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The Group VII elements - Halogens

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The Group VII elements are called HALOGENS. The name Halogen means "salt - former" and is based on the fact that the elements combine with most metals to form salts. (halides) ELEMENT FLUORINE CHLORINE BROMINE IODINE State at 20oC GAS GAS LIQUID SOLID Colour PALE YELLOW PALE GREEN RED-BROWN BLACK Melting Point (0C) - 220 - 101 - 7 113 Boiling Point (0C) - 188 - 35 59 183 ELECTRON AFFINITY Electron affinity is the enthalpy change when gaseous atoms of an element gain electrons to become negative ions. The FIRST ELECTRON AFFINITY of an element is the ENTHALPY CHANGE WHEN ONE MOLE OF GASEOUS ATOMS GAINS ELECTRONS TO FORM ONE MOLE OF GASEOUS IONS. Electron affinity DECREASES as you DESCEND the group VII: F --> 333kJ Cl --> 346kJ of energy is released for every one mole of F/Cl/Br/I Br --> 324kJ atoms changing to a mole of F/Cl/Br/I ions. I --> 295kJ This trend can be explained by the fact that as you descend the group, the X- ion becomes less stable and is therefore less likely to form. H is less stable because electrons are less strongly attracted due to the fact that the valence electrons are further from the attractive positive nucleus, and because there is an increase in the number of inner shells which, in turn, increases the shielding effect. HALOGENS AS OXIDISING AGENTS Since their atoms accept electrons, the halogens are oxidising agents (OIL), and in a reaction they are reduced (RIG). ...read more.


The oxidising ability of the halogens decreases s you descend the group. If chlorine water is added to a salt of bromine, eg.NaB. (aq) or Iodine, eg.KI. (aq), then the chlorine will displace the halogen from it's compound. The colourless Solution will turn brown as bromine is formed, or red as iodine is formed. Cl2 (aq) + 2NaBr (aq) --> 2NaCl (aq) + Br2 (aq) Bromine water (BROWN) Cl2 (aq) + 2KI (aq) --> 2KCl (aq) + I2 (actually KI3 (aq) as I2 does not dissolve In water.) (RED) Bromine water could be used to displace iodine from an iodide salt, such as KI, but it could not displace chlorine from a chloride salt such as KCl. Br2 (aq) + 2KI (aq) --> 2KBr (aq) + I2 (aq) Brown Colourless Colourless Red Halogens form diatomic molecules. They form HOMO - NUCLEAR DIATOMIC MOLECULES : Br - Br I -- I i.e., they have no permanent dipole and are therefore non - polar. They therefore dissolve better in non - polar solvents. Having said this, the halogens DO dissolve in water, a polar solvent, because of instantaneous dipoles. They dissolve much more readily and easily, however, in non - polar solvents. Hexane is a non - polar solvent. In this experiment, hexane is put in a test tube with the aqueous solution of a halide ion: Hexane (non - polar) Aqueous (polar) The test tube is shaken and the hexane is physically mixed through the aqueous solution. ...read more.


--> S + 3I2 + 4H20 CONCENTRATED SULPHURIC ACID ALONG WITHMANGANESE (IV) OXIDE AS THE OXIDISING AGENT. This is an even stronger oxidising solution and it is capable of oxidising HCl as well as HBr and HI. Concentrated sulphuric acid alone is not a sufficiently strong oxidising agent to oxidise HCl(g) to Cl2(g). This isdue to the relatively strong H - Cl bond. In combination with MnO2it can oxidise HClto Cl2. 2HCl + H2SO4(l) MnO2 CL2 + SO2 + 2H2O A COMBINATION OF HYDROGEN PEROXIDE ANDDILUTE SULPHURIC ACID AS AN OXIDISING SOLUTION. This oxidisingsolution is only sufficiently strong to oxidise iodide to iodine. Itcannotoxidise solutions of theother halide ions. REACTIONWITH WATER OF THE HYDROGEN HALIDES All the hydrogenhalides dissolve in water to form acid solutions: Hydrochloric Acid Hydrobromic Acid Hydroiodic Acid HX (g) + H2O --> H3O+(aq) + X-(aq) (Where X = Cl, Br or I) The hydrogen halides are covalently bonded, whereas, their corresponding acids are ionically bonded. Hydroiodic acid is a stronger acid than Hydrobromic acid, which, in turn, is stronger than hydrochloric acid. In other words, acid strength increases from HCl (aq) to HI(aq). This can be explained by virtue of the fact that the H - I bond is weaker than the H - Cl bond. Therefore, HI dissociates more fully in water. All three acids are STRONG ACIDS and display the typical reactions of strong acids. Dilute Hydrofluoric Acid is a WEAK ACID. Only about 10% of the HF molecules are dissociated in a 0.1 mol dm-3solution. This is because the H - F bond is very strong and the presence of strong intermolecular hydrogen bonds hinders dissociation. ...read more.

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