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# 1.8 GROUP VII - THE HALOGENS **Learning Objectives:** By the end of this topic, you should be able to: * Recall the colours of the elements and explain the trends within the Group, limited to physical state at room temperature, melting and boiling points. * Compare the solubility and colours of...

# 1.8 GROUP VII - THE HALOGENS **Learning Objectives:** By the end of this topic, you should be able to: * Recall the colours of the elements and explain the trends within the Group, limited to physical state at room temperature, melting and boiling points. * Compare the solubility and colours of the halogens in water and non-aqueous solvents, e.g. hexane. * Describe the reaction of the halogens with cold, dilute and hot, concentrated sodium hydroxide and explain the disproportionation in these reactions. * Recall the reaction of chlorine with water to form chloride ions and chlorate(I) ions. * Describe the trend in oxidising ability of the halogens down the Group applied to displacement reactions of the halogens with other halide ions in solution. * Demonstrate understanding of the reactions of solid halides with concentrated sulfuric and phosphoric acid in relation to the relative reducing ability of hydrogen halides/halide ions. * Compare the advantages and disadvantages of adding chlorine or ozone to drinking water. **General Characteristics of Halogens:** The halogens are a group of reactive non-metals which are essentially similar with only gradual changes in character as the group is descended. They exist as covalently bonded diatomic molecules. The non-polar molecules have weak van der Waals forces between the molecules. The strength of these forces depends on the number of electrons per molecule. The larger the relative formula mass of the molecule the more electrons there are, therefore the stronger the van der Waal's forces are. Thus melting points and boiling points increase down the group. **Physical Properties of the Halogens:** | Element | Appearance | Intermolecular Forces | Melting Point (°C) | Boiling Point (°C) | |---|---|---|---|---| | Fluorine (F2) | Yellow gas at rtp | van der Waals forces when liquefied | -220 | -188 | | Chlorine (Cl2) | Green-yellow gas at rtp | van der Waals forces when liquefied | -101 | -35 | | Bromine (Br2) | Red-brown liquid at rtp; Red-brown vapour | van der Waals forces | -7 | 59 | | Iodine (I2) | Grey-black crystals at rtp; Vapour is violet | van der Waals forces | 114 (sublimes) | 184 | **Note:** The table shows the physical properties of the halogens. 'rtp' likely means 'room temperature'. The 'sublimes' entry for Iodine indicates it goes directly from solid to gas phase without becoming liquid.