Group 7

• Colour change when chlorine displaces iodide ions:

  Brown solution formed
• Colour change when bromine displaces iodide ions:

  Brown solution formed
• Colour change when chlorine displaces bromide ions:

  Orange solution formed
• Halogen colours & states (room temperature):

  Fluorine- pale yellow gas Chlorine- green gas Bromine- red-brown liquid Iodine- grey solid
• Boiling points down group 7:

  Increase- increasing strength of van der Waals forces as size & relative mass of molecules increases.
• Reactivity down group 7:

  Halogens gain electron when they react (oxidising agents). Less reactive down group as atoms get larger & outer shell gets further from nucleus (harder to gain electron). Less oxidising down group.
• Electronegativity down group 7:

  Decreases- larger atoms attract electrons less than smaller atoms, as their outer electrons are further from nucleus & are more shielded as they have more inner electrons.
• Displacement reactions between halogens & halide ions:

  Halogen displaces halide from solution if halide is below it in periodic table. Add few drops aqueous halogen to solution containing halide ions (colourless)- colour change if there's a reaction.
• What happens when you mix chlorine & water?

  Chlorine undergoes disproportionation. End up with mixture of chloride ions & chlorate (I) ions. Cl2 + H2O <--> 2H+ + Cl- + ClO-
• Risks of chlorine use in water treatment:


  Cl gas & liquid harmful (toxic). Reacts with organic compounds in water to form chlorinated hydrocarbons, many are carcinogenic. (Increased cancer risk small compared to risks from untreated water e.g, cholera).
• How do you make bleach?

  Mix Cl gas with cold, dilute sodium hydroxide solution at room temperature- makes sodium chlorate (I) solution (NaClO (aq) = bleach. Cl oxidised & reduced (disproportionation).
• Bleach production equation:


  2NaOH + Cl2 --> NaClO + NaCl + H2O
• Benefits of chlorine use in water treatment:

  Chlorate (I) ions kill bacteria. Cl kills disease-causing microorganisms. Some Cl persists in water & prevents reinfection. Prevents growth of algae (no bad tastes & smells) & removes discolouration caused by organic compounds.
• Reducing power of halides down group 7:

  Increases
• Cl2 + H2O <--> 2H+ + 2Cl- + 1/2 O2

  In sunlight, Cl can decompose H2O to form chloride ions & oxygen.
• Reactions of halide ions with conc. H2SO4:


  Gives hydrogen halide to start with. Some halide ions strong enough reducing agents so can reduce sulfuric acid to water & SO2. Iodide very strong reducing agent so can reduce to SO2 or H2S or S.