Group 7

3.1.3 Halogens Fluorine (F2 ): very pale yellow gas. It is highly reactive Chlorine : (Cl2 ) greenish, reactive gas, poisonous in high concentrations Bromine (Br2 ) : red liquid, that gives off dense brown/orange poisonous fumes Iodine (I2 ) : shiny grey solid sublimes to purple gas. Trend in melting point and boiling point Increase down the group As the molecules become larger they have more electrons and so have larger induced dipole-dipole forces (London forces) between the molecules. As the intermolecular forces get larger more energy has to be put into break theses intemolecular forces. This increases the melting and boiling points All halogens exist as diatomic molecules Redox reactions and reactivity of halogens and their compounds Electronic configuration. All group 7 elements have the outer shell s2p 5 electron configuration. The will often react by gaining of one electron in redox reactions to form 1– ions

1. The displacement reactions of halide ions by halogens. A halogen that is more reactive will displace a halogen that has a lower reactivity from one of its compounds The reactivity of the halogens decreases down the group as the atoms get bigger with more shielding so they less easily attract and accept electrons. They therefore form -1 ions less easily down the group Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions Chlorine (aq) Bromine (aq) Iodine (aq) potassium chloride (aq) Very pale green solution, no reaction Yellow solution, no reaction Brown solution, no reaction potassium bromide (aq) Yellow solution, Cl has displaced Br Yellow solution, no reaction Brown solution, no reaction potassium iodide (aq) Brown solution, Cl has displaced I Brown Solution, Br has displaced I Brown Solution, no reaction The colour of the solution in the test tube shows which free halogen is present in solution. Chlorine =very pale green solution (often colourless), Bromine = yellow solution Iodine = brown solution (sometimes black solid present) know these observations ! Cl2 (aq) + 2Br – (aq)
2Cl – (aq) + Br2 (aq) Cl2 (aq) + 2I – (aq)
2Cl – (aq) + I2 (aq) Br2 (aq) + 2I – (aq)
2Br – (aq) + I2 (aq) Explanation of reactivity Chlorine is more reactive than bromine because it will gain an electron and form a negative ion more easily than bromine. The is because an atom of chlorine is smaller than bromine and the outermost shell of chlorine is less shielded than bromine so the electron to be gained is attracted more strongly to the nucleus in chlorine than bromine. Observations if an organic solvent is added Chlorine (aq) Bromine (aq) Iodine (aq) potassium chloride (aq) colourless, no reaction yellow, no reaction purple, no reaction potassium bromide (aq) yellow, Cl has displaced Br yellow, no reaction purple, no reaction potassium iodide (aq) purple, Cl has displaced I purple, Br has displaced I purple, no reaction The colour of the organic solvent layer in the test tube shows which free halogen is present in solution. Chlorine = colourless Bromine = yellow Iodine = purple

2. The disproportionation reactions of chlorine and chlorate(I). Disproportionation is the name for a reaction where an element simultaneously oxidises and reduces. Chlorine with water: Cl2 (aq) + H2O(l)
HClO(aq) + HCl (aq) Chlorine is both simultaneously reducing and oxidising. It changes from 0 in Cl2 to -1 in HCl and +1 in HClO If some universal indicator is added to the solution it will first turn red due to the acidity of both reaction products. It will then turn colourless as the HClO bleaches the colour. Chlorine is used in water treatment to kill bacteria. It has been used to treat drinking water and the water in swimming pools. The benefits to health of water treatment by chlorine by its killing of bacteria outweigh its risks of toxic effects and possible risks from formation of chlorinated hydrocarbons . Reaction of Chlorine with cold dilute NaOH solution: Cl2 ,(and Br2 , I2 ) in aqueous solutions will react with cold sodium hydroxide. The chlorine is reacting by disproportionation. The colour of the halogen solution will fade to colourless Cl2 (aq) + 2NaOH(aq)
NaCl (aq) + NaClO (aq) + H2O(l) The mixture of NaCl and NaClO (sodium chlorate (I)) is used as Bleach and to disinfect/ kill bacteria. If the hot sodium hydroxide is used a different disproportionation reaction occurs forming sodium chlorate (v) 3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O

3. The reactions of halide ions with silver nitrate. This reaction is used as a test to identify which halide ion is present. The test solution is made acidic with nitric acid, and then Silver nitrate solution is added dropwise. The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3 . This would mask the desired observations 2 HNO3 + Na2CO3
2 NaNO3 + H2O + CO2 Fluorides produce no precipitate Chlorides produce a white precipitate Ag+ (aq) + Cl- (aq)
AgCl(s) Bromides produce a cream precipitate Ag+ (aq) + Br- (aq)
AgBr(s) Iodides produce a pale yellow precipitate Ag+ (aq) + I- (aq)
AgI(s) The silver halide precipitates can be treated with ammonia solution to help differentiate between them if the colours look similar: Silver chloride dissolves in dilute ammonia to form a complex ion AgCl(s) + 2NH3 (aq)
[Ag(NH3 )2 ] + (aq) + Cl- (aq) Colourless solution Silver bromide dissolves in concentrated ammonia to form a complex ion AgBr(s) + 2NH3 (aq)
[Ag(NH3 )2 ] + (aq) + Br – (aq) Colourless solution Silver iodide does not react with ammonia – it is too insoluble