Introduction

The Relevance of the Topic

Electrochemistry, a branch studied within the field of Chemistry, is essential to understand the chemical behavior involving electron transfer. Within the vast spectrum of this topic, we have electrolysis, a process that allows the manipulation of chemical reactions through the passage of electric current. Electrolysis is therefore a crucial tool in various industries, such as metal production, manufacturing of chemical products, and even in water treatment.

Contextualization

In the High School Chemistry curriculum, the topic of Electrochemistry is usually explored after the study of thermochemistry and chemical kinetics. Knowledge of electrolysis is thus an essential step in understanding the functioning of chemical reactions and the transformation of chemical energy into electrical energy (and vice versa). In addition, electrolysis also enhances the student's understanding of the principle of conservation of mass and the balancing of chemical equations. By mastering this topic, students will be prepared to face more advanced concepts in Chemistry and even in Engineering and Materials Science.

Theoretical Development

Components

• Reduction and Oxidation Reactions (Redox): Key concept to understand electrolysis. In these reactions, electrons are transferred between atoms. The agent that donates electrons is oxidized, while the one that receives electrons is reduced. This is remembered through the mnemonic 'lose electrons - oxidation; gain electrons - reduction' (LEOGER).

• Faraday's Law: Establishes the amount of substance released or deposited in an electrolysis, directly proportional to the amount of electric charge passing through the circuit. In its most common form, Faraday's law is expressed as: mass (m) = charge (Q) × electrochemical equivalent (E).

• Fundamental Equation of Electrolysis: The equation that summarizes the relationship between the quantities of electrons used (n), Faraday's constant (F), and the amount of substance released (m) in an electrolysis. Mathematically, it is written as: n = Q/F

• Electrolysis Cell: Set of equipment used for electrolysis, composed of an electrolyte, two electrodes (anode and cathode), and an external circuit connected to a source of direct current.

• Electrolysis Voltage and Standard Electrode Potentials: The voltage applied in the electrolysis cell must be higher than the reduction potential of the cation (or oxidation potential of the anion). These potentials are measured in relation to hydrogen (which has a standard potential of 0) and are tabulated in the literature.

Key Terms

• Electrolysis: Non-spontaneous process of separating a substance (be it ionic or molecular) through the passage of electric current.

• Cathode: Negative electrode of an electrolysis cell, where cations (positive ions) migrate.

• Anode: Positive electrode of an electrolysis cell, where anions (negative ions) migrate.

• Electrochemical Equivalent: Amount of substance released per mole of electrons passing through an electrolysis.

Examples and Cases

• Electrolysis of Sodium Chloride: In this classic example, Na+ migrates to the cathode and is reduced to metallic Na. Cl- migrates to the anode and is oxidized to gaseous Cl2. It is noted that the migration rate of ions is proportional to their charges, not their masses.

• Electrolysis of Water: Another important case, where water is electrolyzed to produce gaseous H2 and O2. The reaction at the cathode is: 2H2O + 2e- -> H2 + 2OH-, and at the anode is: 4OH- -> O2 + 2H2O + 4e-. Here, we see the importance of knowing the standard electrode potentials - without this, we could try direct electrolysis of water, which would not be efficient.

• Electrolysis of Potassium Fluoride: In this case, the products are different depending on the electrolyte concentration. At 10% KF, we have the formation of gaseous F2 at the anode and K at the cathode, while at 50% KF, we have the formation of KF at the anode and HF at the cathode.

Detailed Summary

Key Points

• Electrolysis Process: It is a non-spontaneous process that separates a substance into its components through the passage of electric current. It is a fundamental process in the industry, allowing the production of metals and other chemical compounds.

• Faraday's Law: It is a physical law that establishes the amount of a substance released during electrolysis. This amount is directly proportional to the amount of electric charge passing through the circuit.

• Reduction and Oxidation Reactions (Redox): These reactions allow the transfer of electrons between atoms. During electrolysis, positive ions migrate to the negative electrode (cathode) where they undergo reduction, and negative ions migrate to the positive electrode (anode) where they undergo oxidation.

• Electrolysis Cell: It is composed of an electrolyte, two electrodes (anode and cathode), and an external circuit connected to a source of direct current. Electrolysis occurs within this cell, with the electrodes serving as sites for oxidation and reduction reactions.

Conclusions

• With an understanding of electrochemistry, specifically the process of electrolysis, the student acquires the ability to manipulate chemical reactions through electric current, expanding their understanding of chemical reactions and their practical applications.

• Faraday's Law and the Fundamental Equation of Electrolysis are essential tools in the quantitative determination of the products of an electrolysis. These laws allow the calculation of quantities of substances released or deposited, expanding the understanding of the electrolysis process.

Exercises

1. Describe the electrolysis process of sodium chloride. What are the products formed in the decomposition of this salt?

2. Write the electrochemical reaction that occurs at the electrodes during the electrolysis of water. What products would you expect to find at the cathode and anode?

3. The electrolysis of an aqueous solution of potassium fluoride (KF) 10% results in different products than a 50% KF solution. Explain which products are formed in each case and why there is this difference.