodes?"

The sodium stays in solution as an ion, but the hydrogen ion (from the hydrolysis of water) can be reduced to form hydrogen gas at the cathode, and the chlorine ion can be oxidized to form chlorine gas at the anode. Electrons can be passed through the solution as solvated electrons under some circumstances, but this is unlikely due to the larger energy barrier to this process (around 3 eV). With sufficient energy, these electrons (called solvated electrons) flow through metal and solutions. The resistance to electron flow in metals is much less than in electrolyte solutions, though.

"Would it cause the water to split into hydrogen gas and hydroxide ions?" and "I would expect the sodium, once given an electron, to be looking for a way to give that electron back."

Any additional redox reaction would require electric potential. You mentioned the splitting of water or the formation of sodium metal. This would not occur according to the standard electrode potential unless the electric potential provided is sufficiently high to overcome the "uphill" energy barrier and as you mention electrolysis would be involved.

The Drude Model

In 1900, Paul Drude used the following schematic of a lattice of metal ions sitting in a sea of electrons to derive a relationship between electron flow and electric potential applied.

Drude Model electrons (shown here in blue) constantly bounce between heavier, stationary crystal ions (shown in red). -From wikipedia entry on Drude model.

A metal's electrical resistance, R, (as seen in Ohm's law above) should take the form:

$$R = \left(\frac{m}{nq^2\tau}\right)$$

where, $m$ is mass of electron; $n$ is number density of electrons; $q$ is charge of electron and $\tau$ is the mean free path. While this classical model does not explain the resistances for all substances perfectly and quantum mechanics can be used to do a better job, it does suggest that if the mean free path for electron flow is increased and the assumptions of the model are valid, the electrical resistance of the media should be reduced.

One way to understand why there is lower electrical resistance (higher electrical conductance) in aqueous solutions of electrolytes is that there is less water in the way of the Drude ion lattice! In other words, in the limit that the solution is a molten salt, we approach a Drudian model. As you add more and more molecules of water to this model, you are reducing the mean free path of the electron and increasing the resistance of the solution.

Notes

1. The fact that an electrolyte solution is comprised of positive and negative ions does not diminish the utility of Drude's model: electrons are equally attracted (and speed up) moving toward a (metal) cation as they are attracted (and slow down) moving away from a metal cation. The situation is reversed but the effect is the same for a nonmetal anion.

2. For more evidence of the utility of note 1., read about the additive nature of the molar electrical conductance (inverse of resistance) of ions (cations and anions): Kohlrausch's Law of independent migration of ions. The additive nature suggests that both cations and anions improve the conductance of an electrolyte solution as the note above suggests.

Answer 2

Actually if you look at electroplating you would see that the metal ions once give an electron at the cathode do turn into elemental metals and plate the electrode. This is how you electroplate things. You can't conduct forever as eventually you will deplete the ions in the solution, much like a battery will run out of charge if it is driving a current rather than the reverse.

If you use molten salt, you will get sodium at one electrode and chlorine gas at the other. If you use a solution you will either get O2 and H2 at the two electrodes, or the ions precipitating out depending which reaction is more favourable.

Answer 3

In this answer, I'll discuss conducting electricity in electrolytes for three different scenarios, an electrolytic cell, a Voltaic cell, and an electrophoresis setup.

Electrolytic cell

Let's take the synthesis of elemental sodium from a sodium chloride melt as example. The net reaction is:

$$\ce{NaCl(l) -> Na(s) + 1/2 Cl2(g)}$$

The electrical power source provides electrons to the cathode, where they reduce sodium cations that traveled from the melt. Chloride anions from the melt travel to the anode, and turns into chlorine gas, providing electrons to the anode going back to the electrical power source. "Ion" designates movement, cations travel to the cathode and anions travel to the anode.

I would expect the sodium, once given an electron, to be looking for a way to give that electron back. Would it cause the water to split into hydrogen gas and hydroxide ions? On the chlorine side, would the chlorine atoms simply turn into chlorine gas.

The purpose of this setup is to produce elemental sodium. You run this in the absence of water, and cool down the sodium and store it away from water, air, and other substances that would react with it.

The same textbook chapter I linked to also discusses the electrolysis of brine, salt water. Here, the net reaction is:

$$\ce{2Cl-(aq) + 2H2O -> 2OH-(aq) + H2 + Cl2(g)}$$

Again, chloride ions are oxidized to chlorine gas. Sodium ions, however, do not undergo reduction, even if they are the most concentrated cations in the brine. Instead, water provides hydrogen ions that are reduced.

Voltaic cell

Let's take the Daniell cell as an example. The net reaction is:

$$\ce{Zn(s) + Cu^2+(aq)-> Zn^2+(aq) + Cu(s) }$$

You have the following half-reaction at the anode:

$$\ce{Zn(s) -> Zn^2+(aq) + 2 e-}$$

There is a charge separation. The electron remains on the electrode (making it more negative), and the zinc ion goes into solution (making it more positive). In this case, there are no anions traveling all the way to the anode; instead, the cations travel from anode into the solution.

For completeness, here is the half-reaction at the cathode:

$$\ce{Cu^2+ + 2 e- -> Cu(s)}$$

But what happens once the charges get to the electrodes?

The charge transport between electrode and solution is already accomplished by the half reaction. The role of the ions in solution is the charge transport between the two half cells. This does not involve any redox process.

Source: by Rehua, https://commons.wikimedia.org/wiki/File:Galvanic_cell_labeled.svg

In the Daniell cell, adding zinc ions to the left half cell results in an excess of cations in the solution (excess of positive charge). Removing copper ions from the right cell, on the other hand, results in an excess of anions in the solution (excess of negative charge, to be accurate).

This imbalance is restored by ions (no matter which) traveling through the salt bridge in the appropriate direction. Depending on the mobility of these ions, there might be more cations or more anions moving, but the charge balance is restored in either case.

In this scenario, a sodium ion can help with conducting electricity. However, the sodium never receives an electron (nor does the zinc ion or the sulfate ion in my example). Ions transport charge just by going from A to B. They are charged already, they don't need to "carry" an extra electron. This is different from an electron carrier that shuttles back and forth between two locations, picking up electrons in one and delivering in the other. In the Daniell cell example, you want to avoid zinc ions reaching the other half cell because they would react directly with the copper electrode, without generating a current. One solution to this problem is adding a high concentration of KCl to the salt bridge. The charge imbalance between half cells will be restored by potassium and chloride ions exiting the salt bridge rather than ions going through it from one to the other side.

I keep trying to figure out how current is conducted through an electrolyte but all I can find are incomplete answers.

Your misconception is that you suppose there is a need for electrons to be transported through the solution. However, there is no closed circuit of electrons. The anode acts as a source, and the cathode acts as a sink for the electrons traveling through the wire. This is different from the electrons moved by a generator (electromagnet in a magnetic field), where you do have a closed circuit for the electrons (if the load also lets the electrons go all the way through, like in a incandescent bulb or a resistor, but not like a capacitator or a battery you are charging with the generator).

Electrophoresis

In electrophoresis, you want to separate ions based on charge and shape differences. This is often done in a gel that is easier for small particles to travel through. An electrical field is used to move ions in one direction. Here, you need ions to buffer the solution and to stabilize the sample, and the conductance ideally is as low as possible. Some biochemical methods use bulky ions with low mobility (as in TBE buffer), avoiding the need for buffer recirculation. The samples (protein, DNA) move along the field gradient, contributing a tiny amount to electric conductivity. Most of the conductivity is due to the buffer components, and they contribute to heating the gel, which is often not desirable. The redox reactions at either electrode are a nuisance, but are unavoidable.

Answer 4

An electrolysis occurs when two electrodes are dipped into a NaCl solution, and when the two electrodes are charged. One of the electrodes is positively charged and attracts negative ions like $\ce{Cl-}$, also called anions. This electrode is called anode. The other electrode is negatively charged, and attracts the cations, or positive ions, like $\ce{Na+}$. This electrode is called cathode. Remember : AN-AN, CAT-CAT. Anions go to the anode, and cations to the cathode, during electrolysis.

When they arrive at their corresponding electrode. anions and cations have a tendency to loose their charge, be discharged and become neutral species. This happens with solutions of hydrochloric acid, for example. where the half-equations occurring at each electrode are : $$\ce{Cathode : 2 H+ + 2e^- -> H2 \tag{1}}$$ $$\ce{Anode : 2Cl- -> Cl2 + 2 e- \tag{2}}$$ This type of reaction occurs often, but not always. It often happens that water interferes with the previous process. Another reaction happens with water that replaces the preceeding ones. This is the case when the redox potential of water is more adapted to the reaction. The new half-equations are then : $$\ce{Cathode : 2H2O + 2e^- -> 2OH^- + H2 \tag{3}}$$ $$\ce{Anode : 2 H2O -> 4 H+ + O2 + 4 e- \tag{4}}$$ In the electrolysis of NaCl, $\ce{Na+}$ is not produced at the cathode because ($3$) replaces ($1$), and pure $\ce{Cl2}$ is not produced at the anode, but ($4$) replaces partly ($2$), because the redox potential of water giving $\ce{O2}$ is $+1.13 $ Volt, which is smaller than the redox potential of $\ce{Cl2}$ ($1.36$ Volt).

As a consequence, the electrolysis of $\ce{NaCl}$ produces $\ce{H2}$ at the cathode according to ($3$) and $\ce{Cl2}$ (+ a little $\ce{O2}$) at the anode according to ($2$) and ($4$). Simultaneously the solution is enriched in $\ce{OH^-}$ around the cathode, and in $\ce{H+}$ (or $\ce{H3O^+}$) around the anode. If the electrolysis had been done only with half-equations ($3$) and $(2)$, the solution would remain neutral by mixing, as the same amount of $\ce{H+}$ and $\ce{OH-}$ had been produced in solution. But, due to the relative simultaneity of reactions ($2$) and ($4$) around the anode, more $\ce{OH-}$ are produced at the cathode by ($3$) than $\ce{H+}$ (or $\ce{H3O+}$) at the anode by ($4$). As a consequence, the solution becomes slowly basic.

In the long range, the concentration of $\ce{OH-}$ becomes high enough to allow a new reaction happening in solution between the chlorine $\ce{Cl2}$ produced by ($2$) at the anode and the $\ce{OH-}$ ions produced by($3$) at the cathode : $$\ce{Cl2 + 2 OH- -> OCl- + Cl- + H2O}$$ The solution gets enriched in the hypochlorite ion $\ce{OCl^-}$. It is then called "bleach".

Answer 5

I would expect the sodium, once given an electron, to be looking for a way to give that electron back. Would it cause the water to split into hydrogen gas and hydroxide ions? On the chlorine side, would the chlorine atoms simply turn into chlorine gas.

If a nonreactive electrode material such as platinum is used in aqueous NaCl solution, sodium ions are unlikely to gain electrons because their reduction would compete with other more favorable reactions, but in the chlor-alkali process involving mercury electrolysis cells and sodium chloride solution, sodium is reduced at a liquid mercury cathode to form an amalgam and is later allowed to react with water in a second step to form sodium hydroxide. As other answers explain, different reactions may occur depending on the voltage applied and redox conditions determined by the composition of the solution, the electrodes used, and the temperature. A reaction can occur if its redox potential is smaller than the applied voltage. Secondary reactions might happen afterwards. For instance, reactive chlorine may dissipate and react with hydroden collected above the cell forming HCl gas, or react with the electrode to form metal chlorides which may oxidize on exposure to oxygen, or form hypochlorite (as explained in other answers).

In any case, after participating in the redox reactions at the electrodes, the ions have contributed to the transport of charge through the circuit connecting the voltage source.

If this is all correct, it seems one would end up with sodium and hydroxide ions. Does it still conduct then? If so, how so?

Conductance describes how readily a substance (metal, solution, gas, etc) allows charged species to move through it, for instance the ease with which an electric current flows through a metal in response to application of a voltage, and is related to the familiar ohmic resistance (in the case of a constant voltage) and more generally to the impedance (for the AC case). For electrolyte solutions containing simple salts the conductance may be estimated from the composition or can be measured with a conductivity meter. In an electrolyte solution ions facilitate movement of charge and account for conductance. Sodium and hydroxide ions would contribute to this.

And don't you have a solution of sodium hydroxide? It seems this process necessitates that the water be split.

Yes, see the previous comments and other answers. Fittingly since the word "electrolysis" means "splitting with electrons". Accordingly hydrolysis means "splitting water".

Is there any salt or other compound which would act similarly but not result in electrolysis (just conducting)?

Faradaic currents can be minimized by applying a low voltage. If the voltage is less than the potential of possible redox reactions the cell will behave like a capacitor. The faradaic current due to electrolysis will then be negligible and any current will be primarily capacitive. A DC voltage source will result in a temporary current as observed when a capacitor charges. As an increasing counter-charge of ions associates with the electrodes and the double layer capacitance at the electrodes builds up the current will decrease. Once a double layer has completely formed there no net electric field will remain in the solution to drive a current. When this happens depends on the size (surface area) of the electrodes, the geometry of the cell, and yes, the composition of the electrolyte.

A conductivity meter applies an alternating voltage between a set of non-reacting electrodes immersed in the fluid and has a very high internal impedance, thus generating only a tiny current (a few mA at most) so that limited reaction or polarization occurs at the electrodes. This would be very close to the ideal scenario of "just conducting".

Answer 6

Matter is ENERGY, the lowest states of energy. Each stable compound, particle, atom, ion, is in its own relative energy minimum surrounded by relative maxima of activation energies. These activation energies can be as small as Van der Waals energies, as large as chemical bond energies, nuclear energies and extreme gravity, or simply the entropy of separation. Energy, in the forms of radiation and molecular kinetic, rotational, vibrational, and electronic energy differences, is the catalyst that nudges or propels each of these entities from their local energy minima. This is well known to all the living; when things are together, THEY REACT. Chemists and other scientists have been working for millennia to discover what, how, and why this happens.

Matter is composed of nuclear particles and electrons. The nuclear particles coalesce to form atomic nuclei of which are the several main types, the elements. Each element has its number of protons, positive charges, and the same number of electrons negative charges, each particle in its own relative minimum energy level. Chemistry is concerned mostly with the electrons and their energies. When energy is removed everything will end up in the lowest energy minima, fortunately these are still relative minima and energy is still available. Electrons will attain the lowest energy levels when energy is removed; they will be excited from these levels when energy is supplied. The electrons in each atom or molecule or ion are in different energy levels. When there is a possible mechanism and an appropriate energy flux the electrons will rearrange to the lowest energy levels. The nuclei are involved also and will rearrange position. The negative electrons attract the positive nuclei and repel each other forming the bonds that hold it all together.

A chemical symbol, formula, equation is a symbol for all the properties of each moiety, both known, yet to be discovered and the unknown. We use the easiest set of properties for a situation. With the application of energy anything can be changed into anything. In the chem lab this is restricted to rearrangement of the orbitals of electrons and the position of nuclei. Sodium metal can be easily changed into hydrogen, but sodium nuclei cannot easily be changed into free protons. Since every substance is at a different energy level if there is a possible mechanism and either a natural or imposed energy difference anything can be changed into anything.

Metals are an array of nuclei bound together by electrons in conduction bands The electrons are labile to a different degree in each metal and this is exhibited in their redox potentials that is really their ability to react with H3O+ ion or H2 gas. A metal placed in water will react to form ions and achieve a slight negative charge. The amount depends on the energy level of the metal. Eventually the charge buildup and the ion concentration reach equilibrium. If the electrons are enough and of high enough energy such as in sodium metal they will react with water, lower energy electrons such as from zinc or iron react slower, very low energy gold, platinum hardly react at all. If a supply of electrons is added a reaction will happen and a chemical capacitor forms until there is an electric discharge or dielectric breakdown. This is what happens in a lightning bolt or when one puts one's finger in a live socket. This involves the propagation of the electric field at the speed of light and the motion of ions that depends on the viscosity and temperature and the specific ions. One electrode results in either an uncontrolled corrosion reaction, no apparent reaction or possible chaos when the other electrode is encountered. [In corrosion there are two electrodes usually very close together.] What has been established is there is an electrode reaction furnishing ions and electrons. Ideally this is a reversible reaction and can be an anode, oxidation M = M+ + e- or cathode, reduction e- + M+ = M. A cathode and anode that are in appropriate physical contact result in a chemical reaction.

The chemical or electrical cell is a method of separating the oxidation, anode, reaction from the reduction, cathode, reaction both in space and in time so the energy can be extracted in forms other than heat. A cell consists of an anode, a cathode, an electrical connection between the two and an ionic pathway between the two. The physical differences can be intimate in a corrosion cell, reasonable in a battery or lab electrical cell or miles apart in a lightning bolt. There must be an anode reaction and a cathode reaction.

In batteries or laboratory cells the electrodes are physical entities that are electrical conductors and are capable of donating electrons to ions or molecules in solution or of transferring ions to or from the solution. They can be inert to the reaction or can be actively involved or be both if several reactions are involved. Oxidation happens at the anode. Electrons are removed from a chemical entity. This entity can be a positive ion, a neutral molecule or a negative ion. The chemistry requires that the process happens at a reasonable rate. These electrons leave the anode into the electrical connector or wire and establish a potential the electrons migrate either thru a device or a short circuit to a lower potential, the cathode. The cathode again is a conductor that can release electrons to species in the solution or transfer ions to or from the solution. The cathode again can be inert or reactive.

Potentials are established at the anode and cathode; to complete the cell there must be a cathode reaction, an anode reaction, and a method of maintaining ionic neutrality. The cathode reaction is reduction. Electrons from the anode accumulate on the cathode, the cathode acquires negative charge, and the electrons are transferred to a species that can accept them: a positive ion, a neutral molecule, some limited negative ions, a solid material with a lower oxidation state. As long as electrons are supplied to the cathode the charge must be neutralized, by loss of anions or the gain of cations. Cations, positive ions migrate to the cathode; anions, negative ions, migrate from the cathode. This flow of ions extends across the solution by diffusion. The rate is determined by the viscosity of the solution, the mobility of the various ions, physical size and structure, and the potential difference between the cathode and anode. Since these processes are partially diffusion-controlled concentrations can accumulate and back voltages are generated, and cell potentials can differ from ideal. In nature this is accommodated by dielectric breakdown, static electricity and lightning discharge; in the lab by careful cell design, stirring and making voltage measurements at minimal current draw. These changes in ion concentrations propagate thru the liquid and would stop except at the anode there is an anode reaction. Oxidation happens at the anode. Electrons are removed from anions, neutral molecules or oxidizable cations and flow into the wire back to the lower potential cathode. The solution potentials are neutralized by migration of ions. A simplified net result is that anions migrate to the anode and cations migrate to the cathode where each reacts to maintain electrical neutrality.

Since the solution electrodynamics are governed by the motion of particles the processes require time. The electric fields propagate at near the speed of light but the ion mobility is closer to the speed of sound. This is why batteries lose voltage under heavy usage to partially recover after rest. The polarization is reduced but the loss of reactants lowers the voltage. Battery construction is art as well as science.

An electrochemical cell is a chemical potential difference between an anode and cathode, an appropriate anode material, an electrical connection to an appropriate cathode material, a cathode reaction, an ionic connection between the cathode and anode, and an anode reaction. These can be subtle as in nerve synapses, reasonable as in dry cells, cell phone batteries or Tesla batteries or dramatic in lightning bolts.

The reaction at the anode is oxidation, electrons leave the cell. The reaction at the cathode is reduction, electrons enter the cell. The voltage of the cell depends on the energy differences or chemical potential differences between the anode and cathode. Anode and cathode voltages are a function of the material and the concentrations of the involved chemicals. The cell voltage is the sum of the anode and cathode voltages modified by any physical constraints such as electrode shape, size, location; ionic conductivity, current draw. The ideal voltage is given by the Nernst equation, https://en.wikipedia.org/wiki/Nernst_equation . Ion mobility is the result of a continuing attempt to maintain electrical neutrality in an electrical field resulting from a chemical potential difference between 2 substances with a possible mechanism to be interconverted. When the chemical potentials, voltages, of the anode and cathode reactions are equal the cell has reached equilibrium. The battery has died.