2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||chlorine, Cl, 17|
|Group, Period, Block||17, 3, p|
|Atomic mass||35.453 (2) g/mol|
|Electron configuration||[Ne] 3s2 3p5|
|Electrons per shell||2, 8, 7|
|Density||(0 °C, 101.325 kPa)
|Melting point||171.6 K
(-101.5 ° C, -150.7 ° F)
|Boiling point||239.11 K
(-34.04 ° C, -29.27 ° F)
|Critical point||416.9 K, 7.991 MPa|
|Heat of fusion||(Cl2) 6.406 kJ·mol−1|
|Heat of vaporization||(Cl2) 20.41 kJ·mol−1|
|Heat capacity||(25 °C) (Cl2)
|Oxidation states||±1, 3, 5, 7
(strongly acidic oxide)
|Electronegativity||3.16 (Pauling scale)|
| Ionization energies
|1st: 1251.2 kJ·mol−1|
|2nd: 2298 kJ·mol−1|
|3rd: 3822 kJ·mol−1|
|Atomic radius||100 pm|
|Atomic radius (calc.)||79 pm|
|Covalent radius||99 pm|
|Van der Waals radius||175 pm|
|Electrical resistivity||(20 °C) > 10 Ω·m|
|Thermal conductivity||(300 K) 8.9 mW·m−1·K−1|
|Speed of sound||(gas, 0 °C) 206 m/s|
|CAS registry number||7782-50-5|
Chlorine ( IPA: /ˈklɔːriːn/, Greek: χλωρóς chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its elemental form under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable suffocating odour and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants.
Chlorine gas is diatomic with the formula Cl2. It combines readily with nearly all other elements, although it is not as extremely reactive as fluorine. At 10 ° C one litre of water dissolves 3.10 litres of gaseous chlorine and at 30 °C only 1.77 litres.
This element is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. As the chloride ion, Cl–, it is also the most abundant dissolved species in ocean water.
Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it dephlogisticated marine acid (see Phlogiston theory) and mistakenly thought it contained oxygen. Chlorine was given its current name in 1810 by Sir Humphry Davy, who insisted that it was in fact an element.
Chlorine gas, also known as bertholite, was first used as a weapon against humans in World War I by Germany on April 22nd, 1915. It was pioneered by a German scientist later to be a Nobel laureate, Fritz Haber. It is alleged that his role in the use of chlorine as a deadly weapon drove his wife to suicide. After its first use, it was utilised by both sides as a chemical weapon.
In nature, chlorine is found mainly as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans—about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits. Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Common chloride minerals include halite ( sodium chloride), sylvite ( potassium chloride), and carnallite (potassium magnesium chloride hexahydrate).
Industrially, elemental chlorine is usually produced by the electrolysis of sodium chloride dissolved in water. Along with chlorine, this chloralkali process yields hydrogen gas and sodium hydroxide, according to the chemical equation
Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.
Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 700x10-15 to 1 with stable isotopes. 36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic ray protons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to 36S and to 36Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, including dating ice and sediments.
Chlorine gas extraction
Chlorine can be manufactured by electrolysis of a sodium chloride solution ( brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2). These two products, as well as chlorine are highly reactive. There are three industrial methods for the extraction of chlorine by electrolysis.
Mercury cell electrolysis
Mercury cell electrolysis was the first method used to produce chlorine on an industrial scale. Titanium anodes are located above a liquid mercury cathode and a solution of sodium chloride is positioned between the electrodes. When an electrical current is applied, chloride is released at the titanium anodes and sodium dissolves into the mercury cathode forming an amalgam.
The amalgam can be regenerated into mercury by reacting it with water, producing hydrogen and sodium hydroxide. These are useful byproducts. However, this method consumes vast amounts of energy and there are also concerns about mercury emissions.
Diaphragm cell electrolysis
In diaphragm cell electrolysis, an asbestos diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing.
This method uses less energy than the mercury cell, but the sodium hydroxide is not as easily concentrated and precipitated into a useful substance.
Membrane cell electrolysis
The electrolysis cell is divided into two by a membrane acting as an ion exchanger. Saturated sodium chloride solution is passed through the anode compartment leaving a lower concentration. Sodium hydroxide solution is circulated through the cathode compartment exiting at a higher concentration. A portion of this concentrated sodium hydroxide solution is diverted as product while the remainder is diluted with deionized water and passed through the electrolyzer again.
This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution.
- Cathode: 2 H+(aq) + 2 e– → H2 (g)
- Anode: 2 Cl– → Cl2 (g) + 2 e–
Overall equation: 2 NaCl + 2H20 → Cl2 + H2 + 2 NaOH
Before electrolytic methods were used for chlorine production, the direct oxidation of hydrogen chloride with oxygen or air was exercised in the Deacon process:
- 4 HCl + O2 → 2 Cl2 + 2 H2O
This reaction was accomplished with the use of CuCl2 as a catalyst. Due to the extremely corrosive reaction mixture, industrial use of this method is difficult.
Another earlier process to produce chlorine was to heat brine with acid and manganese dioxide.
- 2 NaCl + 2H2SO4 + MnO2 → Na2SO4 + MnSO4 + 2 H2O + Cl2
Using this process, chemist Carl Wilhelm Scheele was the first to isolate chlorine in a laboratory. The manganese can be recovered by the Weldon process.
In a laboratory, small amounts of chlorine gas can be created by adding concentrated hydrochloric acid (typically about 5M) to sodium chlorate solution.
Applications and Uses
Purification and Disinfection
Chlorine is an important chemical for some processes of water purification, in disinfectants, and in bleach. Ozone can also be used for killing bacteria, and is preferred by many municipal drinking water systems because ozone does not form organochlorine compounds and does not remain in the water after treatment.
Chlorine is also used widely in the manufacture of many every-day items, or to purify water in various forms.
- Used (in the form of hypochlorous acid) to kill bacteria and other microbes from drinking water supplies and swimming pools. However, in most non-commercial swimming pools chlorine itself is not used, but rather the mixture sodium hypochloride, a mixture of sodium and chlorine. Even small water supplies are now routinely chlorinated. (See also chlorination)
- Used widely in paper product production, antiseptic, dyestuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products.
Chlorine is used extensively in organic and inorganic chemistry as an oxidizing agent and in substitution reactions because chlorine often imparts many desired properties to an organic compound when it is substituted for hydrogen (as in synthetic rubber production) because of its high electron affinity.
World War I
Chlorine became the first killing agent to be employed during World War I. German chemical conglomerate IG Farben had been producing chlorine as a by-product of their dye manufacturing. In cooperation with Fritz Haber of the Kaiser Wilhelm Institute for Chemistry in Berlin, they developed methods of discharging chlorine gas against an en trenched enemy.
For general references to the chloride ion (Cl−), including references to specific chlorides, see chloride. For other chlorine compounds see chlorate (ClO3−), chlorite (ClO2−), hypochlorite(ClO−), and perchlorate(ClO4−), and chloramine (NH2Cl).
- Fluorides: chlorine monofluoride (ClF), chlorine trifluoride (ClF3), chlorine pentafluoride (ClF5)
- Oxides: chlorine dioxide (ClO2), dichlorine monoxide (Cl2O), dichlorine heptoxide (Cl2O7)
- Acids: hydrochloric acid (HCl), chloric acid (HClO3), and perchloric acid (HClO4)
It is also used in the production of chlorates, chloroform, carbon tetrachloride, and in bromine extraction.
Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials. For more information see an MSDS.