Water electrolysis: what is it, example, applications –

The electrolysis of water consists of the decomposition of this substance by means of an electric current and the addition of an electrolyte. Let’s better understand how this occurs?

Read too: What is electrolysis?

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How does electrolysis of water happen?

Water molecules are capable of self-ionization, generating H+ (or H3O+) and OH- ions:

H2O ↔ H+ + OH-

or
2 H2O ↔ H3O+ + OH-

However, water is a very weak electrolyte and, despite having these ions, it cannot conduct electric current. Thus, to carry out its electrolysis, that is, its decomposition by means of an electric current, it is necessary to add an electrolyte, an ionic solute that can be a salt, a base or an acid.

Priority order for selective unloading

However, as explained in the text Aqueous electrolysis, in this case, we will not only have the ions coming from the water, but also those from the substance that was dissolved in it. In electrolysis, only one cation and one anion are discharged at the electrode, that is, it is a selective discharge that follows an order of priority.

Thus, so that the cathode and anode that will be discharged are those of the water, and not those of the dissolved substance, it is necessary to choose an acid, a base or a salt whose ions are less likely to undergo discharge at the electrodes than the ions from water. To do so, we need to query the priority queue shown below:

Note that the cations listed under H+ are less easily discharged than H+. In the table on the right, we see that anions below OH- are less easily discharged. Therefore, we can choose, for example, a salt, a base or an acid that forms the ions Na+, K+, NO3-, SO42- and so on, in addition to also forming the same ions as water, that is, H+ and OH -. Some examples are: sulfuric acid (H2SO4), sodium hydroxide (NaOH) and potassium nitrate (KNO3).

Don’t stop now… There’s more after the publicity 😉

Example with the reactions that occur in the electrolysis of water

Let’s say that an electrolysis of water is performed with the addition of sulfuric acid. In this case, we will have the formation of the following ions in the medium:

Note that the only existing cation is H+, so it will undergo reduction (gain electrons) at the negative electrode (cathode) and produce hydrogen gas.O (Htwo).

Now, speaking of anions, there are two anions in between, which are SO42- and OH-. As the table above shows, SO42- is more reactive and less easily discharged. In this way, the OH- will be discharged, oxidizing (losing electrons) in the positive electrode (anode) and producing oxygen gas (Otwo):

Adding this whole process together, we arrive at the global equation:

• Water ionization: 8 HtwoO → 4H3O+ + 4 OH–

• Cathode half-reaction: 4 H3O+ + 4 and– → 4HtwoO+2Htwo

• Anode half-reaction: 4 OH– → 2 hourstwoO + 1 Otwo + 4 and–

• Overall equation: 2 HtwoO → 2Htwo +10two

Note that the volume of hydrogen produced is twice that of oxygen. However, in practice, this strict ratio is not verified because oxygen is more soluble than hydrogen gas.

Read too: Obtaining aluminum through electrolysis

Water electrolysis applications

The electrolysis of water is a very important process, considering that hydrogen is a gas that can be used as fuel. As fuels derived from petroleum are not renewable, hydrogen gas can become an important alternative.

In addition, gasoline production methods already exist that use the water electrolysis process. See how this is done in the text Scientists have managed to turn carbon dioxide into gasoline.

By Jennifer Fogaça
Chemistry teacher