Principles of Electrochemistry and Batteries

Dissecting Redox Reactions with Half-Reactions

Half-reactions are a useful tool for analyzing redox reactions by breaking them down into their oxidation and reduction components. These half-reactions, also referred to as half-equations, illustrate the individual processes of electron loss or gain by a species. In a complete redox reaction, these half-reactions occur simultaneously, but they are often represented separately for clarity and ease of understanding. By convention, half-reactions are written as reduction processes, and electrode potentials are listed as reduction potentials, which simplifies the comparison of the relative strengths of oxidizing and reducing agents.

Galvanic Cells and Electrical Energy Generation

Galvanic cells, also known as voltaic cells, are devices that convert chemical energy into electrical energy through spontaneous redox reactions. A galvanic cell consists of two half-cells, each containing an electrode and an electrolyte where either oxidation or reduction occurs. The electromotive force (EMF) generated by the cell is the driving force for the movement of electrons from the anode, where oxidation takes place, to the cathode, where reduction occurs. The EMF of the cell, also known as the cell potential, is calculated by taking the difference between the reduction potential of the cathode and the oxidation potential of the anode.

Anatomy of a Galvanic Cell

A typical galvanic cell is composed of two half-cells, each containing an electrolyte and an electrode, connected by a salt bridge and an external electrical circuit. The salt bridge is essential for maintaining electrical neutrality within the cell by allowing ions to flow between the half-cells, thereby preventing charge buildup that would otherwise stop the reaction. The electrodes, usually made of metals or other conductive materials, facilitate the redox reactions; the anode is the site of oxidation and the cathode is the site of reduction. The flow of electrons through the external circuit from anode to cathode is the electrical current that can be harnessed for work.

The Standard Hydrogen Electrode as a Reference

The Standard Hydrogen Electrode (SHE) is the universal reference for measuring electrode potentials in electrochemistry. It is assigned a potential of 0 volts under standard conditions, providing a baseline against which the potentials of other electrodes can be compared. To determine the standard electrode potential of a species, a galvanic cell is constructed with the SHE as one half-cell and the species of interest as the other half-cell. The measured potential difference is the standard electrode potential of the species, indicating its propensity to gain or lose electrons when compared to hydrogen.

Measuring and Interpreting Electrode Potentials

The electrode potential is the voltage difference between an electrode and its surrounding electrolyte, resulting from the separation of charges. It reflects the ability of a species in a half-cell to either donate or accept electrons. The standard electrode potential is measured under standard conditions with a concentration of 1 M for all reactants and products. To measure this potential, a galvanic cell is set up with the species in question and a reference electrode, often the SHE. The EMF of the cell, also known as the cell potential or E°cell, is indicative of the electrode potential for the species, which is a key parameter for predicting the feasibility and direction of redox reactions in electrochemical cells.