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Related Concept Videos

Hess's Law03:40

Hess's Law

45.1K
There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.
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There are two criteria that favor, but do not guarantee, the spontaneous formation of a solution:
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Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum of a system’s internal energy (E) and the mathematical product of its pressure (P) and volume (V):
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Standard Entropy Change for a Reaction03:00

Standard Entropy Change for a Reaction

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Entropy is a state function, so the standard entropy change for a chemical reaction (ΔS°rxn) can be calculated from the difference in standard entropy between the products and the reactants.
20.3K
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The Nernst Equation

40.8K
Nonstandard Reaction Conditions
The interconnection between standard cell potentials and various thermodynamic parameters such as the standard free energy change ΔG° and equilibrium constant K has been previously explored. For example, a redox reaction involving zinc(II) and tin(II) ions at 1 M concentration with Eºcell = +0.291 V and ΔG° = −56.2 kJ is spontaneous.
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Redox Equilibria: Overview01:23

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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...
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Entropy-Enthalpy Compensation in Electron-Transfer Processes.

Henrik Burda1, Isabelle Hsieh1, Clemens Burda1

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Summary
This summary is machine-generated.

Solvent reorganization energies were measured for photoexcitation and electron transfer processes. Entropy-enthalpy compensation is complete for low reorganization energies, becoming less complete as energy increases.

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Area of Science:

  • Physical Chemistry
  • Photochemistry
  • Computational Chemistry

Background:

  • Solvatochromic dyes are sensitive to solvent polarity.
  • Understanding solvent reorganization is crucial for electron transfer and photoexcitation processes.

Purpose of the Study:

  • To determine solvent reorganization energies, free energies, and entropies for photoexcitation and electron transfer.
  • To investigate the relationship between solvent reorganization energy and entropy-enthalpy compensation.

Main Methods:

  • Optical absorption spectra measurements at variable temperatures (150-300 K) in diverse solvents.
  • Computer simulations of intramolecular electron transfer reactions.
  • Analysis of entropy-enthalpy compensation effects.

Main Results:

  • Solvent reorganization energies, free energies, and entropies were quantified for photoexcitation of Nile red, 5-(dimethylamino)-5'-nitro-2,2-bisthiophene, and Reichardt's dye B30.
  • Similar parameters were obtained for charge separation/recombination in Zn-porphyrin-quinone cyclophane and charge transfer in a bis-biphenylandrostane radical anion.
  • Complete entropy-enthalpy compensation was observed for solvent reorganization energies below approximately 0.1 eV.

Conclusions:

  • Entropy-enthalpy compensation in solvent reorganization is highly dependent on the magnitude of the reorganization energy.
  • A semiclassical model effectively explains the observed solvent reorganization entropy trends.
  • These findings provide insights into the fundamental interactions governing photoinduced and electron transfer events in solution.