Halogens

• How do halogens exists ?

  As diatomic molecules
• What are the uses of chlorine ?

  used to manufacture bleach, and disinfect drinking water
• How many electrons do halogens have on their outer shell ?

  7
• Which block are halogens apart of ? Why ?

  P block because the outer electrons are found in the p subshell
• Does the boiling point increase or decrease going down the group ?

  increases
• Explain the trend in the boiling points of Cl2, Br2 and I2, in terms of London forces.

  As you go down the group the number of electrons increases. The London forces between molecules strengthen. Therefore the boiling point increases and volatility increases.
• What does volatility mean ?

  How easily it evaporates
• Do halogens gain or lose electrons ?

  Gain
• Are halogens strong reducing or oxidising agents ?

  Oxidising
• What is an oxidising agent ?

  Oxidises something else but becomes reduced itself
• Write an equation for the reduction of Fluorine.

  F2 + 2e- --> 2F-
• Define oxidising power.


  A measure of the strength with which an atom is able to attract and capture electrons.
• Describe and explain the trend in reactivity going down G17 ?

  As you go down the group the reactivity decreases which means the oxidising power decreases. This is because, as you go down the group the atomic radius increases as the number of shells increases, and the shielding increases as the number of outer most electrons increases. The nuclear attraction between the outer most electrons and nucleus decreases.Therefore the ability of an atom to attract and capture an electron to form a halide ion decreases.
• What type of reactions can be used to demonstrate the decrease in reactivity going down G17 ?

  Redox and Displacement reactions
• What is a displacement reaction ?

  When a more reactive element displaces a less reactive element
• How would you carry out a displacement reaction ?

  By adding aqueous solutions of the halogens to aqueous solutions of the halide ions.
• Chlorine oxidises Br-. Write the equation and identify what has been reduced and oxidised using oxidation states.

  Cl2 + 2Br- --> 2Cl- + Br2

  Oxidised: Bromine from -1 to 0

  Reduced : Chlorine from 0 to -1
• Chlorine oxidises I-. Write the equation and identify what has been reduced and oxidised using oxidation states.

  Cl2 + 2I- --> 2Cl- + I2

  Oxidised: Iodine from -1 to 0

  Reduced : Chlorine from 0 to -1
• What does chorine oxidise ?

  Br- and I-
• Bromine oxidises I-. Write the equation and identify what has been reduced and oxidised using oxidation states.

  Br2 + 2I- --> 2Br- + I2

  Oxidised: Iodine from -1 to 0

  Reduced : Bromine from 0 to -1
• What does Bromine oxidise ?

  I-
• What does iodine oxidise ?

  nothing
• What colour are halide ions in aqueous solution ?

  Colourless
• How would you identify a displacement reaction ?

  colour change
• How would you carry out a displacement reaction to see a colour change ?

  Shake the solution with a non-polar solvent such as cyclohexane
• What colour is Bromine in aqueous solution ?

  orange
• What colour is Bromine in cyclohexane in aqueous solution?

  orange
• What colour is Iodine in aqueous solution ?

  Brown
• What colour is Iodine in cyclohexane ?

  Purple
• What colour is Chlorine in aqueous solution ?

  Pale green
• What colour is Chlorine in cyclohexane ?

  Pale green
• What is a Disproportionation reaction ?

  A redox reaction in which the same element is both oxidised and reduced at the same time ?
• Why is chlorine added to water ?

  To kill bacteria to make it safer to drink.
• Write the equation to show the disproportionation of chlorine when it is added to water ?

  Cl2 (aq)+ H2O (l) -> HCl (aq) + HClO (aq)
• Write the equation to show the disproportionation of chlorine when added to dilute NaOH at RTP to form bleach.

  Cl2 (aq) + NaOH (aq) --> NaCl (aq) + NaClO (aq) + H2O (l)
• What are the 2 problems associated with the use of chlorine in drinking water ?

  1. Chlorine is toxic
  2. Chlorine can react with organic materials - Chlorinated hydrocarbons = carcinogenic
• Describe the test used to identify Halide ions.


  1. Dissolve the halide salt in water
  2. Add aqueous silver ions, using AgNO3 (aq)
  3. The Ag+ (aq) reacts with any halide ions X- (aq) to form a precipitate AgX (s)
• Write the overall equation for the Halide ions test ?

  Silver NItrate + Sodium Halide --> Silver Halide + Sodium Nitrate
• Write the ionic equation for the test for Halide ions.

  Aq+ (aq) + X-(aq) --> AgX (s)
• What is the colour of the Chlorine precipitate ?

  White