C4 - Predicting and Identifying Reactions and Products

C4 - Predicting and Identifying Reactions and Products

1. Group 1 - alkali metals

   • One outer shell electron
   • Similar physical properties
     • Low melting/boiling points relatively
     • Low density (Li, Na and K float in water)
     • Very soft (can be cut with a knife)
   • Form ionic compunds
   • Very reactive
     • readily lose their single outer shell electron
       • Form a 1+ ion
     • As you move down group 1, the metals become more reactive
       • Outer electron more easily lost as it's further away form the nucleus so is less reactive
   • reacts with water
     • Metal + water => metal hydroxide + hydrogen
       • 2Na + 2H20 => 2NaOH + H2
     • React vigorously in water
     • As reactivity increases, the metals react differently
       • Lithium moves around and fizzes furiously
       • Sodium and potassium also fizz like lithium but they melt in the heat of the reaction
       • Rubidium and Caesium react violently and tend to explode
   • Can also react with dilute acids
     • produces a salt and hydrogen gas
       • 2Na + 2HCl => 2NaCl + H2
     • More violent than reacting with water so can be dangerous

2. Group 7 - halogens

   • 7 electrons in the outer shell
   • Exist as diatomic molecules
     • Exist as Cl2 or Br2
     • Covalently bonded
   • Melting/boiling points INCREASE as you go down the group
     • Chlorine is a fairly reactive poisinous green gas
     • poisonous red-brown liquid, gives off an orange vapour
     • Iodine is a dark grey crystalline solid which gives of a purple vapour when heated
   • Reactivity decreases going down the group
     • Only needs to gain one electron to form a 1- ion
     • The easier it is to attract the electron, the more reactive the halogen will be
       • it's harder to attract an electron as it's further away from the nucleus as the atomic radius grows
   • React with alkali metals to form salts
     • Sodium + chlorine => sodium chloride
       • 2Na + Cl2 => 2NaCl
     • alkali metal + halide => metal halide
   • Halogens undergo displacement reactions
     • a more reactive halogen can displace a less reactive one from a salt solution
     • If you add chlorine water to potassium bromide solution, the chlorine will displace the bromine from the solution
       • Cl2 + 2KBr => Br2 + 2KCl
     • These show the reactivity trends of the halogens
       • measure the halide salt solution into a testube
         • add a few drops of the halide solution and gently mix
           • a colour change indicates a reaction has happened

3. Group 0 - noble gases

   • All Group 0 elements are INERT
     • They don't really react at all. They have a full outer shell of electrons meaning they don't easily give up or gain electrons
     • They are not flammible
     • These properties means it's difficult to observe them, they took a long while to be discovered
   • There are patterns in properties
     • boiling/melting point and density increase as you move down the group
     • You can estimate values of elements:
       • He (0.2kg/m^3) and Ar (1.8kg/m^3) means Ne is in the middle so 0.1kg/m^3

4. Transition metals

   • "Everyday" metals, eg copper, iron, gold...
   • Have typical metallic properties
     • Hard, strong, shiny materials that conduct heat and electricity very well
     • HIgh melting points (apart from mercury)
   • Make good catalysts
     • Iron is used in the Haber process (ammonia)
     • Vanadium pentoxide (V2O5) is used in the contact process (H2SO4)
   • Can have multiple ions
     • Fe2+ and Fe3+
     • Cu+ and Cu2+
     • Cr2+ and Cr3+
   • Colourful compounds
     • the colour depends on the transition metal they contain
       • Fe2+ is usually light green
       • Fe3+ is usually orange/brown (rust)
       • Cu2+ is usually blue
   • Transition metals are fairly unreactive
     • Much less reactive than group 1 and 2 metals
     • Most react with dilute acids to form metal salts but they happen much slower than g1 or g2 metals

5. Reactivity of metals

   • How metals react with acid can tell you about their reactivity
     • place pieces of various metals in HCl. The more reactive, the faster the reaction will happen
     • reactive metals will fizz vigorously, whereas less reactive metals might bubble a bit
       • You can show that hydrogen is formed by using the burnt split test
   • Metals can react with water
     • metal + water => metal hydroxide + hydrogen
   • A reactivity series shows how reactive metals are
     • For example it will show that Potassium is more reactive than Magnesium
   • More reactive metals displace less reactive ones
     • if you put an iron nail into a solution of copper sulfate, the more reactive iron displaces the copper
       • CuSO4 + Fe => FeSO4 + Cu
     • if you put a less reactive metal into a solution of a more reactive metal salt, nothing will happen
     • You can use displacement reactions to work out a reactivity series

6. Test for gases

   • Oxygen
     • The gas will relight a glowing splint
       • This is because combustion only happens when oxygen is present so if the gas is not oxygen, the flames will smother
   • Testing for gases can be dangerous
     • Some gases are pretty nasty
     • Tests should be carried out in fume cupboards for safety
   • Carbon dioxode
     • Limewater will turn cloudy if CO2 is bubbled through
   • Hydrogen
     • Makes a "squeaky pop" with a lighted splint
       • The noise comes from hydrogen burning with oxygen in the air to form oxygen
   • Chlorine
     • Damp blue litmus paper will bleach and turn white
       • The paper may turn red first as chlorine is acidic

7. Tests for anions

   • Sulfates
     • Add barium chloride solution then a white barium sulate precipitate will form
       • Ba2+ +SO42 => BaSO4
       • The BaSO4 will not react with the HCl, so the white precipitate will not dissolve
   • Halide ions
     • Add dilute nitric acid (HNO3) then some silver nitrate solution and precipitates should form
       • Chloride gives a white precipitate AgCl
       • Bromide gives a cream precipitate AgBr
       • Iodide gives a yellow precipitate AgI
   • Carbonates
     • Add barium chloride solution then a white barium carbonate precipitate will form
       • Next add HCl and if carbonates are present, it will fizz producing CO2 gas
         • BaCO + 2H+ => Ba2+ + CO2 + H2O
         • You can test for Carbon dioxide with the limewater test

8. Tests for cations

   • The flame test can identify metal ions
     • Lithium, Li+, crimson red
     • Sodium, Na+, yellow
     • Potassium, K+, Lilac
     • Calcium, Ca2+, brick red
     • Copper, Cu2+, blue-green
   • Some metal ions form a coloured precipitate with NaOH
     • you add a few drops of sodium hydroxide solution to your compound. If a hydroxide precipitate forms, you can use the colour to figure out the ion.
       • Calcium, Ca2+, white
         • Ca2+ + 2OH- => Ca(OH)2
       • Copper, Cu2+, Blue
         • Cu2+ + 2OH- => Cu(OH)2
       • Iron, Fe2+, Green
         • Fe2+ + 2OH- => Fe(OH)2
       • Iron, Fe3+, Brown
         • Fe3+ + 3OH- => Fe(OH)3
       • Zinc, Zn2+, white at first then dissolves in excess NaOH to a colourless solution
         • Zn2+ + 2OH- => Zn(OH)2
           • Zn(OH)2 + 2OH- => Zn(OH)4 2-