Elements of the sea

• What is atom economy?


  Atom economy is a measure of the efficiency of a chemical reaction, calculated by comparing the mass of desired product to the total mass of all reactants.
• What’s the formula for atom economy?

  %atom economy = Mr of desired product / Mr of reactants x 100
• Group one metals lose an electron to form 1+ ions, Group two metals lose 2 electrons to form 2+ ions, Group 7 elements gain an electron to form a 1- ion, Group 7 elements are known as the halogens and their negative ions are known as Halide ions
• What’s a redox reaction?

  a reaction which oxidation and reduction take place
• What is oxidation?


  the loss of electrons or increase in oxidation number.
• What is reduction?

  the gain of electrons or decrease in oxidation number
• What’s disproportionation?

  When a species is simultaneously reduced and oxidised to give two different products.

  For example: 2H2O2 —> 2H2O + O2
• The oxidation number of an element is zero. In neutral compounds, the sum of the oxidation states of all the atoms is 0
• What is hydrogens oxidation number?

  +1
• What is Oxygens oxidation number?

  -2
• What are the halogens oxidation number?

  -1
• What oxidation state do group 1 metals?

  +1
• What oxidation number do group 2 metals have?


  +2
• What oxidation number does oxygen have in peroxides?


  -1
• What oxidation number does hydrogen have in metal hydrides?


  -1
• Oxidation numbers

  
• What’s an oxidising agent?

  Accepts electrons from the species that is being oxidised therefore it gains electrons and is reduced
• What is a reducing agent?

  Donates electrons to the species being reduced. It loses electrons and is oxidised
• What is electrolysis?

  the decomposition of a molten or aqueous ionic compound by passing an electric current through it
• what is the order of the electrochemical series?

  • potassium
  • sodium
  • lithium
  • calcium
  • magnesium
  • aluminium
  • zinc
  • iron
  • copper
  • silver
  • gold
• When the electrolyte is molten cations move to the cathode and gain electrons to form atoms. Anions move towards the anode and lose electrons to form atoms
• When using inert electrodes if the metal ions are below hydrogen in the electrochemical series the metal atoms will be produced at the cathode. If metal is above hydrogen, hydrogen gas will be produced at the cathode.
• Generally when a solution is using inert electrodes, oxygen is produced at the anode however if concentrated chlorine ions are present, chlorine will form.
• Non-inert electrodes: the same reaction takes place at the cathode but at the anode atoms lose electrons to form ions in the electrolyte.
• What is a net charge?

  a transfer of an element from the anode to the cathode
• Where does reduction take place in electrolysis?

  Cathode
• Where does oxidation take place during electrolysis?

  Anode
• How to extract halogens:

  e.g bromine from the sea

  1. oxidation of Br- to Br2 - water is first acidified using sulfuric arm if to stop halogens reacting with water as this would lead to a loss of bromine product by the reaction X2 + H2O <-> HX + HXO ( X = Br or Cl) then excess chlorine is used to displace bromine by the reaction Cl2 + 2Br- <-> 2Cl- + Br - chlorine is reduced and bromine is oxidised
  2. The bromine vapours are removed
  3. Reduction of Br2 to HBr - sulfur dioxide, water and bromine vapours react to form hydrogen bromide (more concentrated than bromine in seawater)
• Group 7 elements are highly reactive nonmetals. They exist as diatomic molecules with single covalent bonds. In reactions in order to achieve a full outer shell, they must gain an electron to form a 1- ion
• what are the appearances of the halogens at room temperature and pressure?

  Fluorine - pale yellow gas

  Chlorine - pale green gas

  Bromine - red/brown liquid

  Iodine - dark grey solid which sublimes to purple vapour
• The volatility of the halogens decreases down the group as the boiling point increases. This trend is highlighted by the physical state of the halogens at room temp
• Halogens solubility in water:

  Fluorine reacts violently so hard to measure and the others have no trend

  Chlorine - S = 0.091 - Green solution

  Bromine - S = 0.21 - Orange/Red

  Iodine - S = 0.0013 - Brown solution
• Halogens are more soluble in organic solvents because they are non-polar molecules so are more soluble in non-polar solvents like hexane
• Halogens have s2p5 outer shell configuration. When react tend to gain an electron. As their atomic radius increases this becomes harder as the positive attraction of the nucleus is weakened by additional shielding as a result reactivity decreases down group 7
• Why do halogens act as good oxidising agents?

  They accept electrons from species being oxidised and themselves are reduced. Oxidising power decreases down the group as reactivity decreases
• Halogens basic displacement trend

  
• Halide reactions with silver nitrate and further by ammonia

  
• All halide ions react with concentrated sulfuric acid to produce a hydrogen halide.

  For HF and HCl they can be identified as misty fumes

  HF and HCl are not strong enough reducing agents so no further reaction occurs. HBr misty fumes are produced but it’s a strong enough reducing agent to react with H2SO4 so secondary reaction produces SO2 and brown fumes of Br2 in a redox reaction
• Reaction of NaI with H2SO4:

  misty fumes of HI are produced and as HI is a strong enough reducing agent it reacts with H2SO4 so SO2 is produced but HI is very strong reducing agent so the SO2 is further reduced to H2S which smells like rotten eggs
• What is thermal stability of a hydride?


  How easily a hydrogen halide is broke. Up into its constituent elements when heated