Photochemical Processes An activated molecule does not necessarily experience a reaction. exist processes that compete with each other. Photochemical sequence is understood as set of possible paths that an excited molecule can follow...
morePhotochemical Processes
An activated molecule does not necessarily experience a reaction. exist
processes that compete with each other. Photochemical sequence is understood as
set of possible paths that an excited molecule can follow after
the absorption of radiation. In general you can talk about the following processes:
Primary Process
Excited electronic species, whether singlet or triplet, by owning a
excess energy can be considered species, activated from the point of view
cinetochemistry They can, therefore, undergo chemical reactions that compete with the
physical photo processes that have just been studied.
A primary photochemical process is an elementary chemical process suffered by a
molecular entity electronically excited and resulting in a product photo
primary.
a) Excitation: as a consequence of the absorption of a photon, the molecule
happens to an excited electronic state, in general of the same multiplicity
(singlet S, or triplet T) that the fundamental state
b) Deactivation: The excited molecule can lose its excess energy,
by different mechanisms, that make you return to the same fundamental state
(physical photo processes) or suffer a chemical reaction (photochemical process).
Secondary Process
Frequently the products of the primary photochemical processes are species
very reactive chemistries, such as atoms or free radicals, sometimes even in states
excited These species can, in turn, initiate other chemical reactions that
they are known as secondary photochemical processes. A typical example is the
gas phase reaction between Br2 and H2, with the sample exposed to radiation from
<511 nm, region in which it absorbs Br2. It is a chain reaction, in
The first stage consists of a photo dissociation of the Br2 molecule.
Primary and secondary contaminants
The primary atmospheric pollutants come from very diverse sources,
so its physical nature and its chemical composition are very varied, although
we can group them according to their most characteristic peculiarity, their state
physical activity (case of particles and metals), or common chemical element (case of
gaseous pollutants).
The pollutants most frequently emitted into the atmosphere, which are the cause of
Environmental alterations are:
- Aerosols, which includes dust with sediment and suspended particles and humas.
- Sulfur oxides, SOx (S02 and S03, fundamentally).
- Carbon monoxide, CO.
- Nitrogen oxides, NOx (NO and N02, fundamentally).
- Hydrocarbons, HnCm.
- Ozone, 03
In addition to these substances there are a series of pollutants that occur more rarely but that can affect certain areas because they are very harmful. These are:
- Other derivatives of sulfur and nitrogen.
- Halogens and their derivatives.
- Arsenic and its derivatives.
- Organic components.
- Particles of heavy and light metals, such as lead, mercury, copper, zinc.
- Particles of mineral substances, such as asbestos and asbestos.
Another type of pollutants present in the atmosphere are called secondary, have the characteristic that they are not emitted directly into the atmosphere. They are substances produced through atmospheric reactions that take place between primary pollutants and favored by a series of environmental factors, such as the reaction between nitrogen oxides, hydrocarbons and oxygen (precursors) in the presence of strong radiation solar (photochemical reactions), which form a series of complex substances such as ozone, aldehydes, hydrogen peroxide, peroxyacetyl nitrile (PAN), free radicals, solid particles, etc., called oxidants and that lead to pollution photochemistry.
Another class of secondary pollutants are sulfates and nitrates formed from the emissions of sulfur oxides (SOx) and nitrogen oxides (NOx), although they can sometimes appear as human pollutants. There are also other contaminating effects such as ionizing radiation from radioactive elements present in the atmosphere, which lead to radioactive contamination, as well as the noise produced by noise pollution.
From Primary Contaminant To Secondary Contaminant
Primary pollutants such as nitrogen oxides, sulfur oxides, hydrocarbons when they come into contact with atmospheric oxygen and favored by ultraviolet radiation, generate the so-called secondary pollutants among which are HNO3, H2SO4, O3
ATMOSPHERIC (which is not good O3, because it generates oxidation reactions in the atmosphere) And in general the so-called SMOT PHOTOCHEMICAL.
The S02 by photochemical effects is oxidized and transformed into SO3 (sulfur trioxide), that is, a primary pollutant by chemical reaction in the air becomes a secondary pollutant. SO3 can react with moisture in the air to form sulfuric acid (H2SO4) mist.
Example:
When the atmosphere receives strong doses of oxides of sulfur and nitrogen, which are the primary pollutants, these compounds by complex chemical reactions are partially converted into sulfuric and nitric acid. Some of these acid particles disappear by gravity or by impact against the ground, buildings, plants, etc.: it is called dry precipitation. Others, remain in the atmosphere, combine with the humidity of the clouds and fall with rain, snow and dew: it is acid rain. The gases produced after the mixing of water vapors and polluting substances are the secondary pollutants. Acid rain is a phenomenon that is produced by the combination of oxides of nitrogen and sulfur from human activities, with the water vapor present in the atmosphere, which then fall to the ground acidifying the soil, but which can be dragged at great distances from their place of origin before being deposited in the form of rain. Coal, as well as other mineral fuels, are responsible for pouring sulfur oxide into the atmosphere.
