2.3 Group 7
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What is the trend in bpt down group 7? Why?
Increases down the group because: size of atom increases as more occupied electron shells → stronger van der Waals forces of attraction between molecules, take more energy to break
What is the trend in electronegativity down group 7? Why?
Decreases because: more occupied electron shells → greater atomic radius and outer electrons are further from the positive charge of the nucleus → lower force of attraction between the nucleus and electron pair in the covalent bond
What do you use to test for halide ions?
Acidified AgNO3
Why do you add HNO3 ? Why not HCl?
To remove CO₃ 2- as adding HCl would add Cl- ions, giving a false positive result
What happens (+ equations) to each of the silver halide precipitates when dilute/conc NH3 are added?
AgCl- dissolves in both dilute and conc AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + Cl AgBr- only dissolves in conc AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + Br AgI- will not dissolve in either
What is the trend in oxidising ability down the group? Why?
Decreases down group (Cl best, I worst) Because: Cl has fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus, easiest to gain electrons and be reduced → best oxidising agent
What is the trend in reducing ability of the halides down the group? Why?
increases down the group (Cl- worst, I- best) because: 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 nucleus → easiest to be oxidised and lose electrons → best reducing agent
Why is chlorine added to drinking water? 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 using it
What are 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, evaporates from water more quickly
What type of reaction is the reaction of chlorine with water?
Disproportionation- the chlorine is both oxidised and reduced
What are the two forms of the chlorate ion?
ClO- is chlorate (I) ClO₃- is chlorate (V)
What is desalination?
Converts saltwater into clean, potable water Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low pressure and low 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