Aqueous electrolysis: what is, how, exercises

THE aqueous electrolysis is redox reaction not spontaneous that occurs with the passage of electric current through a solution of ions dissolved in Water. To understand it well, it is important to know what electrolysis itself is. Follow up!

See too: What is igneous electrolysis?

What is electrolysis and what is it for?

Electrolysis is the name given to the chemical redox reaction that occurs caused by the passage of an electrical current. This reaction can happen in two ways: a igneous electrolysis and the electrolysis in aqueous ionic solution. The latter being of interest to us in this text.

In both types of electrolysis there are ions, the difference is that, in the first type, the ionic compound is cast and there is no water in the process, and in the second, as the name says, the ionic compound is dissolved in water.

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Electrolysis is a chemical process used for obtaining of chemical elements (like metals, hydrogen, beryllium, chlorine

, among others), for the galvanizing process, such as chrome plating and nickel plating, and also for electrolytic metal purification. If you are curious about this topic, read our text: Electrolysis concept.

Galvanizing process of metallic structures in a zinc bath.

Aqueous electrolysis

In aqueous electrolysis, we have an ionic compound dissolved in water, and this one, by dissociation or ionization, releases its ions into the solution, allowing the passage of electrical current. In addition to the ions released by the ionic compound, we must take into account the ions from the water autoionization:

H₂O → H⁺ + OH^-

Because there is a need for electric current for electrolysis to occur, we say, then, that it is a non-spontaneous process. which happens exactly to the contrary to the process seen in a stack, which, in turn, transforms the chemical energy derived from a reaction, to the production of electrical energy.

How does aqueous electrolysis occur

As already said, during aqueous electrolysis, we must take into account the water derived ions and the ions derived from the dissolved compound. See the example of dissociation of sodium chloride:

NaCl_(here) → In⁺_(here) + Cl^-_(here)

So the solution has two cations (H⁺ and on⁺) and two anions (OH^- and Cl^-). However, only one cation and one anion will undergo oxidation-reduction by the electrical discharge. To identify which of the two will be affected, we have a priority queue, represented below, in ascending order:

- Cations: Metals of the 1,2 and 13 family < H⁺ < other metals

- anions: Oxygen and F anions^- < OH^- < non-oxygenated anions

So, for the example of electrolysis in aqueous solution of sodium chloride, we have that the H ions⁺ and Cl^- will suffer an electrical discharge. Now, we will do the analyze of what happens at each of the poles:

• Cathode and anode

At the cathode, the negative pole of the electrolytic cell, the electrons reach the electrode and this is where the cations present in the solution migrate to. Therefore, this is where the discharge of the H cation occurs.⁺ and its reduction, according to the following equation:

2h⁺ + 2e → H_2(g)

At the anode, the positive pole of the electrolytic cell, the cations present in the solution Discharge and lose their electrons. Because it has the priority of downloading over the OH^-, the Cl^- migrates to the anode, where it undergoes oxidation according to the following equation:

2Cl^-_(here) → 2e + Cl_2(g)

We can write the general equation of the electrolysis process adding up the reactions of each step of the process: dissociation; the self-ionization of water; cation reduction; and the oxidation of the anion.

NaCl_(here) → In⁺_(here) + Cl^-_(here)

H₂O → H⁺ + OH^-

2h⁺ + 2e → H_2(g)

2Cl^-_(here) → 2e + Cl_2(g)

Balancing the equations and eliminating the items that are repeated in the reactants and products, we have:

2NaCl_(here) + 2H₂O_(liquid.) → 2Na⁺_(here) + 2OH^-_(here) + H_2(g) + Cl_2(g)

Analyzing the global equation, we still have the Na ions in solution.⁺_(here) and oh^-_(here), forming caustic soda (NaOH), one of the products of the reaction, in addition to the hydrogen gas, formed at the cathode, and the gas chlorine, formed at the anode.

See too:Quantitative aspects of electrolysis

solved exercises

Question 01 (UEG) Galvanizing is a process that allows giving a metallic coating to a particular piece. Shown below is an experimental apparatus, set up to enable nickel plating of a key.

In the process of coating the key with nickel, an X reaction will occur, represented by a Y half-reaction. In this case, the XY pair can be represented by:

a) reduction, Ni⁺ + 1e^– → Ni(s)

b) reduction, Ni(s) → Ni²⁺ + 2e^–

c) oxidation, Ni²⁺ + 2e^– → Ni(s)

d) oxidation, Ni(s) → Ni²⁺ + 2e^–

e) reduction, Ni²⁺ + 2e^– → Ni(s)

Resolution: Letter e". The ions present in the solution are: cations: Ni²⁺ and H⁺; anions: SO₄^2- and oh^-. For cations, Ni²⁺ it has priority in the discharge and, therefore, it will suffer a reduction in the cathode, according to the equation: Ni²⁺ + 2e^– → Ni(s).

Question 02 (FMABC-SP) Consider the following system used in the purification of metallic copper:

In this process:

a) II represents the cathode where oxidation occurs.

b) II represents the anode where the reduction occurs.

c) I represents the cathode where oxidation occurs.

d) I represents the cathode at which reduction occurs.

e) I represents the anode where oxidation occurs.

Resolution: Letter e". In electrolysis, the electrode connected to the positive pole of the generator is called an anode, and in it the anions lose electrons and undergo oxidation, according to the equation: Cu⁰ → Cu²⁺ + 2e.

Question 03 (Fatec-SP) To chrome a steel ring, a student assembled the electrolytic circuit, shown in the following figure, using a direct current source.

During the operation of the circuit, it is correct to say that it occurs

a) release of chlorine gas at the anode and metallic chromium deposit on the cathode.

b) release of chlorine gas at the cathode and deposit of metallic chromium on the anode.

c) release of oxygen gas at the anode and deposit of metallic platinum on the cathode.

d) release of hydrogen gas at the anode and corrosion of the metallic platinum at the cathode.

e) release of hydrogen gas at the cathode and corrosion of metallic steel at the anode.

Resolution: Letter a". The ions present in the solution are: cations: Cr³⁺ and H⁺; anions: Cl^- and oh^-. For cations, Cr³⁺ it has priority in the discharge and, therefore, it will suffer reduction at the cathode, according to the equation: Cr³⁺ + 3e^– → Cr(s).

For anions, Cl- has priority in the discharge and, therefore, will undergo oxidation at the anode, according to the equation: 2Cl^-_(here) → 2e + Cl_2(g).

That is, at the anode (platinum part) we will have the release of chlorine gas Cl_2, and, in the cathode (steel ring), the deposition of metallic chromium.

By Victor Ferreira
Chemistry teacher