Group 7

• Atomic radius

  Increases down group. There are more shells so more shielding so attraction between nucleus and outer e- is weaker so outer e- held less tightly
• Why is the ionic radius bigger than the atomic radius?

  In the halide ion (X-), the number of e- is greater than the number of protons, so the attraction between the protons and the e- is weaker so e- are held less tightly. Cl- is larger than Cl2
• Boiling point


  Increases down group. Halogens become less volatile because as the Mr increaes/molecule gets bigger, there are more e- so stronger vdws' between molecules so more energy needed
• Electronegativity


  Decreases down group, more shells more shielding so weaker attraction between nucleus and electrons in covalent bond
• First ionisation energy


  Decreases- more shells, more shielding so weaker attraction between nucleus and outer e- to be removed
• Colour of the halogens

  Cl2- green

  Br2- Red-brown

  I2- black
• State at room temperature

  Cl2- gas

  Br2-liquid

  I2-solid
• Colour of aqueous solution

  Cl2-pale green

  Br2-orange

  I2- brown
• Colour in cyclohexane


  Cl2-pale green

  Br2-orange

  I2-pink purple
• Why are the halogens more soluble in non-polar solvents than in water?

  Halogens are non polar so they dissolve in non polar solvents like cyclohexane
• Solubility of the halogens in water decreases down the group as the halogen molecule becomes larger. Cyclohexane can be used to tell i2 and br2 apart
• Cl2 + KCl


  no reaction
• Br2 and KCL


  orange solution, orange in cyclohexane
• I2 and KCL


  brown solution, cyclohexane= pink
• Cl2 and KBr


  orange, cyclohexane=orange
• Br2 and KBr


  no reaction
• I2 and KBr


  stays brown solution, pink in cyclohexane
• Cl2 and KI


  brown solution, cyclohexane=pink
• Br2 and KI


  Brown solution, cyclohexane=pink
• I2 and KI


  No reaction
• Why are halogens oxidising agents?


  they accept e- from X- to form X- themselves. e.g. X2 + 2e- -> 2X-
• Why does the ability to act as oxidising agents decrease the group for halogens?

  There are more shells, so more shielding so attraction between nucleus and e- being accepted is weaker
• Disproportionation


  a redox reaction in which the same element is both oxidised and reduced
• The reaction of chlorine and water

  Cl2 + H2O -> HCl + HClO

  Chlorine is reduced. Its oxidation state decreases from 0 to -1 in HCl

  Chlorine is oxidised. Its oxidation state increases from 0 to +1 in HClO
• Observations of chlorine and water

  Cl2- pale green solution

  HCl- litmus paper turns red

  HClO- litmus paper is bleached white
• What happens when chlorine and water react in presence of sunlight?

  Cl2 + H2O -> 2HCl + 1/2 O2

  This is because the chloric(i) acid initially formed decomposes in sunlight to form hcl and o2
• Uses of chlorine in water treatment

  Kills bacteria

  however, cl2 is toxic in large quantities
• Reaction of chlorine with cold dilute sodium hydroxide

  Cl2 + 2NaOH -> NaCl + NaClO + H2O

  the green colour of chlorine fades and the smell is less pungent, this is another disproportionation reaction

  Uses: As a bleach which kills bacteria
• Test for halide ions


  Make a solution (aq) of the substance to be tested

  Add dilute nitric acid to remove other ions that interfere with the test results by also giving ppts (carbonates: CO3 2- + 2H+ ->CO2 +H2O)

  Add silver nitrate
• Observations of halide ions with AgNO3

  Cl-: white ppt AgCl

  Br-: cream ppt AgBr

  I-: yellow ppt AgI
• What would be observed when F- reacts with AgNO3?

  A colourless solution because AgF is soluble in water
• General ionic equation


  Ag+(aq) + X- -> AgX (s)
• Trends in solubility of the silver halides ppt in ammonia solution

  Add dilute ammonia and see if it dissolves, if not, add concentrated ammonia and see if it dissolves

  AgCl: dissolves with dilute nh3

  AgBr: cream ppt remains with dilute, dissolves in conc. nh3

  AgI: yellow ppt remains with both dilute and conc. NH3
• Equations of silver halides with ammonia

  • AgCl (s) + 2NH3 (aq) -> {Ag(NH3)2 +} (aq) + Cl- (aq)
• Why can halides act as reducing agents?


  X- ions donate e- to form X2 molecule: 2X- -> X2 + 2e-
• H2SO4- Oxidation number of sulfur=+6

  SO2-o.n of s=+4, gas with a choking smell

  S- o.n=0, yellow solid

  H2S- o.n of s=-2, smell of bad eggs

  If sulfuric acid is reduced by the halide ion, then the halide ion X-, will be oxidised to the halogen molecule
• acid-base reaction


  H2SO4 acts as an acid, it donates H+ to form HSO4-
• redox reaction


  H2SO4 acts as an oxidising agent to form SO2, S, or H2S
• acid-base observations 


  NaCl- steamy white fumes of HCL

  NaBr-steamy white fumes of HBr

  NaI- steamy white fumes of HI
• redox observations- oxidation product


  NaCl- no redox reaction

  NaBr- orange fumes of Br2

  NaI- black solid, I2