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Module 3 Chapter 8 (8.2 The halogens (Reactivity decreases down the group,…

- - - - Add nitric acid to confirm that the solution could contains Carbonate ions if effervescence takes place
      - Bubble through limewater (Ca(OH)2), if the solution contains carbonates a white precipitate is formed (goes cloudy)
    - - Barium sulfate is insoluble in water
      - Add aqueous barium nitrate to solution
      - If a white precipitate forms the solutions contains sulfate ions
      - Application-Barium meal.-A drink containing barium sulphate that coats the lining of a part of the body to detect problems or anomalies.
    - - Silver halides are insoluble in water
      - Add silver ions (AgNO3) added to solution
      - If solution contains Cl- = white precipitate, Br-= cream precipitate, I-=Yellow precipitate
      - Hard to distinguish colours
        
        Add dilute Ammonia and Conc. ammonia
        
        Chlorine dissolves in dilute
        
        Bromine dissolves in conc.
        
        Iodine doesn't dissolve
  - - - Need to use a fresh sample
      - Add aqueous NaOH to solution, hold a damp piece of pH indicator paper over test tube
      - If the solution contains ammonium ions then ammonia gas (alkaline) will be produced- will turn indicator paper blue/purple
- - - - Chlorine- Pale green in water, Pale green in cyclohexane
      - Bromine Orange in water, Orange in cyclohexane
      - Iodine Brown in water, Violent in cyclohexane
  - - - Chlorine used to treat drinking water- kills harmful bacteria
      - Cl2(g) + H2O(l) → HClO(aq) + HCl(aq)
      - HClO is chloric(I) acid
        HCl is hydrochloric acid
    - - Cl2(g) + 2NaOH(aq) → NaClO(aq) + NaCl(aq) + H2O(l)
      - Dilute NaOH so more Cl can dissolve
  - - - Pros
        
        Stops diseases like typhoid and cholera
      - Cons
        
        Chlorine toxic gas
        
        Chlorine reacts with methane in water- forms chlorinated hydrocarbons- carcinogen
- - - - General formula= MO
      - Metal oxidised oxygen reduced
    - - Form alkaline hydroxide
      - General formula M(OH)2
      - Reaction becomes more vigorous going down the group
    - - metal + acid -> salt + hydrogen
  - - - Reactivity increases down the group
        
        More shells= more electron shielding and less nuclear attraction so it's easier to lose an electron
      - Ionisation energy decreases down group
        
        Bigger atomic radius and increasing shielding so less nuclear attraction for outer electron
    - - Atomic radius increases down the group
        
        More filled energy levels between nucleus and outer electron and more shielding= lower IE
        
        Outer electrons- more shielded so bonding electrons less strongly attracted to nucleus
        
        Reactions of Group 2 Compounds
        
        Oxide with water
        
        Release OH- ions- alkaline solution of metal hydroxide is formed
        
        Group 2 hydroxide only sparingly soluble in water- when solution saturated precipitate forms
        
        Solubility
        
        Solubility increases down the group
        
        If it's more soluble there are more OH- ions so solution is more alkaline
        
        Mg(OH)2 forms a solution of low concentration with pH 10.
        
        Ba(OH)2 forms a more concentrated solution (more soluble) with pH 13.
        
        Uses of group 2 compounds as bases
        
        Agriculture
        
        Calcium hydroxide is added to fields by farmers to raise the pH of acid soil
        
        Ca(OH)2(s) + 2H+(aq) →Ca2+(aq) + 2H2O(l)
        
        Medicine
        
        Group 2 bases are used to treat acid indigestion (antacid).
        
        Mainly magnesium and calcium carbonates are used-Magnesium hydroxide can also be used
        
        Stomach acid is mainly HCl(aq).
        
        Mg(OH)2 is used as a suspension in water as the solubility of magnesium hydroxide is low
        
        Mg(OH)2 + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
        
        CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)