*Elements from the Sea

• An oxidising agent accepts electrons and is reduced itself
• A reducing agent loses electrons and is oxidised itself
• Electrolysis is the decomposition of molten or aqueous compounds by passing an electric current through it which contains a cathode which is negative and an anode which is positive
• If gold, copper, silver and platinum are in electrolysis they will be formed in place of hydrogen
• A halogen will always be formed if present in electrolysis
• Al3+ + 3e- -> Al

  reduction
• 2Cl- -> Cl2 + 2e-

  oxidation
• Oxidation of Br- to Br2:

  • acidified using sulfuric acid to stop reaction with water
  • Excess chlorine is used to displace bromine
  • Bromine vapours are removed
  • Reduction of Br2 to HBr y sulfur dioxide, waer and bromine
  • Steam and chlorine are used to oxidise hydrogen bromide to bromine
• Group 7 elements are highly reactive non-metals which exist as diatomic molecules
• The appearance of group 7 at room temp:

  • Fluorine = pale yellow gas
  • Chlorine = pale green gas
  • Bromine = red/brown liquid
  • Iodine = dark grey solid (purple vapour)
• Volatitlity of halogens decreases down the group as the boiling point increases
• Group 7 solubility in water:

  • chlorine = green
  • bromine = orange/red
  • iodine = brown
• Group 7 elements are more soluble in organic solvents than water because the halogens are non-polar molecules so are more soluble in non-polar solvents
• Reactivity of halogens decreases down the group due to an increase in electron shielding causing their ability to attract electrons to decrease
• identifying halogens with AgNO3:

  • chlorine = white ppt
  • bromine = cream ppt
  • iodine = yellow ppt
• Halogens in dilute NH3:

  • chlorine = ppt dissolves
  • iodine and bromine = no change
• Halogens in conc NH3:

  • chlorine and bromine = ppt dissolves
  • iodine = no change
• Concentrated phosphoric acid is used to prepare HCl, HBr and HI as it donates a hydrogen ion to the halogen ion
• Sulfuric acid can be used to make HCl but not HBr or HI as it would further oxidise them due to it being a strong oxidising agent
• Thermal stability of hydrides decreases down group 7
• Hydrogen hallides react with ammonia gas to form ammonium salts in an acid-base reaction e.g. HCl + NH3 -> NH4Cl
• Hydrogen halides react with water to form dilute acids which dissociate to release halide and hydrogen ions

  e.g. HCl + H2O -> Cl- + H3O+
• Chlorine is used in bleach and kills bacteria. In water it reacts in a disproportionation reaction producing Cl- and ClO-. The reaction produces HCl so an alkali is usually added to reduce acidity
• Chlorine can be toxic and can potentially react to form cancer-causing compounds so debates are happening as to whether it should be used in water
• Dynamic equilibriums is when the forward and backwards reaction are happening at the same rate in a closed system
• In equilibrium the concentration will remain constant
• Le Chateliers Principle says that if reaction conditions are altered the reaction will shift the equilibria to counteract the change
• If you increase the temperature of a equilibrium it will shift to the endothermic side
• If you increase the pressure of an equilibrium it will shift to the side with fewer moles of gas
• Increasing concentration of the reactants will shift the equilibrium to the side of the products
• Kc= products/reactants
• Volatility is how readily a substance vapourises