Galvanic and Electrolytic Cells

Electrochemistry is the branch of chemistry that deals with the study of the relationship between electrical energy and chemical change. The essence of electrochemistry is galvanic and electrolytic cells, both of which are devices that convert chemical energy into electrical energy or vice versa. Although they serve different purposes in the field of electrochemistry, they are fundamentally interconnected. Let's explore each type of cell, their mechanisms, similarities, differences, and practical applications.

What is a galvanic cell?

A galvanic cell, also known as a voltaic cell, is an electrochemical cell that produces electrical energy from spontaneous redox reactions that occur within the cell. Its primary purpose is to convert chemical energy into electrical energy. The oxidation and reduction reactions in a galvanic cell occur in separate compartments, and electricity is produced by the flow of electrons from one compartment to another.

Components of a galvanic cell

• Anode: The electrode where oxidation occurs. Electrons are lost by the substance at the anode. In a galvanic cell, the anode is negatively charged.
• Cathode: The electrode where reduction takes place. Electrons are gained by the material at the cathode. In a galvanic cell, the cathode is positively charged.
• Salt bridge: A passage designed to allow the movement of ions between two compartments, thereby maintaining electrical neutrality. This usually contains a salt solution such as KCl or NaNO₃.
• Electrolyte solutions: Solutions that conduct electricity due to the presence of ions.
• External circuit: A wire through which electrons flow. It connects the anode and cathode externally.

Working principle of galvanic cell

The redox reaction in a galvanic cell takes place in two separate half-cells. These half-cells are connected in the following manner:

1. The oxidation reaction occurs at the anode, where electrons are released. Consider the following reaction at the anode:

Zn (s) → Zn2+ (aq) + 2e-

2. The cathode accepts these electrons, and reduction occurs. Consider the reduction reaction:

Cu2+ (aq) + 2e- → Cu (s)

3. The salt bridge helps ions to migrate to maintain charge balance. Positive ions move toward the cathode, and negative ions move toward the anode.
4. The flow of electrons from the anode to the cathode through an external circuit produces electric current, which can be measured or used to do work.

Illustration of a galvanic cell

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Features of galvanic cell

• Spontaneity: The reactions in a galvanic cell are spontaneous, meaning they occur naturally and release free energy.
• Cell potential: The voltage or electrical potential difference between two electrodes. It depends on the nature of the reactants and their concentration.
• Positive cell potential: For a galvanic cell, the cell potential ((E_{text{cell}})) is positive, which indicates the spontaneous reaction.

Example of a galvanic cell reaction

In a typical zinc–copper cell:

Anode reaction: Zn (s) → Zn2+ (aq) + 2e- 
Cathode reaction: Cu2+ (aq) + 2e- → Cu (s) 
Overall cell reaction: Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s) 
Ecell = +1.10 V

What is an electrolytic cell?

An electrolytic cell is an electrochemical cell that uses electrical energy to drive a non-spontaneous reaction. It essentially functions the opposite of a galvanic cell, converting electrical energy into chemical energy. Electrolytic cells are used in processes that require an external energy input to proceed.

Components of electrolytic cell

• Anode: The electrode where oxidation occurs, characterized by the loss of electrons. In electrolytic cells, the anode is positively charged.
• Cathode: The electrode where reduction occurs, characterized by the gain of electrons. In an electrolytic cell, the cathode is negatively charged.
• Electrolyte: Substance containing free ions that carry electric current between the anode and cathode.
• Power source: A battery or other source that provides the energy needed to drive the non-spontaneous reaction.

Working principle of electrolytic cell

An electrolytic cell uses electrical energy to facilitate a redox reaction that would otherwise not occur on its own. Here's how the process unfolds:

1. An external power source moves electrons away from the cathode and toward the anode, reversing the direction of electron flow compared to a galvanic cell.
2. Oxidation occurs at the anode, often involving the formation of a gas or precipitate and the emission of electrons.
3. Reduction occurs at the cathode, often involving the deposition of a metal or other substance and the capture of electrons.
4. The power source continuously provides the energy needed to continue this process.

Illustration of an electrolytic cell

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Characteristics of electrolytic cell

• Non-spontaneous: Electrolytic cells depend on an external power source to drive the non-spontaneous process.
• Cell Potential: The cell potential is negative, indicating that energy must be consumed to drive the reaction.
• Applications: Electrolytic cells are commonly used in electroplating, electrolysis, and the production of chlorine and sodium hydroxide.

Example of an electrolytic cell reaction

Consider the electrolytic cell used for the electrolysis of water:

Anode reaction: 2H2O (l) → O2 (g) + 4H+ (aq) + 4e- 
Cathode reaction: 4H+ (aq) + 4e- → 2H2 (g) 
Overall cell reaction: 2H2O (l) → 2H2 (g) + O2 (g)

Comparison: Galvanic vs. Electrolytic Cells

Speciality	Galvanic cell	Electrolytic cell
Objective	Convert chemical energy into electrical energy	Convert electrical energy into chemical energy
Feedback type	Spontaneous	Non-intuitive
Cell potential	Positive ((E_{text{cell}} > 0))	Negative ((E_{text{cell}} < 0))
Functional role	Battery (Power source)	Electricity consumer (requires power source)
Anode polarity	Negative	Positive
Cathode polarity	Positive	Negative
Example	Batteries, fuel cells	Electroplating, electrolysis

Practical applications

Galvanic cells

• Batteries: Galvanic cells are used in batteries to power electronic devices, cars, and toys.
• Fuel cell: A special type of galvanic cell that uses hydrogen as a fuel to produce electricity, with water as a byproduct, which is used in power generation and transportation.

Electrolytic cell

• Electroplating: The process of applying a layer of metal to an object. This enhances beauty and improves corrosion resistance.
• Industrial electrolysis: It is used to produce elements such as chlorine and hydrogen through the electrolysis of brine and also aluminum from its ores.

Conclusion

Galvanic and electrolytic cells highlight the fascinating interplay between electrical and chemical energy transformations. Understanding these cells not only provides deep insights into fundamental chemical processes, but also opens up avenues for essential applications for industry and technology. Awareness and understanding of these processes paves the way for innovations in sustainability, energy storage, and chemical production.

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