Lesson Explainer: Electrolysis of Salt Solutions Chemistry

In this explainer, we will learn how to predict the products of electrolysis of aqueous salt solutions using the reactivity series.

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.

If we are performing electrolysis, it will generally be because we are performing an endothermic reaction—the products will be less stable than the reactants. This difference in energy is provided by the battery or power supply in order to make the reaction occur.

In this explainer, we will be focusing exclusively on the electrolysis of salt solutions, like .

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.

One way we can make ions mobile is to dissolve them in water.

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

In practice, some salts are more soluble than others. Salts are ionic compounds, with strong ionic bonds between their cations and anions. Ionic compounds conduct electricity when molten or in aqueous solution as the mobile ions are capable of carrying an electric current. Sometimes, the cations and anions stuck together are more stable than if they were dissolved, and we might describe salts of this type as insoluble. However, most simple salts that we will come across here will be soluble to some degree.

We tend to express the solubility of salts in grams (or milligrams) per 100 mL of solvent; this means that if we start with 100 mL of solvent, we can dissolve a maximum of that mass of salt in it. At this point, the solution would be considered to be saturated.

Here is a selection of salts with their solubilities.

When a salt solution is electrolyzed, the anion may be oxidized and the cation may be reduced. However, the presence of water makes things a little more complex. Also, the materials the electrodes are made of can make a big difference.

When we electrolyze a molten simple salt, like sodium chloride, it is very easy to predict what the products will be:

The products will be the elemental forms of the anion and cation (sodium metal and chlorine gas in this case).

However, when electrolyzing a solution, water itself can be electrolyzed:

So, when electrolyzing a solution of sodium chloride, there appear to be four possible products:

However, we know that sodium is extremely reactive. If, somehow, sodium is formed, it would immediately react with water, producing sodium hydroxide:

This eliminates sodium from our list of products and suggests that hydrogen should be produced instead:

Water molecules can dissociate into hydrogen ions and hydroxide ions:

So, we can write the production of hydrogen gas as the reduction of hydrogen ions that happens at the cathode:

As for chlorine and oxygen, the hydroxide ions migrate to the positive electrode (the anode) where oxidation takes place. At the anode, there is a competition between the hydroxide ions and the chloride ions to be oxidized.

Generally, for solutions with a reasonable concentration of chloride ions, the main product is chlorine gas. The full details of this are beyond the scope of this explainer. However, the overall process is below:

If, instead, the hydroxide ions were oxidized, this is the equation for the process:

The general rule we use is that in the electrolysis of halide salts, the halogen is produced. In the electrolysis of other salts, oxygen is produced.

The reasons for this are beyond the scope of this explainer.

For the electrolysis of sodium chloride solution, our expected products are chlorine at the anode and hydrogen at the cathode:

So, for the electrolysis of a solution of sodium chloride, we expect the reaction to follow this equation:

The diagram below shows the whole cell:

We can apply what we have learned to any simple salt solution.

As a general rule, if more than one type of ion could be oxidized, the more reactive ion will be oxidized and the remaining ions will remain in the electrolyte.

The only other thing we need is the reactivity series, which tells us generally how reactive a pure element is in relation to the others—a simple version of the reactivity series is shown below.

If an element is above hydrogen in the series, hydrogen gas is produced at the cathode. If an element is below hydrogen in the series, then the pure form of that element is produced instead.

Let’s have a look at some examples.

If we add sulfuric acid to water, we do not end up with sulfur (), hydrogen sulfide (), or sulfur oxides (, ). We end up with the same products as the electrolysis of pure water (, ). In effect, the sulfuric acid makes the water conductive, helping the electrolysis of the water.

What side reactions might occur is of particular importance when it comes to selecting what materials to make the electrodes out of.

It would be helpful if the electrodes did not react with water or the dissolved salt. For instance, it would be unhelpful if the electrodes rusted away, so iron would be a poor choice.

Unreactive metals like platinum are great for this kind of thing; platinum is unreactive, but it is very expensive.

On the other hand, carbon is more reactive, but it is very cheap. Carbon is even higher up the reactivity series than hydrogen, so, theoretically, carbon will react with water to produce carbon dioxide and with salts to produce carbon compounds. In practice, we do not need to worry about this because the activation energies for these reactions are so high that much higher temperatures would be needed. When doing electrolysis of a salt solution, temperatures would not exceed the boiling point of the solution (which would be close to ).

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

When discussing the reactivity or inertness of 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. For electrolysis reactions of salt solutions, we can consider carbon electrodes to be inert.