Group 7

• Halogens exist as diatomic molecules
• Atomic radius increases across down group 7 as there is more shells so more shielding so weaker attraction between nucleus and outer electrons so they are held less tightly
• The ionic radius of the halogens is bigger than the atomic radius as the X- ion has the same number of protons but more electrons so attraction is weaker and electrons are held less tightly
• Boiling point increases down group 7. Halogens become less volatile (able to evaporate easily) down the group because going down the group Mr (molecules) get bigger so there are more electrons so Van der Waal's forces between molecules are weaker
• Electronegativity decreases down group 7 because more shells so more shielding so weaker attraction between nucleus and electrons in a covalent bond
• First ionisation energy decreases down group 7 because more shells so more shielding so weaker attraction between nucleus and electron being removed so less energy needed to remove it
• Chlorine is a pale green gas at room temperature. In aqueous solution it is a pale green solution and in cyclohexane it is a pale green solution.
• Bromine is a red-brown liquid at room temperature. In aqueous solution it is an orange solution and in cyclohexane it is an orange solution.
• Iodine is a black solid at room temperature which sublimes into a purple vapour. In aqueous solution it is a brown solution and in cyclohexane it is a pink-purple solution.
• Halogens are more soluble in non-polar solvents like cyclohexane because they are non-polar.
• Solubility of halogenoalkanes in water decreases down the group has halogen molecule becomes larger.
• Add chlorine to KBr and bromine is formed (orange solution). Add chlorine to KI and iodine is formed (brown solution)
• Add bromine to KCl and solution is still orange as it is still bromine. Add bromine to KI and iodine is formed (brown solution)
• Add iodine to KCl or KBr and solution is still brown.
• Add cyclohexane to the test tubes and orange indicates bromine and pink-purple indicates iodine.
• An oxidising agent is a substance which accepts electrons. Halogens are oxidising agents as they accept electrons to form halide ions.
• Oxidising power of the halogens decreases going down the group as there are more shells so more shielding so weaker attraction between nucleus and electron being accepted so overall going down group they accept electrons less readily.
• Disproportionation is a redox reaction in which the same element is both oxidised and reduced.
• eg An equilibrium is established between chlorine, water, hydrochloric acid and chloric(I) acid: Cl2 + H2O <-> HCl + HClO. Chlorine is both oxidised and reduced.
• Observations: chlorine is a pale green solution, hydrochloric acid turns litmus paper red and chloric(I) acid bleaches litmus paper.
• When chlorine reacts with water in the presence of sunlight, hydrochloric acid and oxygen are produced because the chloric(I) acid initially formed decomposes in sunlight to form HCl and oxygen. (HClO -> HCl + 1/2O2 so Cl2 + H2O -> HCl + 1/2O2)
• Chlorine is useful in water treatment as it kills bacteria but it has risks as it is toxic in large quantities.
• Reaction between chlorine and dilute NaOH solution produces NaCl, NaClO and H2O. During reaction green colour of chlorine fades and smell is less pungent. Cl2 is both oxidised and reduced. It is used to kill bacteria and as a bleach.
• Examples of halide salts include NaCl, CaBr2 and KI.
• To test for halide ions:

  1. make a solution if the substance to be tested.
  2. add dilute HNO3 to remove other ions that may interfere eg CO32-. (Not HCl as don’t want to add Cl-)
  3. add AgNO3 solution (silver nitrate)
  4. Observe colour of ppt.
• Cl- becomes AgCl which forms white precipitate.

  Br- becomes AgBr which forms a cream precipitate.

  I- becomes AgI which forms a yellow precipitate.

  F-ions do not form precipitates in the solution as AgF is soluble in water.
• General ionic equation for the reaction of silver nitrate and halide ions is Ag+(aq) + X-(aq) -> AgX(s)
• AgCl dissolves in both dilute and concentrated ammonia.

  AgBr is insoluble in dilute ammonia (cream ppt remains) but dissolves in concentrated ammonia.

  AgI is insoluble in both dilute and concentrated ammonia (yellow ppt remains).
• A reducing agent is an electron donor. Halides are reducing agents as they can donate electrons to form halogens.
• Concrentrated H2SO4 (+6) reacts with solid halide salts to produce:

  SO2 (choking acidic gas, +4)

  S (yellow solid, 0)

  H2S (gas with bad eggs smell, -2)

  H2SO4 is reduced to one of these products, halide is oxidised to a halogen.
• NaCl reacts with H2SO4 to produce NaHSO4 and HCl. Chloride ions are weak reducing agents so can’t reduce sulfuric acid. Observations are steamy white fumes (HCl) and H2SO4 acts as an acid (proton donor). NOT A REDOX REACTION
• NaBr reacts with H2SO4 to produce HBr and NaHSO4. Steamy white fumes (HBr) and H2SO4 acts as acid.

  Redox: Br- ion reduces acid to SO2 so observations are orange fumes (Br2) and choking acidic gas (SO2). Br- is oxidised and H2SO4 is reduced so Br- is reducing agent and H2SO4 is oxidising agent.
• NaI reacts with H2SO4 to produce HI and NaHSO4. Steamy white fumes (HI) and H2SO4 acts as an acid.

  Redox: I- can reduce H2SO4 to SO2 (black solid and choking acidic gas), to S (black solid and yellow solid) and to H2S (black solid and gas with bad egg smell).
• Halide ions act as reducing agents because they donate electrons. Reducing strength of the halides increases down a group because more shells so more shielding so weaker attraction between nucleus and electron being donated so it is more easily lost.