Lesson Explainer: Electrolysis of Molten Salts Chemistry

In this explainer, we will learn how to describe the components of an electrolytic cell and predict the products of the electrolysis of molten salts.

Some chemicals are so stable that it is very difficult to find chemicals that will break them up. However, we can use electrical fields and electrical currents that are far more powerful than individual chemicals.

“Electrolysis” is a combination of “electro-” (referring to electrons/electricity) and “-lysis” (meaning to set free or to break apart). We can use electricity to break apart substances, turning them into pure elemental substances or other chemicals.

Definition: Electrolysis

Electrolysis is a type of process where an electric current is passed through a liquid or a solution containing ions, which causes the substances inside to decompose.

If we are performing an electrolysis, it will generally be because we are performing a nonspontaneous endothermic reaction—the products will be less stable than the reactants. This difference in energy is provided by the battery or power supply that drives the redox reaction.

In this explainer, we will be focusing exclusively on the electrolysis of molten salts, like NaCl()l.

To sustain an electrolytic reaction, we need to have a complete circuit; we need to be able to continuously get power from the battery or power supply. To do this, we need our ions to be able to move.

In electrolytic cells, we use a direct current power source, meaning that the electrodes are always either positive or negative. With an alternating current power supply, the electrodes would switch between being positive and negative many times a second (which is not very helpful if you want to collect the decomposition products from just the anode or cathode).

Instead of a dc power supply, we could use a cell or a battery.

Inside the electrolytic cell, we want a fresh supply of ions at each electrode so that we can produce the products. This is not going to happen if the ions cannot move.

The most obvious way to make ions mobile it to make them into a liquid.

A substance or mixture that conducts electricity and can undergo electrolysis is called an electrolyte.

Definition: Electrolyte

An electrolyte is a type of substance or mixture that contains mobile ions that can undergo electrolysis.

In practice, melting a salt can be a lot more difficult than it sounds. Salts are ionic compounds and they tend to have very high melting points because of strong ionic bonds between their cations and anions.

Here are the melting points of some common salts.

SaltCationAnionMelting Point (C)
Lithium chloride (LiCl)Li+Cl605
Sodium chloride (NaCl)Na+Cl801
Magnesium chloride (MgCl2)Mg2+Cl714
Calcium chloride (CaCl2)Ca2+Cl772

Example 1: Identifying the Reason for Melting Zinc Iodide before Beginning Electrolysis From a Set of Reasons

In a laboratory demonstration of the electrolysis of zinc iodide, a teacher melts the compound before turning on the electricity. What is the reason for this step in the procedure?

  1. This step ensures good connections between the substance and the electrodes.
  2. The zinc and iodide ions must be free to move.
  3. The reaction is exothermic.
  4. Melting the salt ensures a good rate of reaction.
  5. This step removes water molecules adhering to the zinc iodide powder.

Answer

When performing electrolysis, we want to be able to get our reactants to the electrodes. Our reactants are the zinc ions (Zn2+) and iodide ions (I) in the zinc iodide. However, zinc iodide is a solid at room temperature.

In an ionic solid, the ions are fixed in place. They may vibrate on the spot, but they cannot move, even if put up against a highly charged electrode. By melting zinc iodide, the zinc and iodide ions are able to move over each other and react at the electrodes.

A good test of whether there are mobile ions present is if an electrical current will flow around an external circuit—solid zinc iodide is not conductive, but liquid zinc iodide is.

If we consider these points together, the correct answer is B.

When a simple molten salt composed of only two elements is electrolyzed, we expect the products to be the elemental forms of the anion and cation:

These may go on to react with other things around them, but we will come to that later.

Two half-equations match this process. The first is the reduction of lithium ions: 2Li()+2e2Li()+ll

And the second is the oxidation of chloride ions: 2Cl()Cl()+2e2lg

The total reaction that occurs in the electrolytic cell is the sum of these two half-equations: 2Li()+2Cl()2Li()+Cl()+2lllg

Example 2: Identifying the Molten Salt That Would Produce Sodium Metal and Chlorine Gas When Electrolyzed in a Set of Formulas

Which molten salt would produce sodium metal and chlorine gas when electrolyzed?

  1. MgCl2
  2. NaF
  3. NaCl
  4. NaBr
  5. AlCl3

Answer

If we melt a salt, we will produce a liquid. Ions in a liquid are able to move around.

When we electrolyze a molten salt, cations and anions will be attracted to whichever electrode has the opposite charge (cations to the negative electrode and anions to the positive electrode).

At the electrodes, reduction and oxidation reactions can occur: cations are reduced (they gain electrons), and anions are oxidized (they lose electrons).

For a simple salt containing only two elements, we expect the products to be the pure elements: sodium salts produce sodium metal, and chlorine salts produce chlorine gas.

The simplest salt that contains both sodium and chlorine is sodium chloride (NaCl): NaCl()Na()+Cl()Na()+eNa()Cl()Cl()+ellllllg++212

The melting point of sodium chloride (801C) is much higher than the melting point of sodium (98C), so sodium metal would be produced as a liquid. Thus, the correct answer is C.

In the molten salt, anions (negative ions) are attracted toward the positive electrode, which we call the anode. Cations (positive ions) are attracted to the negative electrode, which we call the cathode.

This diagram shows the movement of cations toward the cathode and anions toward the anode.

Once cations reach the cathode, they can be reduced—they gain electrons.

Once anions reach the anode, they can be oxidized—they lose electrons.

This diagram shows the addition of electrons to cations at the cathode (reduction) and removal of electrons from anions at the anode (oxidation).

The reaction at the cathode can involve more than one electron. The most common cations involved in electrolysis are metal cations, so the product is an elemental metal:

This is the half-reaction at the cathode (reduction):

And this is the half-reaction at the anode (oxidation):

The atoms produced at the anode and cathode can combine further to produce lumps or pools of metal, or molecules.

The simultaneous consumption of electrons at the cathode and production of the electrons at the anode provides a continuous “circuit.”

This diagram shows the complete circuit of an electrolytic cell.

The names anode and cathode refer to either the electrode that electron flow comes from (anode) or goes to (cathode).

Definition: Anode

An anode is the electrode of an electrochemical cell that provides electrons to the external circuit.

In an electrolytic cell, the anode is the positive electrode.

Anions in the electrolyte travel toward the anode and are oxidized.

Definition: Cathode

A cathode is the electrode of an electrochemical cell that accepts electrons from the external circuit.

In an electrolytic cell, the cathode is the negative electrode.

Cations in the electrolyte travel toward the cathode and are reduced.

Example 3: Identifying the Anode or Cathode Based on an Experimental Setup Involving Mobile Ions

In the diagram shown, which electrode is on the right?

  1. The anode, as it is attracting an anion
  2. The cathode, as it is attracting an anion
  3. The cathode, as it is attracting a cation
  4. The anode, as it is attracting a cation

Answer

In an electrolytic cell, an external direct current power source (like a dc supply or a battery or cell) causes the decomposition of an electrolyte. The positive end of an electrical cell is drawn as a long line, and the negative end is drawn as a short line.

In the diagram, the negative side of the power source is on the right, and the positive side is on the left.

In an electrolytic cell like this, we expect cations in the electrolyte to move toward the negative electrode and anions to move toward the positive electrode. This is what we see: the cation in the electrolyte is moving to the right, toward the negative electrode, and the anion is moving to the left, toward the positive electrode.

At the electrode on the right, we expect cations to gain electrons (to be reduced). These electrodes will flow from the power source. This means that the electrode on the right is the cathode, and we know this because we are looking at an electrolytic cell and the cation in the diagram is moving toward it. Thus, the correct answer is C.

When designing an electrolytic cell, there are many factors to consider such as how quickly the products will be produced, what temperature is required, and the side reactions that might occur.

That third point is of particular importance when it comes to selecting what materials to make the electrodes out of.

It would be nice if the electrodes did not degrade and had much higher melting points that the molten salts. And, of course, they need to be conductive.

Unreactive metals like platinum are great for this kind of thing; platinum is unreactive and has a melting point of 1768C. But it is very expensive.

On the other hand, carbon is more reactive, and it is very cheap.

In the electrolysis of aluminum, massive carbon electrodes are used because they provide a large surface area and can be cheaply replaced. They do react with oxygen, turning into carbon dioxide, but in the long run, they are still much cheaper than platinum electrodes.

Electrodes made of more reactive metals are going to cause problems. For instance, an iron cathode would react with salts of copper, producing iron salts and copper metal, even without a power supply to drive the reaction.

When discussing the reactivity or inertness of an electrode material, we often consider the reactants we are going to use it with. If the material would not react with these particular reactants, we consider an electrode made from the material to be inert.

Key Points

  • Electrolysis is a type of process where an electric current is passed through a liquid or a solution containing ions, which causes the substances inside to decompose.
  • An electrolyte is a substance or mixture that contains mobile ions that can undergo electrolysis.
  • In electrolysis, we use direct current power sources (like power supplies, batteries, and cells) to provide the current.
  • The cations and anions that make up salts are mobile when in liquid form and are attracted to the cathode or the anode.
  • The anode is the electrode of an electrochemical cell where oxidation takes place.
  • The cathode is the electrode of an electrochemical cell where reduction takes place.
  • Cations are attracted to the negative electrode (the cathode) and are reduced.
  • Anions are attracted to the positive electrode (the anode) and are oxidized.
  • When a molten simple salt is electrolyzed using inert electrodes, the products are the elemental forms of the anion and cation: lithiumchloridelithiummetal+chlorinegas

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