Chlorine
Chlorine is one of the most important and most produced chemicals.
As chlorine reacts with almost all elements and compounds, with a few exceptions, it occurs in nature almost exclusively as a chemical compound. Large chlorine deposits are therefore widespread on earth, especially in the form of salts, for example in salt deposits in the earth's interior or dissolved in seawater - chlorine is the most common element in seawater after oxygen and hydrogen. In its ionic form of chloride as a mineral, chlorine also controls many important processes in the human body, such as the water balance. Chloride is also required for the production of stomach acid and for impulse conduction in nerve fibres.
In its elemental form, chlorine is present in very small quantities in volcanic gases and in the ozone layer, where it is split off from chlorofluorocarbons (CFCs) produced by humans and contributes significantly to the formation of the ozone layer.
Properties of chlorine
Chlorine is a diatomic molecule of yellow-green colour with the chemical formula Cl2. It has a pungent odour and is classified as toxic in the list of hazardous substances. The gas reacts violently with almost all other elements and many compounds, generating a lot of heat. Only with oxygen, nitrogen, carbon and noble gases does no direct reaction take place. Many metals, such as manganese and zinc or the precious metals gold, silver and platinum, only react with chlorine at elevated temperatures. The presence of water also plays an important role in the reaction of chlorine with metals.
Due to its high reactivity, chlorine must be handled with care. It is very reactive, especially when wet. In particular, the combination with metals to form metal chlorides is usually very violent, often even causing fire. Chlorine forms explosive mixtures with hydrogen (chlorine oxyhydrogen).
Chlorine is an element with atomic number 17, which is in the seventh main group in the periodic table and belongs to the 17th IUPAC group, the halogens. Chlorine is a mixture of two stable isotopes (atomic species): 35Cl, which makes up about 75 per cent of the element, and 37Cl.
With a density of 3.214 g/l at 0 °C, chlorine is about 2.5 times heavier than air. It condenses to a yellow liquid at -34.6 °C, at -195 °C it is almost colourless. At -101 °C, chlorine changes to the solid state and forms yellow crystals. Chlorine is liquid at a pressure of 6.7 bar at 20 °C and can be transported in steel cylinders or tank wagons.
The gas can be dissolved in water. One litre of water dissolves 2.3 litres of chlorine at 20 °C. A 0.5 percent solution of chlorine in water is known as chlorinated water. The element dissolves well in compounds containing chlorine and in some organic solvents.
Chlorine exists with oxidation numbers from -1 to +7. Inorganic compounds in which chlorine is present in the oxidation state -1 and therefore as an anion are called chlorides. They are the most important compounds with chlorine. The best-known chloride is sodium chloride, i.e. common salt. It is formed in a violent reaction of chlorine and the alkali metal sodium (Na) under bright, yellow light. The most important Cl-1 compound is hydrogen chloride (HCl), the aqueous solution of which produces hydrochloric acid, one of the most important acids. Other chlorous acids are hypochlorous acid (HClO), chlorous acid (HClO2), chloric acid (HClO3) and perchloric acid (HClO4), an extremely strong, so-called super acid.
In the chemical industry, chlorine is obtained exclusively by chlor-alkali electrolysis: an aqueous sodium chloride solution is electrolysed. Chlorine is formed at the positive pole. At the negative pole, water decomposes into hydrogen and hydroxide ions, which form caustic soda with the sodium ions. In all chlorine production processes, it should be noted that the anode, where the chlorine is formed, is separate from the cathode, where hydrogen and hydroxide ions are formed: chlorine reacts explosively with hydrogen in the so-called chlorine oxyhydrogen reaction if irradiation with short-wave light or localised heating takes place at the beginning. The energy supplied causes the Cl2 molecule to split. If produced in the same vessel, the explosive chlorine-hydrogen mixture hydrogen chloride, also known as chlorine oxyhydrogen, would be formed. In addition, a reaction of chlorine with the hydroxide ions to form hypochlorite would take place.
