2.03 Group VII Halogens

• What is the trend in Tb going down group 7? Why?

  Increases down the group

  • Size of atoms increase as more occupied electron shells, therefore stronger VDW forces of attraction between molecules, therefore it requires more energy to break
• What is the trend in electronegativity down group 7? Why?

  Decreases.

  • More occupied electron shells therefore greater atomic radius and outer electrons are further from the positive charge of the nucleus, so a weaker force of attraction between the nucleus and the electron pair in the covalent bond
• What do you use to test for halide ions?

  Acidified AgNO3
• Why do you add HNO3/nitric acid? Why not HCl?

  • To remove CO32-
  • Adding HCl would add Cl- ions, giving a false positive result
• What is the result and equation for the Cl- test?

  • White precipitate
  • Ag+ + Cl- → AgCl(s)
• What is the result and equation for the Br- test?

  • Cream precipitate
  • Ag+ + Br- → AgBr(s)
• What is the result and equation for the test for I-?

  • Yellow precipitate
  • Ag+ + I- → AgI
• What happens to each of the silver halide precipitates when dilute/conc NH3 are added?

  • AgCl dissolves in both dilute and conc:$AgCl\left(s\right)+$$2NH_3\left(aq\right)\ \rightarrow\ \left[Ag\left(NH_3\right)_2\right]^+$$\left(aq\right)\ +$$\ Cl^-$
  • AgBr only dissolves in conc: $AgBr\left(s\right)+$$2NH_3\left(aq\right)\ \rightarrow\ \left[Ag\left(NH_3\right)_2\right]^+$$\left(aq\right)\ +$$\ Br^-$
  • AgI will not dissolve in either
• What is the trend in oxidising ability down the group? Why?


  Decreases down the group (Cl = best, I = worst)

  • Cl as fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus, easiest to gain electrons and be reduced therefore it is the best oxidising agent
• Write the equation for Cl2 oxidising 2I-

  Cl2 + 2I- → 2Cl- + I2
• What is the trend in reducing ability of the halides down the group? Why?

  Increases down the group (Cl- = worst, I- = best)

  • I- has the most occupied electron shells, so outer electrons are further from the nucleus, weakest force of attraction between outer electrons and positive charge of the nucleus therefore it is the easiest to be oxidised and lose electrons, so it is the best reducing agent
• What products are formed when I- reduces H2SO4?

  H2SO4 + 2I- → SO4 2- + 2HI

  H2SO4 + 2H+ + 2I- → SO2 + I2 + 2H2O (SO2=choking gas with pungent odor)

  H2SO4 + 6H+ + 6I- → S + 3I2 + 4H2O (S=yellow solid)

  H2SO4 + 8H+ + 8I- → H2S + 4I2 + 4H2O (H2S=smells like rotten eggs)
• What are the products of Br- and H2SO4?

  HBr and So2
• Does Cl- reduce H2SO4?

  No, not a powerful enough reducing agent - only HCl is formed
• Why is chlorine added to drinking water, and why is it safe?

  • Forms ClO- ions which oxidise/kill all microorganisms in water
  • Once it has done its job, little remains, and the health benefits outweigh the risks of it
• What are the potential risks of adding chlorine to drinking water?

  Chlorine is toxic and damages the respiratory system in large enough quantities; can form carcinogens with hydrocarbons
• Why is ozone not used to purify water in the UK?


  More expensive than chlorine and evaporates from water more quickly
• What is the equation for the reaction of Cl2 with water?

  Cl2(g) + H2O(l) → HClO(aq) + HCl(aq)
• What is the type of reaction between chlorine and water?

  Disproportionation - chlorine is both oxidised and reduced
• What are the two forms of chlorate ion?

  • ClO- is chlorate (I)
  • ClO3- is chlorate (V)
• What is the equation for making bleach?

  Cl2 + 2NaOH → NaCl + NaClO + H2O

  • NaClO is bleach
• Give the equation for the reaction of chlorine and water in the presence of sunlight

  2Cl2 + 2H2O → 4HCl + O2
• What is desalination?

  Converts saltwater into clean, potable water.

  • Either by reverse osmosis by using a smart membrane or...
  • By using vacuum distillation at a low pressure and temperature
• What are the advantages and disadvantages of desalination?

  • Advantages - safe, clean, drinkable water produced in places where it might not otherwise be available
  • Disadvantages - uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystems