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.

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 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 NaCl()aq.

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.

Definition: Electrolyte

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

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.

SaltCationAnionSolubility (Grams/100 Milliliters of Water at 20C)
Lithium sulfate (LiSO24)Li+SO4234.8
Sodium chloride (NaCl)Na+Cl35.9
Magnesium nitrate (Mg(NO)32)Mg2+NO3139
Calcium carbonate (CaCO3)Ca2+CO320.0006

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: 2NaCl()2Na()+Cl()llgSodiumchlorideSodiumChlorineelectrolysisC2

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: HO()H()+O()222lggWaterHydrogenOxygenelectrolysis12

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: Na()+HO()NaOH()+H()slaqgSodiumWaterSodiumhydroxideHydrogen2212

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: HO()H()+OH()2+laqaqWaterHydrogenionHydroxideion

So, we can write the production of hydrogen gas as the reduction of hydrogen ions that happens at the cathode: 2H()+2eH()+2aqg

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: 2Cl()Cl()+2e2aqg

If, instead, the hydroxide ions were oxidized, this is the equation for the process: 4OH()2HO()+O()+4e22aqlg

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.

Type of Salt in SolutionAnionProduct at Anode
Fluoride, chloride, bromide, and iodideF, Cl, Br, IF2, Cl2, Br2, I2
Nitrate and sulfate (and other salts)NO3, SO42O2

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

Example 1: Identifying the Ions Present in a Solution of Strontium Chloride

What ions are present in an aqueous solution of strontium chloride?

  1. Sr2+, H+, Cl, OH
  2. Sr2+, Cl, OH
  3. Sr2+, H+, Cl
  4. Sr2+, Cl
  5. H+, OH

Answer

An aqueous solution of strontium chloride is what we get when we dissolve strontium chloride (a metal salt) in water.

The element strontium is in group 2, so we would expect strontium to form 2+ ions. This is our first ion: Sr2+.

The element chlorine is in group 17, so we would expect chlorine to form 1 ions. This is our second ion: Cl.

Is that everything?

Well, not yet.

Water can dissociate into hydrogen ions and hydroxide ions: HO()H()+OH()2+laqaqWaterHydrogenionHydroxideion

While the concentration in pure water is very low (only 0.0000001M at room temperature), these ions have an important role in processes like electrolysis, so they should be accounted for.

Therefore, the ions present in an aqueous solution of strontium chloride are Sr2+, Cl, H+, and OH.

The correct answer is A.

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: 2NaCl()+2HO()2NaOH()+H()+Cl()aqlaqggSodiumchlorideWaterSodiumhydroxideHydrogenChlorine222electrolysis

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.

SolutionCationAnionIs Cation Element More Reactive Than Hydrogen?Product at the CathodeIs Anion a Halide?Product at the Anode
Copper(II) chloride
(CuCl()2aq)
Cu2+ClNoCu()sYesCl()2g
Sodium chloride
(NaCl()aq)
Na+ClYesH()2gYesCl()2g
Copper(II) sulfateCu2+SO42NoCu()sNoO()2g
Sodium sulfate
(NaSO()4aq)
Na+SO42YesH()2gNoO()2g
Acidified water
(HSO()24aq)
H+SO42H()2gNoO()2g
Concentrated hydrochloric acidH+ClH()2gYesCl()2g

If we add sulfuric acid to water, we do not end up with sulfur (S), hydrogen sulfide (HS2), or sulfur oxides (SO2, SO3). We end up with the same products as the electrolysis of pure water (H2, O2). In effect, the sulfuric acid makes the water conductive, helping the electrolysis of the water.

Example 2: Identifying the Equation That Shows the Reaction at the Cathode during the Electrolysis of Potassium Chloride Solution Using Inert Electrodes in a Set of Equations

What equation shows the reaction at the cathode during the electrolysis of potassium chloride solution using inert electrodes?

  1. 2H+2eH+2
  2. 4OH2HO+O+4e22
  3. 2ClCl+2e2
  4. 2HH+2e+2
  5. KK+e+

Answer

Potassium chloride is a salt consisting of potassium ions (K+) and chloride ions (Cl). When performing electrolysis of an aqueous solution of potassium chloride, the positive potassium ions are attracted to the cathode and the negative chloride ions are attracted to the anode.

If the electrodes are inert, that means that (for this system) they will not react. All they will do is channel electrons into or away from the electrolytic cell.

Let’s focus on the cathode.

At the cathode, potassium ions could be converted into potassium metal, but potassium is more reactive than hydrogen. So, instead, hydrogen is displaced from water and the potassium ions remain in solution.

We can think of water molecules as being made of hydrogen ions and hydroxide ions, and there would be small concentrations of both in our solution. So we can show the production of hydrogen gas by writing the reduction of hydrogen ions: 2H()+2eH()+2aqg

This corresponds to the equation 2H+2eH+2

We would not see potassium metal as it takes more energy to convert potassium ions to potassium metal than to convert hydrogen ions to hydrogen gas.

Therefore, the correct answer is A.

Example 3: Writing the Equation for the Reaction at the Anode during the Electrolysis of Copper Sulfate Solution Using Inert Electrodes

What equation shows the reaction at the anode during the electrolysis of copper sulfate solution using inert electrodes?

  1. SOSO+O+2e2422
  2. CuCu+2e2+
  3. SO+O+2eSO2224
  4. 4OH2HO+O+4e22
  5. 2H+2eH+2

Answer

When discussing copper sulfate, we can safely assume we are dealing with the more common copper(II) sulfate (CuSO4). If you electrolyze a solution of copper sulfate, you will break it down.

The positive Cu2+ ions will be attracted to the cathode. There, they will be reduced to form copper metal. But what about the reaction at the anode?

Water can dissociate into hydrogen ions and hydroxide ions.

When negative ions migrate to the anode, one of two things can happen: they will be oxidized or hydroxide ions from water will be oxidized.

The way we can tell is if the anion is a halide; if we had copper(II) chloride, chlorine gas will be produced at the anode as chloride ions are oxidized.

However, we have copper(II) sulfate. Instead of oxidizing the sulfate anion, hydroxide ions from the dissociation of water will be oxidized.

When hydroxide ions are oxidized, the products are water, oxygen, and electrons: 4OH2HO+O+4e22

The correct answer is D.

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 100C).

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.

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 dissolved in water 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 simple salt solution is electrolyzed using inert electrodes, the products depend on two things:
    • If the cation is of an element that is more reactive than hydrogen, hydrogen gas will form at the cathode; otherwise, the pure elemental form of the cation will form.
    • If the anion is a halide, the equivalent halogen will form at the anode; otherwise, oxygen gas will form.
  • The reactivity series can be used to predict which materials will be inert; if the electrode material is less reactive than elements that make up the salt, the electrode will be unreactive during electrolysis.

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