Group 7

• Atomic Radius

  Atomic radius of group 7 elements increases down the group
• Reactivity

  As atomic radius increases this becomes harder as the positive attraction of the nucleus is weakened by additional shielding, harder to attract an electron so reactivity decreases
• Ionisation Energy

  The first IE decreases down the group due to greater atomic radius and increased amounts of shielding
• Boiling Point

  The group 7 elements are simple covalent molecules held together by VDW forces. The strength of these intermolecular forces increases down the group, more energy required so higher b.p

  Fluorine is a gas at room temp whilst iodine is a solid
• Oxidising Power

  Halogens are good oxidising agents as they accept electrons from the species being being oxidised and reduced

  Oxidising power decreases down the group as their ability to attract electrons decreases due to shielding and a greater atomic radius

  The relative oxidising strengths mean a halogen will displace any halide beneath it in the periodic table

  E.g. Cl2 will displace Br- and I- ions. Br2 will displace I- ions. I2 won't displace any halide ions
• Halide Ions

  Good reducing agents, as they donate electrons to the species being reduced and are themselves oxidised

  This reducing power increases down the group as electrons easier to lose from larger ions (due to shielding and atomic radius)
• Redox reactions of Halide Ions with H2SO4

  Fluoride and Chloride Ions

  NaF + H2SO4 ---> NaHSO4 + HF

  NaCl + H2SO4 ---> NaHSO4 + HCl

  Bromide Ions

  NaBr + H2SO4 ---> NaHSO4 + HBr

  2HBr + H2SO4 ---> Br2 + SO2 + 2H2O

  Iodide Ions

  NaI + H2SO4 ---> NaHSO4 + HI

  2HI + H2SO4 ---> I2 + SO2 + 2H2O

  6HI + SO2 ---> H2S + 3I2 + 2H2O
• Test for Halide Ions

  Acidified silver nitrate used as it forms different coloured precipitates depending on ion present - Can be tested further using Ammonia

  With AgNO3

  Cl-: White ppt (AgCl)

  Br-: Cream ppt (AgBr)

  I-: Yellow ppt (AgI)

  With dilute NH3

  Cl-: Ppt dissolves

  Br-: No change

  I-: No change

  With conc NH3

  Cl-: Ppt dissolves

  Br-: Ppt dissolves

  I-: No change
• Chlorine and Chlorate Ions

  Chlorine reacts with cold water to produce chlorate ions and chloride ions in a disproportionation reaction as chlorine is reduced and oxidised

  E.g. Cl2 + H2O ---> ClO- + Cl- + 2H+

  In the presence of UV light, chlorine decomposes to produce oxygen and hydrochloric acid (chlorine is reduced)

  2Cl2 + 2H2O + (uv) --->4HCl + O2
• Bleach Production

  Chlorine can be mixed with cold, aqueous NaOH to produce sodium hypochlorite - key ingredient of bleach

  2NaOH + Cl2 ---> NaClO + NaCl + H2O
• Test for Sulfates

  Use BaCl2 which forms white ppt
• Test for Hydroxide (OH-) 


  These ions indicate that the substance is alkaline - can be identified with red litmus which turns blue
• Test for Carbonate (CO32-)

  When acid like HCl is added, the substance containing the carbonate ions will produce effervescence and CO2 gas is given off

  Gas can be collected and bubbled through limewater which will turn cloudy, confirming CO2
• Group 2 Ions Test

  Calcium (Ca2+): Brick red

  Strontium (Sr2+): Red

  Barium (Ba2+): Pale green
• Test for Ammonium (NH4+)

  Presence of ammonium ions can be tested by holding red litmus over a petri dish of the substance being tested - turns blue if ammonium ions present

  Can be tested same way by adding NaOH
• Chlorine has a low b.p. because intermolecular forces are weak, explain how these forces arise between molecules of chlorine:(3)

  Movement of electrons in one molecule creates a dipole as there is an imbalance in electron density in one molecule

  Induces a dipole in a neighbouring molecule

  There is temporary dipole attraction between delta+ and delta-
• Give an equation for the reaction of chlorine with water

  Give a reason why chlorine is added to drinking water(2)

  2Cl2 + 2H2O ---> 4HCl + O2

  Kills bacteria
• Give an equation for the reaction of solid sodium bromide with conc sulfuric acid to form bromine:

  State one observation:(2)

  
  

  2H2SO4 + 2NaBr ---> Na2SO4 + SO2 + Br2 + 2H2O

  Orange fumes
• Which solid sodium halide does not produce a sulfur-containing gas as one of the products?(1)

  Sodium chloride - NaCl
• Half equation for the conversion of iodide ions to iodine:

  Half equation for the conversion of sulfuric acid to sulfur:

  2I- ---> I2 + 2e-

  H2SO4 + 6H+ + 6e- ---> S + 4H2O

  Overall equation: 6H+ 6I- + H2SO4 ---> 3I2 + S + 4H2O

  Other sulfur-containing reduction product: SO2
• Common q: Silver nitrate is added to the solution

  Suggest why an excess is used (1)

  To ensure all halide ions are removed from the solution
• Ionic equation for the reaction between chlorine and cold dilute sodium hydroxide solution:

  Give the oxidation state of chlorine in each of the chlorine-containing ions formed: (2)

  CL2 + 2HO- ---> OCl- + Cl- + H2O

  OCl- is +1

  Cl- is -1
• Write an equation for the reaction of sodium bromide with conc sulfuric acid

  Explain why bromide ions react differently to chloride ions

  2NaBr + 2H2SO4 ---> Na2SO4 + Br2 + SO2 + 2H2O

  Br- ions are larger than Cl- ions so Br- ions are therefore more easily oxidised
• A colourless solution contains a mixture of sodium chloride and sodium bromide

  Using aqueous silver nitrate, develop a procedure to prepare a pure sample of silver bromide from this mixture

  Explain each step (with equations)

  • Add silver nitrate

  AgNO3 + NaCl ---> AgCl + NaNO3 (ppt is white)

  AgNO3 + NaBr ---> AgBr + NaNO3 (ppt is cream)

  • Add excess dilute ammonia to the mixture
• Describe how to distinguish between aqueous solutions of KNO3 and K2SO4, using one simple test tube reaction

  Reagent: BaCl2

  Observation with KNO3: Colourless solution

  Observation with K2SO4: White ppt (BaSO4)
• Which property increases down Group 7?

  Boiling point
• Which species is the best oxidising agent?


  Cl2
• Which pair of solutions, when mixed, reacts to form a dark brown solution?

  NaBr(aq) + Cl2(aq)
• Explain the trend in electronegativities shown by the halogens

  Power of the molecule to attract an electron pair - fluorine has the least shielding so its reactivity is the highest
• Write a half-equation for the reaction of sulfuric acid to form hydrogen sulfide

  Write a half-equation for the formation of the black solid

  H2SO4 + 8H+ + 8e- ---> H2S + 4H2O (Black solid forms)

  2I- ---> I2 + 2e-
• Suggest one reason why the reaction of sodium fluoride with conc sulfuric acid is different from the reaction with sodium iodide


  Fluoride is a weaker reducing agent