The chlorine It is a chemical element that is represented by the symbol Cl. It is the second of the halogens, located below fluorine, and is also the third most electronegative element of all. Its name derives from its yellowish green color, which is more intense than fluoride.
Popularly when someone hears your name, the first thing they think of is the bleaching products for clothes, and the water in swimming pools. Although chlorine does work effectively in such examples, it is not its gas, but its compounds (especially hypochlorite) that exert the bleaching and disinfecting action..
The image above shows a round flask with chlorine gas. Its density is greater than that of air, which explains why it remains in the flask and does not escape into the atmosphere; as it happens with other lighter gases, to say helium or nitrogen. In this state it is a highly toxic substance, as it produces hydrochloric acid in the lungs..
That is why elemental or gaseous chlorine does not have many uses, other than in some syntheses. However, its compounds, be they salts or chlorinated organic molecules, do cover a good repertoire of uses, going beyond swimming pools and extremely white clothing..
Likewise, its atoms in the form of chloride anions are found within our bodies, regulating the levels of sodium, calcium and potassium, as well as in gastric juice. Otherwise, the ingestion of sodium chloride would be even more lethal.
Chlorine is produced by electrolysis of brine, rich in sodium chloride, an industrial process in which sodium hydroxide and hydrogen are also obtained. And because the seas are an almost inexhaustible source of this salt, the potential reserves of this element in the hydrosphere are very large..
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Due to the high reactivity of chlorine gas, ancient civilizations never suspected its existence. However, its compounds were part of the culture of humanity since ancient times; its history began linked to common salt.
On the other hand, chlorine arose from volcanic eruptions and when someone dissolved gold in aqua regia; But none of those first approaches was even enough to formulate the idea that said yellowish-green gas was an element or compound.
The discovery of chlorine is attributed to the Swedish chemist Carl Wilhelm Scheele, who in 1774 carried out the reaction between the mineral pyrolusite and hydrochloric acid (by then called muriatic acid).
Scheele gets the credit as he was the first scientist to study the properties of chlorine; although previously recognized (1630) by Jan Baptist van Helmont.
The experiments with which Scheele obtained his observations are interesting: he evaluated the bleaching action of chlorine on reddish and blue flower petals, as well as on the leaves of plants and insects that died instantly.
Likewise, he reported its high reactive for metals, its suffocating smell and undesirable effect on the lungs, and that when it was dissolved in water, its acidity increased..
By then chemists considered any compound that had oxygen to be an acid; so they mistakenly thought that chlorine must be a gaseous oxide. That is how they called it 'oxymuriático acid' (muriatic acid oxide), a name coined by the famous French chemist Antoine Lavoisier.
Then in 1809 Joseph Louis Gay-Lussac and Louis Jacques Thénard tried to reduce this acid with charcoal; reaction with which they obtained metals from their oxides. In this way, they wanted to extract the chemical element of the supposed oxymuratic acid (which they called 'muriatic acid deflogsticated air'.
However, Gay-Lussac and Thénard failed in their experiments; but they were right to consider the possibility that said yellowish-green gas must be a chemical element and not a compound.
The recognition of chlorine as a chemical element was thanks to Sir Humphry Davy, who in 1810 carried out his own experiments with carbon electrodes and concluded that such an oxide of muriatic acid did not exist..
And furthermore, it was Davy who coined the name 'chlorine' for this element from the Greek word 'chloros', which means yellowish green..
As they studied the chemical properties of chlorine, many of its compounds were found to be saline in nature; hence they named it as a 'halogen', which means salt former. Then the term halogen was used with the other elements of its same group (F, Br and I).
Michael Faraday even managed to liquefy the chlorine into a solid that, due to being contaminated with water, formed the hydrate CltwoHtwoOR.
The rest of the history of chlorine is linked to its disinfectant and bleaching properties, until the development of the industrial process of electrolysis of brine to produce massive amounts of chlorine..
It is a dense, opaque, yellowish-green gas with an irritating acrid odor (a super-enhanced version of commercial chlorine) and is also extremely poisonous..
17
35.45 u.
Unless otherwise stated, all other properties correspond to quantities measured for molecular chlorine, Cltwo.
-34.04 ºC
-101.5 ºC
-Under normal conditions, 3.2 g / L
-Right at the boiling point, 1.5624 g / mL
Note that liquid chlorine is approximately five times as dense as its gas. Also, the density of its vapor is 2.49 times that of air. That is why in the first image the chlorine does not tend to escape from the round flask, since being denser than air it is located at the bottom. This characteristic makes it an even more dangerous gas..
6.406 kJ / mol
20.41 kJ / mol
33.95 J / (mol K)
1.46 g / 100 mL at 0 ºC
7.67 atm at 25 ° C. This pressure is relatively low compared to other gases.
3.16 on the Pauling scale.
-First: 1251.2 kJ / mol
-Second: 2298 kJ / mol
-Third: 3822 kJ / mol
8.9 10-3 W / (mK)
Chlorine occurs in nature primarily as two isotopes: 35Cl, with an abundance of 76%, and 37Cl, with an abundance of 24%. Thus, the atomic weight (35.45 u) is an average of the atomic masses of these two isotopes, with their respective abundance percentages..
All the radioisotopes of chlorine are artificial, among which the 36Cl as the most stable, with a half-life of 300,000 years.
Chlorine can have various oxidation numbers or states when it is part of a compound. Being one of the most electronegative atoms in the periodic table, it usually has negative oxidation numbers; except when it runs into oxygen or fluorine, in whose oxides and fluorides, respectively, it has to "lose" electrons.
In their oxidation numbers the existence or presence of ions with the same magnitude of charge is assumed. Thus, we have: -1 (Cl-, the famous chloride anion), +1 (Cl+), +2 (Cltwo+), +3 (Cl3+), +4 (Cl4+), +5 (Cl5+), +6 (Cl6+) and +7 (Cl7+). Of all of them, -1, +1, +3, +5 and +7 are the most common found in chlorinated compounds..
For example, in ClF and ClF3 the oxidation numbers for chlorine are +1 (Cl+F-) and +3 (Cl3+F3-). In the CltwoOr, this is +1 (Cltwo+ORtwo-); while in the ClOtwo, CltwoOR3 and CltwoOR7, are +4 (Cl4+ORtwotwo-), +3 (Cltwo3+OR3two-) and +7 (Cltwo7+OR7two-).
In all chlorides, on the other hand, chlorine has an oxidation number of -1; as in the case of NaCl (Na+Cl-), where it is valid to say that the Cl exists- given the ionic nature of this salt.
Chlorine atoms in their ground state have the following electronic configuration:
[Ne] 3stwo 3p5
Therefore, each of them has seven valence electrons. Unless they are overloaded with energy, there will be individual Cl atoms in space, like green marbles. However, their natural tendency is to form covalent bonds between them, in order to complete their valence octets..
Note that they just need one electron to have eight valence electrons, so they form a single simple bond; this is, the one that joins two Cl atoms to create the Cl moleculetwo (top image), Cl-Cl. That is why chlorine in normal and / or terrestrial conditions is a molecular gas; not monatomic, as with noble gases.
The Cl moleculetwo it is homonuclear and apolar, so its intermolecular interactions are governed by the London scattering forces and its molecular masses. In the gas phase, the distance Cltwo-Cltwo it is relatively short compared to other gases which, added to its mass, makes it a gas three times denser than air.
Light can excite and promote electronic transitions within the molecular orbitals of Cltwo; consequently, its characteristic yellowish-green color appears. This color intensifies in the liquid state, and then partially disappears when it solidifies.
As the temperature drops (-34 ºC), the Cl moleculestwo lose kinetic energy and distance Cltwo-Cltwo decreases; therefore, these coalesce and end up defining the liquid chlorine. The same happens when the system is cooled even more (-101 ºC), now with the Cl moleculestwo so close together that they define an orthorhombic crystal.
The fact that chlorine crystals exist is indicative that their dispersive forces are sufficiently directional to create a structural pattern; that is, molecular layers of Cltwo. The separation of these layers is such that their structure is not modified even under a pressure of 64 GPa, nor do they exhibit electrical conduction..
Chlorine in its gaseous state cannot be found anywhere on the Earth's surface, as it is very reactive and tends to form chlorides. These chlorides are well diffused throughout the earth's crust and, furthermore, after millions of years of being washed away by the rains, they are enriching the seas and oceans..
Of all the chlorides, the NaCl of the mineral halite (upper image) is the most common and abundant; followed by the minerals silvin, KCl, and carnalite, MgCltwoKCl 6HtwoO. When water masses evaporate due to the action of the Sun, they leave behind desert salt lakes, from which NaCl can be directly extracted as raw material for the production of chlorine..
The NaCl dissolves in water to produce a brine (26%), which is subjected to electrolysis inside a chlor-alkali cell. There, two half-reactions take place in the anode and cathode compartments:
2Cl-(ac) => Cltwo(g) + 2e- (Anode)
2HtwoO (l) + 2e- => 2OH-(ac) + Htwo(g) (Cathode)
And the global equation for both reactions is:
2NaCl (aq) + 2HtwoO (l) => 2NaOH (aq) + Htwo(g) + Cltwo(g)
As the reaction proceeds, the Na ions+ formed at the anode migrate into the cathode compartment through a permeable asbestos membrane. For this reason, NaOH is on the right hand side of the global equation. Both gases, Cltwo and Htwo, are collected from the anode and cathode, respectively.
The image below illustrates what was just written:
Note that the concentration of the brine at the end decreases by 2% (it goes from 24 to 26%), which means that part of its anions Cl- originals were transformed into Cl moleculestwo. In the end, the industrialization of this process has given a method to produce chlorine, hydrogen and sodium hydroxide..
As mentioned in the history section, chlorine gas can be produced by dissolving pyrolusite mineral samples with hydrochloric acid. The following chemical equation shows the products obtained from the reaction:
MnOtwo(s) + 4HCl (aq) => MnCltwo(aq) + 2HtwoO (l) + Cltwo(g)
Chlorine alloys do not exist for two simple reasons: their gaseous molecules cannot be trapped between metallic crystals, and they are also very reactive, so they would react immediately with metals to produce their respective chlorides..
On the other hand, chlorides are not desirable either, since once dissolved in water they exert a saline effect that promotes corrosion in alloys; and therefore, metals dissolve to form metal chlorides. The corrosion process for each alloy is different; some are more susceptible than others.
Chlorine, therefore, is not a good additive for alloys at all; nor as Cltwo nor as Cl- (and Cl atoms would be very reactive so they can even exist).
Although the solubility of chlorine in water is low, it is enough to produce hydrochloric acid in the moisture of our skin and eyes, which ends up corroding the tissues causing serious irritation and even loss of vision..
Even worse is breathing its greenish-yellow vapors, since once in the lungs it generates acids again and damages the lung tissue. With this, the person experiences a sore throat, cough and breathing difficulties due to the fluids formed in the lungs.
If there is a chlorine leak, you are in a particularly dangerous situation: the air cannot simply "sweep away" its vapors; they stay there until they react or slowly disperse.
In addition to this, it is a highly oxidizing compound, so that various substances can react explosively with it at the slightest contact; just like steel wool and aluminum. That is why where there is stored chlorine, all the necessary considerations must be taken to avoid the risks of fire..
Ironically, while chlorine gas is deadly, its chloride anion is not toxic; It can be consumed (in moderation), it does not burn, nor does it react except with fluorine and other reagents.
About 81% of the chlorine gas produced annually is used for the synthesis of organic and inorganic chlorides. Depending on the degree of covalence of these compounds, chlorine can be found as mere Cl atoms in chlorinated organic molecules (with C-Cl bonds), or as Cl ions- in a few chloride salts (NaCl, CaCltwo, MgCltwo, etc.).
Each of these compounds has its own applications. For example, chloroform (CHCl3) and ethyl chloride (CH3CHtwoCl) are solvents that have come to be used as inhalation anesthetics; dichloromethane (CHtwoCltwo) and carbon tetrachloride (CCl4), meanwhile, are solvents widely used in organic chemistry laboratories.
When these chlorinated compounds are liquid, most of the time they are used as solvents for organic reaction media..
In other compounds, the presence of chlorine atoms represents an increase in the dipole moment, so they can interact to a greater degree with a polar matrix; one made up of proteins, amino acids, nucleic acids, etc., biomolecules. Thus, chlorine also has a role in the synthesis of drugs, pesticides, insecticides, fungicides, etc..
Regarding inorganic chlorides, they are usually used as catalysts, raw material for obtaining metals by electrolysis, or sources of Cl ions.-.
The gaseous or elemental chlorine has no role within living beings other than destroying their tissues. However, this does not mean that its atoms cannot be found in the body. For example, Cl ions- are very abundant in the cellular and extracellular environment, and help to control the levels of Na ions+ and Catwo+, mostly.
Likewise, hydrochloric acid is part of the gastric juice with which food is digested in the stomach; its Cl ions-, in the company of H3OR+, define the pH close to 1 of these secretions.
The density of chlorine gas makes it a lethal substance when spilled or poured into closed or open spaces. Being denser than air, a current of it does not easily carry chlorine, so it remains for a considerable time before finally dispersing.
In World War I, for example, this chlorine was used on battlefields. Once released, he would sneak into the trenches to suffocate the soldiers and force them to surface..
Chlorinated solutions, those where chlorine gas has been dissolved in water and then made alkaline with a buffer, have excellent disinfectant properties, as well as inhibit tissue putrefaction. They have been used to disinfect open wounds to eliminate pathogenic bacteria.
The pool water is precisely chlorinated to eliminate bacteria, microbes and parasites that may harbor in it. Chlorine gas used to be used for this purpose, however its action is quite aggressive. Instead, sodium hypochlorite solutions (bleach) or trichloroisocyanuric acid (TCA) tablets are used..
The above said shows that it is not the Cltwo the one that exerts the disinfectant action but HClO, hypochlorite acid, which produces O radicals that destroy microorganisms.
Very similar to its disinfecting action, chlorine also bleaches materials because the colorants responsible for the colors are degraded by HClO. Thus, its chlorinated solutions are ideal for removing stains from white garments, or for bleaching paper pulp..
The most important chlorine compound of all, which accounts for about 19% of the remaining chlorine gas production, is polyvinyl chloride (PVC). This plastic has multiple uses. With it, water pipes, window frames, wall and floor coverings, electrical wiring, IV bags, coats, etc. are made..
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