Chapter 2: Chemical Equilibria

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2.17:

Redox Equilibria: Overview

JoVE Core
Analytical Chemistry

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JoVE Core Analytical Chemistry

Redox Equilibria: Overview

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An Oxidation-reduction or redox reaction involves the transfer of electrons from the reducing agent to the oxidizing agent. Species that undergo an increase in their oxidation state are oxidized, while those that experience a decrease in their oxidation state are reduced. So, the thermodynamics of the redox reaction is expressed in terms of the moving electron. The free energy change for a redox reaction is –nFE, where n is the number of electrons, F is Faraday's constant, and E is the electrochemical potential. However, the Gibbs free energy of a reaction is a function of the concentrations of the reactants and products. Substituting for ΔG in this equation gives the relationship with the electrochemical potential, and dividing by –nF results in the Nernst equation. As ΔG is zero at equilibrium, E must be zero, so the reaction quotient equals the equilibrium constant. Considering the values of constants at 25°C and expressing the Nernst equation using base-ten logarithms gives the relation between the standard state potential and the equilibrium constant.

2.17:

Redox Equilibria: Overview

A reduction-oxidation reaction is commonly called a redox reaction. In a redox reaction, electrons are transferred from one species to another rather than being shared between or among atoms. The reducing agent or reductant is the species that loses electrons and gets oxidized in the process. The species that gains electrons and gets reduced in the process is the oxidizing agent or oxidant. Redox reactions are represented as two separate equations called half-reactions, where one equation represents the species that gains electrons, and the other represents the species that loses electrons.

Generally, the thermodynamics of a reaction is expressed in terms of the change in Gibbs free energy (ΔG), which is a function of concentrations of reactants and products. However, the thermodynamics of a redox reaction is expressed in terms of electrochemical potential (E) and the Nernst equation, as the reaction involves the movement of electrons. The Nernst Equation expresses the relationship between the electrochemical potential and the concentrations of the reactants and products.