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An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process.
Typical oxidizing agents undergo permanent chemical alteration through covalent or ionic reaction chemistry, resulting in the complete and irreversible transfer of one or more electrons. In many chemical circumstances, however, the transfer of electronic charge from an electron donor may be only fractional, meaning an electron is not completely transferred, but results in an electron resonance between the donor and acceptor. This leads to the formation of charge transfer complexes in which the components largely retain their chemical identities.
The overall energy balance (Î”E), i.e., energy gained or lost, in an electron donor-acceptor transfer is determined by the difference between the acceptor's electron affinity (A) and the ionization potential (I) of the electron donor:
In chemistry, a class of electron acceptors that acquire not just one, but a set of two paired electrons that form a covalent bond with an electron donor molecule, is known as a Lewis acid. This phenomenon gives rise to the wide field of Lewis acid-base chemistry. The driving forces for electron donor and acceptor behavior in chemistry is based on the concepts of electropositivity (for donors) and electronegativity (for acceptors) of atomic or molecular entities.
Examples of electron acceptors include oxygen, nitrate, iron (III), manganese (IV), sulfate, carbon dioxide, or in some microorganisms the chlorinated solvents such as tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethene (DCE), and vinyl chloride (VC). These reactions are of interest not only because they allow organisms to obtain energy, but also because they are involved in the natural biodegradation of organic contaminants. When clean-up professionals use monitored natural attenuation to clean up contaminated sites, biodegradation is one of the major contributing processes.
In biology, a terminal electron acceptor is a compound that receives or accepts an electron during cellular respiration or photosynthesis. All organisms obtain energy by transferring electrons from an electron donor to an electron acceptor. During this process (electron transport chain) the electron acceptor is reduced and the electron donor is oxidized.
- X + eâˆ’â†’ Xâˆ’
The electron affinity, Eea, is defined as positive when the resulting ion has a lower energy, i.e. it is an exothermic process that releases energy:
- Eea = Einitial âˆ’ Efinal
- Xâˆ’â†’ X + eâˆ’
Electron affinities of the elements
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.
Eea generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a group 7A atom releases more energy than a group 1A atom on gaining an electron because it obtains a filled valence shell and therefore is more stable.
A trend of decreasing Eea going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus, and thus would experience a lesser effective nuclear charge. However, a clear counterexample to this trend can be found in group 2A, and this trend only applies to group 1A atoms. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.
The following data are quoted in kJ/mol. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds. Elements marked with a dotted box are synthetically made elementsâ€”elements not found naturally in the environment.
Molecular electron affinities
The electron affinity of molecules is a complicated function of their electronic structure. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silicoexperiments show that the electron affinity ofhexacyanobenzene surpasses that of fullerene.
Electron affinity of Surfaces
The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which the electron affinity is one parameter. For one illustration of the apparent effect of surface termination on electron emission, see Figure 3 in Marchywka Effect.
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Answers:1. oxygen - hence u wrok out, produce water (sweat) 2. reflected, if absorbed, then u can't see it. 3. carbon dioxdie and water 4. this is tricky, my guess is source of electron for photosynthesis (usually its involve in the energy production pathways of an organism) 5. aerobinc respiration
Answers:By accepting electrons, DPIP will speed up electron flow, water splitting and oxygen production. DPIP ain't NADPH so I would imagine thed dark rxn would probably be inhibited.
Answers:Pyruvate! If it is anaerobic, no oxygen would be available.
Answers:I can only find sulfate reduction in the metabolism of archea and bacteria in my bio book. It's biological science by scott freeman. Under sulfate reducers, it lists H2 or organic compounds as the e- donor, (SO4)2- as the e- acceptor. The by products are H2O or CO from the donor and H2S from the acceptor. Hope this helps.