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3.1.3 The Halogens

Physical properties

Trend in boiling points

  • Boiling point increases down the group
  • Halogens exist as diatomic molecules at RTP
  • Number of electron shells in the atom increases going down the group
  • The atom gets bigger + heavier
  • Number of electrons in the diatomic molecules increases
  • Stronger induced dipole-dipole interactions
  • More energy required to break the intermolecular forces
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Appearance under different states

Halogen Colour in natural state Colour in aqueous solutions Colour in organic solvents
\(F_2\) Pale yellow gas / /
\(Cl_2\) Pale green gas Pale green Pale green
\(Br_2\) Red-brown liquid Orange Orange
\(I_2\) Shiny grey-black solid Brown Violet

Redox reactions

Redox reaction of halogens

  • Halogens all have \(s^2p^5\) electron configuration in their outer shell
  • They gain 1 electron to form 1- ion during redox reactions and gain the electron configuration of the nearest noble gas (reduced)
  • Another species loses electrons to halogen atoms so it is oxidised
  • Halogens are oxidising agents as they oxidise other species
  • Halide ions combine with metal ions to form white (ionic) solids which are mostly soluble

Halogen-halide displacement reactions

  • Solution of halogen added to other halide solutions
  • If the halogen added is more reactive than the halide ion in the solution
    • It will displace the halide ion in the solution
    • The solution changes colour (from ... to ...)
  • Results
    • \(Cl_2\) reacts with \(Br^-\) (\(Cl_2(aq) + 2Br^-(aq) \rightarrow 2Cl^-(aq) + Br_2(aq)\) orange)
    • \(Cl_2\) reacts with \(I^-\) (\(Cl_2(aq) + 2I^-(aq) \rightarrow 2Cl^-(aq) + I_2(aq)\) violet)
    • \(Br_2\) reacts with \(I^-\) only (\(Br_2(aq) + 2I^-(aq) \rightarrow 2Br^-(aq) + I_2(aq)\) violet)
    • \(I_2\) doesn't react at all
  • Element displaced can be identified by adding cyclohexane (non-polar solvent, dissolve more readily) + mix
    • Use the colour of the top layer to identify the element being displaced
  • Show reactivity: \(Cl_2 > Br_2 > I_2\)

Trend in reactivity

  • Reactivity decreases down the group
  • Atomic radius increases
  • More inner shells so shielding increases
  • Effect of increasing nuclear charge is outweighed by increasing atomic radius and shielding
  • Less attraction between the nucleus and the outer shell
  • Harder for elements to capture an electron from another species and form 1- ions

Disproportionation reactions

Disproportionation

  • A redox reaction in which the same element is both oxidised and reduced

Chlorinating water

  • Exported image
  • Used in water treatment systems to kill harmful bacteria
  • Bacteria killed by chloric(I) acid (\(HClO\)) and chlorate(I) ions (\(ClO^-\))
  • Chloric(I) acid also acts as a weak bleach e.g. indicator paper will turn red then white

Benefits and risk of chlorinating water

  • Benefits
    • Kill bacteria in water treatment
    • Reduces the risk of waterborne diseases
  • Risks
    • Hazards of toxic chlorine gas \(\rightarrow\) respiratory irritant in small concentrations, can be fatal if in large concentrations
    • Formation of chlorinated hydrocarbons which is carcinogenic

Manufacturing bleach

  • React chlorine with cold and dilute aqueous \(NaOH\) solution
  • Exported image
  • \(NaClO\) solution = bleach

Halide test

  • See 3.1.4