Chemistry - C2.5 - Salts and Electrolysis

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C2.5.1 - Acids and Alkalis

  • Pure water  - nuetral: pH7
  • Acids - substances that produce H+ ions(aq) when dissolved in water - aqueos solution: substance dissolved in water - (aq) state symbol
  • Base + acid -> neutral
  • Alkalis: bases - dissolve in water maing alkaline solution - produce OH- ions (aq) in the solution
  • pH scale - 0-14 - acids: below 7, alkalis: above 7, neutral:7
  • Indicators change colour in acidic and alkaline solutions - universal indicator and others have different colours for different pH values
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C2.5.2 - Making Salts from Metals or Bases

  • Acids - react with metals above H in reactivity series - some to violent to be done safely
  • Metal + acid -> salt + hydrogen: H2SO4(aq) + Zn(s) -> ZnSo4(aq) + H2(g)
  • Metal oxides and hydorxides - bases
  • Acid + base -> salt + water: 2HCl(aq) + MgO(s) -> MgCl2(aq) + H2O(l) - neutralisation
  • Reactions used to make salts
  • Insoluable metal or base added a little at a time to acid until it's all reacted, mixture filtered removing excess solid reactant - leaving solution of salt - solid salt made when water evaporates so it crystallises
  • Chlorides made from hydrochloric acid, nitrates from nitric acid, sulfates from sulfuric acid
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C2.5.3 - Making Salts from Solutions

  • Acid + alkali -> salt + water: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
  • Neutralisation reaction - H+(aq) + OH-(aq) -> H2O(l)
  • No visible change when acid and alkalis react - need indicator or pH meter to show when reaction is complete - solid salt obtained from solution by crystallisation
  • Ammonia solution - alkali without metal in - reacts with acids to make ammonium salts, eg. ammonium nitrate NH3NO3 - used as fertilisers
  • Make insoluable salts by mixing solutions of soluable salts containing ions needed - some form precipitate: filtered from solution, washed with distilled water and dried: Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2KNO3(aq)
  • Some pollutants, eg. metal ions, removed by precipitaion - substances that react with pollunant added forming insoluable solids
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C2.5.4 - Electrolysis

  • Process using electricity to break down ionic compounds into elements - takes place when electricity passes through molten ionic compound or solution containing ions - substance broken down is electrolyte
  • Electrical circuit has two electrodes in contact with electrolyte - made of inert substance so don't react with products
  • Ions in electrolyte move to electrodes where they are discharged to produce elements
  • Positively charged ions attracted to negative electrode - gain electron and form metal or hydrogen
  • Negatively charged ions attracted to positive electrode - lose charge and form non-metallic elements
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C2.5.5 - Changing at the Electrodes

  • Reduction - gaining electrons: positive ions are reduced - go to negative electrode, gain electrons, become neutral atoms
  • Oxidation - losing electrons: negative ions are oxidised - go to positive electrode, lose electrons, become neutral atoms - some non-metallic atoms combine forming molecules
  • Represent changes at electrodes with half equations, eg. lead bromine: negative electrode - Pb2+ + 2e- -> Pb, positive electrode - 2Br- -> Br2 + 2e-
  • Solutions - water has hydrogen and hydoxide ions - hydrogen may be produced at negative electrode if positive ion in solution is a metal more reactive than hydrogen - oxygen usually produced at positive electrode from aqueos solution - halogen will be produced if there's a reasonably high concentration of a halide ion
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C2.5.6 - The Extraction of Aluminium

  • Aluminium - more reactive than carbon: must be extracted by electrolysis - ore contains aluminium oxide - must be purified and melted to be electrolysed - melts at over 2000C - need energy - mixed with cryolite: melts at ~850C - mixture electrolysed at lower temperature producing aluminium and oxygen
  • 2Al2O3(l) -> 4Al(l) + 3O2 - aluminium oxide -> aluminium + oxygen
  • Cryolite remains in cell and fresh aluminium oxide added as aluminium and oxygen are produced
  • Negative electrode - Al3+(l) + 3e- -> Al(l) - molten alumium metal collected from bottom of cell
  • Positive electrode - 2O2-(l) -> 2O2(g) + 4e- - oxygen forms as oxygen molecules - made of carbon - high temperatures: oxygen reacts with carbon -> CO2 - gradually burn away: have to be replaced regularly
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C2.5.7 - Electrolysis of Brine

  • Brine - solution of sodium chloride in water - Na+(aq), Cl-(aq), H+(aq) and OH-(aq) ions - when electrolysed: positive electrode: 2Cl- -> Cl2 +2e-, negative electrode: 2H+ + 2e- -> H2 - sodium and hydroxide ions left (NaOH(aq))
  • Sodium hydroxide - strong alkali - used for making soap, paper and bleach, neutralisation and controlling pH
  • Chlorine - used to kill bacteria in drinking water and swimming pools, make bleach disinfectants and plastics
  • Hydrogen - used to make margarine and hydrochloric acid
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C2.5.8 - Electroplating

  • Electrolysis used to put thin coating of metal onto an object - why?:
    • make object look more attractive
    • protect metal object from corroding
    • increase hardness of surface
    • reduce costs - uses thin layer of metal instead of pure metal
  • Negative electrode: object to be plated, positive electrode: plating metal, electrolyte: solution containing plating metal
  • Positive electrode: plating metal atoms lose electrons forming metal ions that go into solution
  • Negative electrode: metal ions from solution gain ions forming metal ions that are disposed on object being plated
  • Half equations for nickel: positive electrode - Ni(s) -> Ni2+(aq) +2e-, negative electrode - Ni2+(aq) +2e- -> Ni(s)
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