How can halogens be detected in the aqueous layer

Part of the problem was finding an oxidizing agent strong enough to oxidize the F - ion to F 2. The task of preparing fluorine was made even more difficult by the extraordinary toxicity of both F 2 and the hydrogen fluoride HF used to make it. The best way of producing a strong reducing agent is to pass an electric current through a salt of the metal. Sodium, for example, can be prepared by the electrolysis of molten sodium chloride.

In theory, the same process can be used to generate strong oxidizing agents, such as F 2. Attempts to prepare fluorine by electrolysis, however, were initially unsuccessful. Humphry Davy, who prepared potassium, sodium, barium, strontium, calcium, and magnesium by electrolysis repeatedly tried to prepare F 2 by the electrolysis of fluorite CaF 2 , and succeeded only in ruining his health.

Joseph Louis Gay-Lussac and Louis Jacques Thenard, who prepared elemental boron for the first time, also tried to prepare fluorine and suffered from very painful exposures to hydrogen fluoride. George and Thomas Knox were badly poisoned during their attempts to make fluorine, and both Paulin Louyet and Jerome Nickles died from fluorine poisoning.

Finally, in Henri Moissan successfully isolated F 2 gas from the electrolysis of a mixed salt of KF and HF and noted that crystals of silicon burst into flame when mixed with this gas. Electrolysis of KHF 2 is still used to prepare fluorine today, as shown in the figure below. Common Oxidation Numbers for the Halogens. Fluorine is the most electronegative element in the periodic table. As a result, it has an oxidation number of -1 in all its compounds.

General Trends in Halogen Chemistry. The chemistry of fluorine is simplified by the fact it is the most electronegative element in the periodic table and by the fact that it has no d orbitals in its valence shell, so it can't expand its valence shell. Chlorine, bromine, and iodine have valence shell d orbitals and can expand their valence shells to hold as many as 14 valence electrons. These compounds are all colorless gases, which are soluble in water. Each of the hydrogen halides ionizes to at least some extent when it dissolves in water.

Several of the hydrogen halides can be prepared directly from the elements. Mixtures of H 2 and Cl 2 , for example, react with explosive violence in the presence of light to form HCl. Because chemists are usually more interested in aqueous solutions of these compounds than the pure gases, these compounds are usually synthesized in water. Aqueous solutions of the hydrogen halides are often called mineral acids because they are literally acids prepared from minerals.

Hydrochloric acid is prepared by reacting table salt with sulfuric acid, for example, and hydrofluoric acid is prepared from fluorite and sulfuric acid. These acids are purified by taking advantage of the ease with which HF and HCl gas boil out of these solutions. The gas given off when one of these solutions is heated is collected and then redissolved in water to give relatively pure samples of the mineral acid.

Interhalogen compounds are formed by reactions between different halogens. All possible interhalogen compounds of the type XY are known. A results table similar to the one below could be used for the recording of results. It has been completed with expected observations. The halogens are more soluble in the hydrocarbon and move to this top layer when shaken with a hydrocarbon solvent. For chlorine and bromine the colour does not change. You might need a white background to see the colour of the chlorine solution.

However, for iodine there is a colour change, from brown in water to purple in the hydrocarbon layer. Where no displacement reaction takes place between a halogen solution and a halide solution, it may be that some lightening in the colour of the solution is observed and this can be explained by the effect of dilution.

Some students with respiratory problems can show an allergic reaction to chlorine, the onset of which may be delayed. Iodine is the least soluble of the halogens in water. Polar water molecules interact with iodine molecules, altering the wavelengths of light they absorb.

All three halogens react with water to produce a strong acid HX , and a weak acid HOX , which has bleaching properties and is an oxidising agent. The extent of reaction decreases down Group With iodine it is so small that the acidic and bleaching properties of the solution are not seen in this experiment.

In the displacement reactions chlorine displaces both bromine and iodine from their compounds and bromine displaces iodine. For example:. A more advanced treatment identifies the halogens as oxidising agents, accepting an electron to form halide ions:.

Potassium is only present here as very unreactive potassium ions spectator ions in solution. This collection of over practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Demonstrate the movement of positive and negative ions with a simpler, safer version of this classic demo. In this activity students are required to apply their understanding of redox to identify which metal is oxidised and which is reduced in a series of displacement reactions.

Use this practical to investigate how solutions of the halogens inhibit the growth of bacteria and which is most effective. Site powered by Webvision Cloud. Skip to main content Skip to navigation. Five out of five No comments. Equipment Apparatus Eye protection Test tube rack, to hold 10 test tubes Test tubes x10 Cork or rubber bungs to fit, x4 Plastic dropping pipettes x6 White spotting tile White tile Glass rod Paper towel or tissue Chemicals About 10 cm 3 of each of the following halogen solutions in stoppered test tubes see notes 1 and 2 : Chlorine water, 0.

The halogen solutions can be diluted further to minimise the amount of chlorine or bromine fumes given off but should not be so dilute that their distinctive colours are not clearly visible in the test tubes a white background may be needed for chlorine water. At the end of the experiments all mixtures and solutions should be returned to a suitable waste container in a fume cupboard for safe disposal.

Health, safety and technical notes Read our standard health and safety guidance. Wear eye protection throughout. Iodine solution is actually iodine dissolved in aqueous potassium iodide. Essentially, the product of the ionic concentrations is never be greater than the solubility product value. Enough solid is always precipitated to lower the ionic product to the solubility product.

The table below lists solubility products from silver chloride to silver iodide a solubility product for silver fluoride cannot be reported because it is too soluble. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.

The equation for this reaction is given below:. A solution in contact with one of the silver halide precipitates contains a very small concentration of dissolved silver ions. The effect of adding the ammonia is to lower this concentration still further. If the adjusted silver ion concentration multiplied by the halide ion concentration is less than the solubility product, some precipitate dissolves to restore equilibrium.

This occurs with silver chloride, and with silver bromide if the ammonia is concentrated. The more concentrated ammonia pushes the equilibrium even further to the right, lowering the silver ion concentration even more.

The silver iodide is so insoluble that ammonia cannot lower the silver ion concentration enough for the precipitate to dissolve. Adding concentrated sulfuric acid to a solid sample of one of the halides gives the following results:. The only possible confusion is between a fluoride and a chloride—they behave identically under these conditions.