Halogens

• How do halogens occur?

  as stable halide ions (Cl-, Br-, I-)
• Properties of fluorine


  a very reactive

  poisonous yellow gas

  most electronegative elements
• Properties of chlorine


  a green gas

  poisonous gas

  bleaches dyes
• Properties of bromine


  orange liquid

  reactive

  vaporises at room temperature
• Properties of iodine


  purple vapour

  grey crystals

  reactive

  toxic

  used in solution as antiseptic
• Why are halogens oxidising agents?


  halogens gain electrons, these electrons are from another species, that species has been oxidised
• How do halogens exist at room temperature and pressure?

  diatomic molecules, Cl2, Br2, I2
• What happens to melting/boiling point of halogens down the group?

  melting point increases due to stronger van der Waals forces
• What happens to halogen atoms in redox reactions?

  halogen atom is reduced, gaining one electron to form 1- halide ion with electronic config of nearest noble gas

  
  

  Cl2 - 2e- ---> 2Cl- chlorine is reduced
• What is the electronic configuration of the halogens?

  s2p5, two electrons in outer s sub-shell and five in outer p sub-shell
• What is a simple experiment for halogen-halide displacement reactions?

  a solution of a halogen is added to aqueous solution of another halide, the solution changes colour if the halogen displaces the halide

  cyclohexane is then added to make colour more distinguishable
• Why does reactivity decrease down group 7?

  Halogens need to gain one electron to obtain a stable electron configuration. As you go down group 7 atomic radius and electron shielding increase. Attraction between the nucleus and outer shell electrons decreases so it is harder for the atom to gain an electron
• What is the colour of halogen solutions in water (aqueous layer)?

  Cl2 - pale green

  Br2 - orange

  I2 - brown
• What is the colour of halogen solutions in cyclohexane (organic layer)?

  Cl2 - pale green

  Br2 -orange

  I2 - violet
• What happens when aqueous chlorine reacts with bromide ions?

  solution turns orange in water and cyclohexane

  
  

  Cl2 + 2Br- ----> 2Cl- + Br2
• What happens when aqueous chlorine reacts with iodide ions?

  solution turns brown in water and violent in cyclohexane

  
  

  Cl2 + 2I- ----> 2Cl- + I2
• What happens when aqueous bromine reacts with iodide ions?

  solution turns violet

  
  

  Cl2 + 2I- -----> 2Cl- + I2
• What is the equation for the reaction between chlorine and sodium bromide?

  Cl2 + 2NaBr -----> 2NaCl + Br2
• Why does the tendency to gain electrons decrease down group 7?

  due to increased atomic radius and shielding the attraction between valence electrons and nucleus is decreased therefore nuclear attraction is decreased. This makes it harder for the atom to attract an electron to fill its outer shell.
• What is a disproportionation reaction?


  where a single element is simultaneously oxidised and reduced at the same time
• What is the reaction between chlorine and water?

  Cl2 + H2O -> HCl + HClO

  chlorine + water ---> hydrochloric + chloric (i) acid
• What are the risks of chlorine?

  very toxic gas

  respiratory irritant in small concentrations

  large concentration can be fatal

  chlorine in water can form chlorinated hydrocarbons by reacting with hydrocarbons in decaying vegetation, linked to cancer
• What is the test for halide ions?


  aqueous silver halide, precipitate colour determines ion

  chloride - white

  bromide - cream

  iodide - yellow

  then solubility test, aqueous dilute and concentrated ammonia added

  chloride - soluble in dilute ammonia

  bromide - soluble in concentrated ammonia

  iodide - insoluble in concentrated ammonia
• Reaction between chlorine and sodium hydroxide

  Cl2 + 2NaOH ----> NaClO + NaCl + H2O
• Benefits of chlorine use

  purification, kills bacteria
• What conditions are needed for reaction between chlorine and sodium hydroxide?

  cold, dilute sodium hydroxide