electrolysis

• the cathode is the negative electrode
• the anode is the positive electrode
• oxide ions lose 2 electrons at the anode forming oxygen. oxygen reacts with the graphite in the cathode to form carbon dioxide. this is the anode burning.
• equation for anode burning = carbon + oxygen -> carbon dioxide
• why does graphite conduct electricity?

  each carbon atom forms three bonds with other carbon atoms therefore graphite has one free (delocalised) electron per carbon that can move throughout the structure and carry charge .
• why is a mixture used as the electrolyte?

  cryolite reduces the melting point of aluminium oxide to 905 degrees so that less energy is needed to separate the ion, deeming it more cost effective and better globally.
• aluminium oxide oxygen equation = 2O2- -> O2 + 4e-
• aluminium oxide aluminium equation = al3+ + 3e- -> al
• graphite? unreactive, high melting point and conductive
• rate of reaction?

  mass of reactant used up (g) / time (s)
• marble chips?

  the larger the marble chips, the lower the surface area, the slower the rte of reaction.
• explain in terms of collision theory why increasing concentration increases rate of reaction?

  increasing concentration means there are more particles in a fixed volume. collisions are likely so the frequency of collisions increase
• explain in terms of collision theory why increasing pressure increases rate of reaction?

  increasing pressure means there are more particles in a fixed volume. collisions are likely so the frequency of collisions increase.
• explain in terms of collision theory why increasing surface area increases rate of reaction?

  larger SA has more particles exposed to collide with.
• explain in terms of collision theory why increasing temperature increases the rate of reaction: as the particles have more kinetic energy, they collide more frequently. particles have more energy so when they collide, they overcome the activation energy.
• oxide ions lose electrons at the anode and become oxygen atoms the oxygen atom reacts with the carbon in the graphite and forms carbon dioxide, this burns the anode so it needs replacing
• what is electrolysis?


  electrolysis is separating ionic compounds into their elements by using electricity.
• why are the electrodes made using graphite?

  they have a low reactivity, they're conductive, have a high melting point
• why is the ionic substance molten or in solution?

  so that ions are free to move and carry charge through the substance as the electrons are delocalised.
• cons of electrolysis of molten ionic substances?

  uses lots of energy

  very expensive

  specialised vessels are required
• cons of electrolysis of dissolved ionic substances?

  sometimes an undesireable product is made
• what changes at the electrode?

  positive ions gain electrons from the negatively charged cathode. they change from ions to atoms. negative ions lose electrons at the positively charged anode. they change from ions to atoms.
• what happens when negative ions are attracted to the anode?

  if ion is a halide (chlorine, bromine or iodine)the halide is produced is produced, if not, oxygen is produced.

  4OH--> 2H2O+O2+4e-
• what happens when positive ions are attracted to the cathode?


  if they metals are more reactive than hydrogen, hydrogen is produced

  the equation for this is 2H+ +e- -> H2

  if they are less reactive, the metal is produced (copper, gold,silver,platinum)
• what happens when an ionic solution is dissolved in water?

  the ions H+ and OH- are also present.

  e.g. molten zinc oxide ions = Zn2+ and Cl-

  -> dissolved zinc chloride = Zn2+, Cl-, H+, OH-
• what are the products of the electrolysis of brine?

  chlorine gas (discharged at anode)

  hydrogen gas (discharged at anode)

  sodium hydroxide (remains in solution)
• what is chlorine gas useful for?

  killing bacteria in water, bleach, PVC
• what is hydrogen gas used for?

  making margarine, and fertilisers, used for rocket fuel
• what is sodium hydroxide used for?

  used in many chemical reactions (making soap), neutralising acids and making paper