What is meant by negative free energy
The Activation energy is the energy that is required to transfer a particle (molecule, atom, electron, etc.) from a certain energy level (separate energy) to a higher energy level in which the particle receives an "activity" that was not given before.
Physical chemistry understands the activation energy to be the energy barrier that the reaction complex must overcome in order for a reaction to take place. When the reaction partners meet, they temporarily combine to form the activated complex (the area of the maximum in the graph on the left). The higher the temperature of the reaction system, the higher the probability that the reactants will provide the required activation energy and can react further to form the product.
The relationship between activation energy and temperature is quantitatively described by the Arrhenius equation:
What the activation energy is not
Certain simplistic but incorrect assumptions are often repeated. Contrary to these assumptions, the activation energy is
- Not the energy that has to be supplied (from the outside) to an endothermic reaction
- no direct measure of the absolute speed of reaction
- Not identical to the "threshold energy" of the classical impact theory
In fact, Arrhenius' model does not fully describe the processes involved in a chemical reaction; the factor A. is a purely empirical quantity, which in turn depends on the temperature. For some reactions a negative activation energy found what seems physically impossible. According to Eyring, (see also the theory of the transition state) is Free enthalpy of activation the determining factor for the reaction speed.
A (positive) catalyst reduces the activation energy for chemical reactions, but does not change the free enthalpy of reaction ΔR.G. It is assumed that in the presence of a catalyst a complex with a lower activation energy is formed and thus the likelihood of a reaction increases.
- For one undopedSemiconductor is the energy that is necessary to gain an electron or a hole as a free charge carrier. This is identical to the band gap between the valence band and the conduction band, known as the band gap. As a result, electrons that were previously firmly bound in the atoms (valence band) are raised to a state in which they can move freely (conduction band). The electrons can thus contribute to the transport of electricity in the semiconductor, so they are considered to be activated. The activation energy can either be supplied by means of thermal energy or collisions with particles (e.g. photons) (see generation of charge carriers).
- For one endowed Semiconductors, the energy to generate a free charge carrier is much lower. The doping atoms form energy levels in the band gap. The activation energy is therefore the difference between these energy levels and the closest band. For p-doped semiconductors it is the difference between doping level and valence band, for n-semiconductors it is the difference between doping level and conduction band.
Category: Chemical reaction
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