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Reactions at the electrodes

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Open-Book Paper Reactions at the electrodes The membrane cell is used in the extraction of chlorine. The brine containing the chlorine is added to the side with the positive electrode, this is where there is a titanium anode. Here chloride ions are discharged which produces chlorine gas. At the other side of the membrane cell is a steel cathode. The sodium ions, which have passed through the membrane, enter the dilute sodium hydroxide. The steel cathode supplies the water with electrons to liberate hydrogen gas and hydroxide ions. These reactions are redox reactions as there is an oxidation and a reduction. In the reaction that takes place at the anode the chlorine ions are being oxidised and in the reaction that takes places at the cathode the hydrogen is being reduced. It is necessary to keep apart the anode and cathode products of the electrolysis of aqueous sodium chloride to prevent them from reacting with each other. This is achieved by having a type of barrier between the electrodes. Mercury cathode cell In the mercury cathode cell the products do not react with each other because the chlorine is produced in a different container. ...read more.


As the chloride ions could not pass through the membrane the sodium hydroxide solution has a high purity and is not contaminated with salt. Fig. 1: the membrane cell The halogens The methods for producing chlorine and iodine work well due to the chemistry of the elements. Chlorine and iodine both belong to group 7, the halogens. All halogens have seven electrons in the outer shell. The outer shell has to be full to convert ions into molecules, this is why the methods of extracting chlorine and iodine include the transfer of electrons as redox reactions. 600 It is harder for the ions to convert to molecules as iodine is less reactive than chlorine (due to it being larger as it has one extra full shell of electrons). This is why the process of extracting iodine is a slower process then that of chlorine. Iodine is produced by a displacement reaction the chlorine molecules readily lose electrons to the iodide ions. This is not possible in the production of chlorine as the only element that will displace it is fluorine. ...read more.


After cooling a spray of the solution is into sulphur dioxide which reduces the iodate(V) ions to iodide ions. Iodide is liberated by adding a small amount of the original iodate(V) to the resulting solution. The iodide then separates as a solid and is extracted with a hydrocarbon solvent. Another method used today is from extracting iodine from brine using a redox reaction. The brine is reacted with chlorine after purification and acidification with sulphuric acid. This solution is dilute in iodine and is concentrated by blowing a stream of air in to the solution causing the iodine to vaporise. The iodine rich air passes into an absorbing tower containing acid; here adding sulphur dioxide reduces the iodine. When this solution is chlorinated again the concentration of iodine is greater then the solubility so therefore crystals of iodine separate and these can be obtained through filtration. These methods are a lot easier to control then the original way of extracting iodine by using kelp. Iodine can be obtained quicker as there is no need for drying. This is why the kelp industry stopped and the other methods became more popular. 980 The uses of chlorine, sodium hydroxide, hydrogen and iodine in present day industry. ...read more.

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