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CHEMISTRY ASSESSMENT 26/11/2020, GROUP 7 ELEMENTS - HALOGENS, ELECTROLYSIS…

- - - - The halogens have low melting points and low boiling points. This is a typical property of non-metals. Fluorine has the lowest melting and boiling points.
        The melting points and boiling points increase as you go down the group.
    - - Room temperature is usually taken as being around 25*c at this temperature:
        
        Fluorine and chlorine are gases.
        
        Bromine is a liquid.
        
        Iodine and astatine are solids.
        
        There is therefore a trend in state from gas to liquid to solid as you go down the group.
    - - The halogens become darker as you go down the group.
        
        Fluorine is very pale yellow.
        
        Chlorine is yellow-green.
        
        Bromine is red-brown.
        
        Iodine crystals are shiny purple - it can easily turn into dark purple vapour when they are warmed up.
    - - When we see a trend in the properties of some of the elements in a group, it is possible to predict the properties of other elements in that group.
        Astatine is below iodine in Group 7.
        The colour of these elects gets darker as you go down the group.
        Iodine is purple, so astatine is black.
- - - - Redox reactions involve both oxidation (loss of electrons) and reduction (gain of electrons).
      - Halogen displacement reactions are redox reactions because the halogens gain electrons and the halide ions lose electrons.
        When we consider one of the displacement reactions, we can see which element is being oxidised and which is being reduced.
      - bromine + potassium iodide --> iodine + potassium bromide . We can see that bromine has gained electrons, so it has been reduced. The iodide ions have lost electrons, so they have been oxidised.
- - - - Postive Anode Negative Is Cathode
  - - - Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis:
        
        Pb2+ ions gain electrons at the cathode and become Pb atoms.
        
        Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules.
        
        So lead forms at the negative electrode and bromine forms at the postive electrode.
- - - - Whether hydrogen is or a metal is produced at the cathode depends on the position of the metal in the metal reactivity series: (most reactive to least reactive). Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, CARBON, Zinc, Iron, HYDROGEN, Copper, Silver, Gold.
    - - Oxygen is produced (from hydroxide ions), unless halide ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (chlorine, bromine or iodine).
        
        Negative ion: chloride Cl- --> element: Chlorine, Cl2.
        
        Negative ion: bromide, Br- --> element: bromine, Br2.
        
        Negative ion: iodide, I- --> element: iodine, I2.
        
        Negative ion: sulfate, SO42- --> element: oxygen, O2
- - - - energy transferred = mass of water heated x specific heat capacity of water x temperature rise.
- - - - General formula: CnH2n+2
    - - General formula: CnH2n
      - Reactions of alkenes. : Alkenes undergo addition reactions because you can break the C=C double bond and 'add' to molecule.
        
        Reaction 1 - The addition of halogens.
        
        Bromine water can be added to an alkene. A colour change from orange to colourless will occur if an alkene is present. The Bromine molecule (Br2) breaks the C=C bond and adds to the two carbon atoms.
        
        Reaction 2 - The addition of hydrogen.
        
        Alkenes are unsaturated so we can add more hydrogen to make the saturated.
        Hydrogenation needs:
        
        300*c
        
        Nickel catalyst.
        
        Reaction 3 - The addition of water.
        
        Ethanol (and other alcohols) can be made from hydration of ethene.
        This needs:
        
        Concertrated phosphoric acid catalyst.
        
        High temperature and pressure.