Group 7 Elements: Redox Reactions

The Group 7 ElementsThe Group 7 elements are fluorine, chlorine, bromine, iodine and astatine. These elements are generally known as the halogens.Physical PropertiesThe halogens have the following physical properties: they have low melting and boiling points the exist as diatomic molecules X2 As you move down the group the number of electrons increases, leading to an increase in the van der Waals' forces between molecules. the boiling points of the halogens increases down the group the physical states of the halogens at room temperature and pressure show the classic trend of gas to liquid to solid as you move down the group Electronic ConfigurationThe elements in Group 7 have seven electrons in their outer shell. The highest energy electrons are in the p sub-shell and the elements form part of the p-block in the Periodic Table.Each Group 7 element has: one electron fewer than the electron configuration of a noble gas an outer p sub-shell containing 5 electrons Halogens as oxidsing agentsThe halogens are the most reactive non-metals in the Periodic Table and are strong oxidising agetns removing electrons in reactions. The oxidising power of a halogen is a measure of the strength with which a halogen atom is able to attract and capture an electron to form a halide ion.In redox reactions each halogen atom gains one electron into a p sub-shell to for a halide ion with a 1- charge.1/2 Cl2 + e- ----> Cl-The halogens become less reactive down the group as their oxiding power decreases. As halogens gain an electron in their reactions, reactivity decreases down the group because: atomic radius increases electron shielding increases the ability to gain an electron into the p sub-shell decreases to form a halide ion Redox Reactions of the HalogensRedox reactions can show that the halogens become less able to form halide ions down the group. We can show the decrease in reactivity using redox reactions of: aqueous solutions of halide ions: Cl-, Br-, and I- with aqueous solutions of halogens: Cl2, Br2, I2 Each halogen is mixed with aqueous solutions of the different halides. A more-reactive halogen will oxidise and displace a halide of a less-reactive halogen. This is often called a displacement reaction.Halogens form solutions with different colours, so any change in colour will show whether a redox reaction has taken place. The mixture is usually shaken with an organic solvent, such as cyclohexane, to help distinguish between bromine and iodine. Cl2 - pale green in water, pale green in cyclohexaneBr2 - orange in water, orange in cyclohexaneI2 - brown in water, violet in cyclohexaneChlorine oxidises bromine and iodine ions. Cl2  +  2Br- -------> 2Cl-  +  Br2     orange in water and in cyclohexane Cl2  +  2I- ---------> 2Cl-  +  I2        brown in water, purple in cyclohexane Changes in oxidation number for reaction with bromine is: Cl2  +  2Br- --------> 2Cl-  +  Br2 0x2     2x-1            2x-1     2x0 Bromine oxidises iodine ions only:Br2  +  2I- --------> 2Br- + I2       brown in water, purple in cyclohexaneIodine does not oxidise either Cl- or Br- ions.DisproportionationDisproportionation is a reaction in which the same element is both oxidised and reduced.Disproportionation of chlorine in waterSmall amounts of chlorine are added to water to kill bacteria and make the water safer to drink.Chlorine reacts with water, forming a mixture of two acids, hydrochloric acid and chloric (I) acid, HClO.This is a disproportionation reaction in which chlorine is both oxidised and reduced. Cl2  +  H20  ----------> HCl  +  HClO 0                               +1                     chlorine reduced 0                                           -1          chlorine oxidised Disproportionation of chlorine in aqueous sodium hydroxideChlorine is only slightly soluble in water and has a mild bleaching action. Household bleach is formed when dilute aqueous sodium hydroxide and chlorine react together at room temperature.This is also a disproportionation reaction because chlorine is both oxidised and reduced. Cl2  +  NaOH  -------------> NaCl  +  NaClO  +  H2O 0                                       -1                                 chlorine reduced 0                                                     +1                  chlorine oxidised

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