chemical changes

• when a metal reacts with oxygen we form a metal oxide
• gaining oxygen is called an oxidation reaction
• losing oxygen is called a reduction reaction
• reactivity series

  • potassium
  • sodium
  • lithium
  • calcium
  • magnesium
  • carbon
  • zinc
  • iron
  • hydrogen
  • copper
• magnesium, zinc, iron, copper have no reaction with water at room temp
• potassium, sodium, lithium react very rapidly with water at room temp
• potassium, sodium,lithium react dangerously fast with acids
• calcium and magnesium have quite vigorous and fast reactions wiht acid
• zinc and copper a reaction with acids
• copper has no reaction with solids
• when metals react they lose electrons and form positive ions
• unreactive metals such as gold are found in the earth as the metal itself
• a more reactive element will displace out a less reactive element from its compound
• in aqueous solutions acids produce H+ ions
• in aqueous solutions alkalis produce hydroxide ions
• magnesium forms Mg2+ when reacting with acid
• zinc forms Zn+2 when reacting with acid
• iron form Fe+2 when reacting with acid
• strong acids fully ionise in aqueous solution
• HCl --> H+ + Cl-
• H2SO4 --> 2H+ + SO4-2
• HNO3 --> H+ + NO3-
• weak acids partially ionise
• the pH scale gives us an idea of the concentration of hydrogen ions
• as the pH scale deacreases by one unit the concentration of hydrogen ions increases by 10 times
• solid ionic compounds cannot conduct electricity because the ions are locked in place and are not free to move
• when ionic compound is melted or dissolved in water the forces of attraction are broken and the ions are now free to move

  these liquids and solutions can now conduct electricity
• electrolysis of aluminium oxide

  mixed with cryolite which lowers the melting point

  reduces the amount of energy needed and saves money

  aluminium is attracted to the cathode

  oxygen is attracted to the anode
• why are the cathode and anode made of carbon

  good conductor of electricity and has a high melting point
• Al3+ + 3e- --> Al
• 2O2- --> O2 + 4e-
• the anode must be replaced regularly

  the oxygen molecules produced at the anode react with the graphite which forms carbon dioxide
• why is electrolysis very expensive?


  melting the compounds needs a lot of energy

  a lot of energy needed to produce the electric current
• water molecules ionise (split) forming hydrogen ions and hydroxide ions
• hydrogen is produced at the cathode is the metal is more reactive then hydrogen
• Cu2+ + 2e- --> Cu
• 4OH- --> O2 + 2H2O + 4e-
• if the aqueous solution contains halide ions then the halogen will be produced at the anode
• 2H+ + 2e- --> H2
• 2Cl- --> Cl2 + 2e-