Group 7

• Fluorine (F2)

  Very pale yellow gas, 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
• As the molecules become larger

  They have more electrons and so have larger London forces between the molecules
• As the intermolecular forces get larger

  More energy has to be put into overcoming the forces, increasing the melting and boiling points
• Melting Point and Boiling Point of Halogens

  • Fluorine: Melting Point -220°C, Boiling Point -188°C, Physical State Gas
  • Chlorine: Melting Point -101°C, Boiling Point -35°C, Physical State Gas
  • Bromine: Melting Point -7°C, Boiling Point 59°C, Physical State Liquid
  • Iodine: Melting Point +114°C, Boiling Point 184°C, Physical State Solid
• Electronegativity

  The relative tendency of an atom in a molecule to attract electrons in a covalent bond to itself
• As one goes down the group

  The electronegativity of the elements decreases
• As one goes down the group

  The atomic radii increases due to the increasing number of shells, making the nucleus less able to attract the bonding pair of electrons
• Oxidising agent

  An electron acceptor
• A halogen that is a strong oxidising agent will displace a halogen that has a lower oxidising power from one of its compounds
• The oxidising strength of the halogens decreases down the group
• Reactions of halogens

  1. Chlorine will displace both bromide and iodide ions

  2. Bromine will displace iodide ions
• Reactions of halogens with halide solutions

  • Chlorine (aq) with Potassium chloride (aq): Very pale green solution, no reaction
  • Chlorine (aq) with Potassium bromide (aq): Yellow solution, Cl has displaced Br
  • Chlorine (aq) with Potassium iodide (aq): Brown solution, Cl has displaced I
  • Bromine (aq) with Potassium iodide (aq): Brown Solution, Br has displaced I
• 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)
• All the halide salt solutions (KI, NaBr, KCl etc.) are colourless
• Reactions of halogens with halide solutions in organic solvent

  1. Chlorine (aq) with Potassium chloride (aq): colourless, no reaction

  2. Chlorine (aq) with Potassium bromide (aq): yellow, Cl has displaced Br

  3. Chlorine (aq) with Potassium iodide (aq): purple, Cl has displaced I

  4. Bromine (aq) with Potassium iodide (aq): purple, Br has displaced I
• 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
• Oxidation half equation for halogens

  2Br- (aq) Br2 (aq)+ 2e-
• Reduction half equation for halogens

  Cl2 (aq) + 2e- 2Cl- (aq)
• Oxidation reactions of metals and metal ions by halogens

  1. 3Cl2(g) + 2 Fe (s) 2 FeCl3 (s)

  2. Br2(l) + 2Na (s) 2NaBr (s)

  3. Cl2(g) + 2Fe2+ (aq) 2 Cl- (aq) + 2Fe3+ (aq)

  4. 2I- (aq) + 2Fe3+ (aq) I2 (aq) + 2Fe2+ (aq)
• In all reactions where halogens are reacting with metals, the metals are being oxidised
• Chlorine and bromine can oxidise Fe2+ to Fe3+, but iodine is not strong enough an oxidising agent to do this reaction
• Disproportionation reaction of chlorine with water

  Cl2(aq) + H2O(l) ⇌ HClO(aq) + HCl (aq)
• Chlorine is both simultaneously reducing and oxidising, changing its oxidation number from 0 to -1 and 0 to +1
• If universal indicator is added, the solution will first turn red due to the acidity, then colourless as the HClO bleaches the colour
• Chlorine is used in water treatment to kill bacteria, the benefits outweigh its toxic effects
• Reaction of chlorine with water in sunlight

  2Cl2 + 2H2O 4H+ + 4Cl- + O2
• The greenish colour of chlorine water fades as the Cl2 reacts and a colourless gas (O2) is produced
• Reaction of halogens with cold dilute NaOH solution

  1. Cl2(aq) + 2NaOH(aq) NaCl (aq) + NaClO (aq) + H2O(l)

  2. 3I2 (aq) + 6OH- (aq) 5 I- (aq) + IO3- (aq) + 3H2O (l)
• The mixture of NaCl and NaClO is used as bleach and to disinfect and kill bacteria
• NaClO

  Sodium chlorate(I)
• NaClO3

  Sodium chlorate(V)
• K2SO4

  Potassium sulfate(VI)
• K2SO3

  Potassium sulfate(IV)
• Reaction of halogens with hot dilute NaOH solution

  1. 3Cl2 (aq) + 6 NaOH(aq) 5 NaCl (aq) + NaClO3 (aq) + 3H2O (l)

  2. 3I2 (aq) + 6KOH (aq) 5 KI (aq) + KIO3 (aq) + 3H2O (l)
• Thiosulfate redox titration

  2S2O3^2-(aq) + I2 (aq) 2I- (aq) + S4O6^2-(aq)