group 17-halogens

• a halogen is a non-metallic element belonging to group 17 of the periodic table, none of the halogens are found in nature in their elemental form as they are too reactive. they are found as mineral salts containing halide ions
• a halide is a compound containing a halogen like sodium chloride
• oxidation is when a species loses electrons
• oxidising agent is a substance that has the ability to oxidise other substances and in the process gains electrons and itself is reduced
• reduction is when a species gains electrons
• reducing agent is a substance that has the ability to reduce other substances and in the process loses electrons
• a displacement reaction for halogens is when a halogen molecule (the oxidising agent) removes electrons from a halide ion
• an oxidation number is a hypothetical number assigned to an individual atom or ion present in a substance, they relate to how many electrons were added or removed from an element to get to its present state
• disproportionation is a reaction in which a substance is simultaneously oxidised and reduced giving two different products
• a spectator ion is an ion that isn’t involved in the main reaction
• reactivity is the likelyhood of an atom of an element to forms its ion
• common properties of halogens:

  • non-metallic
  • low melting/boiling points
  • poor conductors of heat and electricity
  • have coloured vapours
  • simple molecular structure
• chlorine is pale yellow in cyclohexane and water
• bromine is orange/brown in cyclohexane and orange/yellow in water
• iodine is pink in cyclohexane and orange/brown in water
• chlorine is the strongest oxidising agent
• iodine is the strongest reducing agent
• rules for assigning oxidising numbers:

  • oxidation number of atoms in uncombined elements is 0
  • in neutral molecules the sum of the oxidation numbers is 0
  • in ions the sum of the oxidation numbers equals the charge on the ion
  • in any substances the more electronegative atom has the negative oxidation number and the less electronegative atom has the positive oxidation nunber
• trends in boiling point:

  • increases down the group
  • the only intermolecular forces are induced dipole forces
  • the more electrons in a molecule the stronger the induced dipole forces and the more energy required to overcome these forces
  • as group 7 descends the number of electrons increase
• trends in atomic radius:

  • increases down the group
  • despite increased nuclear charge as the group descends the increased shielding results in a decreased nuclear attraction and therefore increased atomic radius
• trends in electronegativity:

  • decreases down the group
  • as the group descends the nuclear attraction to the shared pair of electrons decreases due to increased atomic radius, increased shielding despite increase in nuclear charge
• chlorine is rarely present in nature by itself as elemental chlorine due to its chemical reactivity.
• chlorine kills bacteria and waterborne pathogens in order to keep drinking water and swimming pools safe
• chlorine is partly soluble in water, it also reacts with water to form an acidic solution
• chlorine reacts with water to form hydrochloric acid and chloric acid. in this reaction chlorine is both oxidised and reduced. this is a disproportionation
• bleach is manufactured by adding chlorine to cold sodium hydroxide solution
• chlorine reacts with cold aqueous sodium hydroxide to from sodium chloride, sodium chlorite and water. chlorine is simultaneously oxidised and reduced in this disproportionation reaction
• Cl2 + H2O -> HCl + HOCl
• 2NaOH + Cl2 -> NaCl + NaClO + H2O
• to test for halide ions in solution silver nitrate (AgNO3) is added after nitric acid and different colour precipitates are formed based on the halide
• a white precipitate indicates that chloride ions are present
• a cream precipitate indicated that bromide ions are present
• a yellow precipitate indicates that iodide ions are present
• if chloride ions are suspected, dilute ammonia is added and the white precipitate dissolves as it is soluble
• if bromide ions are suspected, concentrated ammonia is added and the cream precipitate dissolves as it is soluble
• if iodide ions are suspected, add concentrated ammonia and the yellow precipitate will not dissolve as it is insoluble
• nitric acid is used to remove carbonate ions from the solution as they can react with silver nitrate to form a white precipitate and give a false positive result for halide ions
• the oxidation state for oxygen is always -2 in most of its covalent compounds
• in covalent compounds with non-metals the oxidation state of hydrogen is +1 and with metals it is -1
• the oxidation state for halides is usually -1 but can be +1, +3, +5 or +7