Chemistry

• Electrolysis

  Used to extract some metals
• Extracting aluminium by electrolysis

  1. Mix aluminium oxide with compound cryolite to lower melting point

  2. Apply electric current to molten aluminium oxide

  3. Aluminium ions attracted to cathode and gain 3 electrons to form aluminium atoms

  4. Oxide ions attracted to anode and lose 2 electrons to form oxygen atoms
• Cathode reaction


  • Aluminium ions (Al3+) gain 3 electrons to form aluminium atoms
• Anode reaction


  • Oxide ions (O2-) lose 2 electrons to form oxygen atoms
• Oxygen atoms formed at anode pair up to form oxygen molecules (O2)
• Anode must be replaced regularly as the oxygen reacts with the graphite to form carbon dioxide
• Producing metals by electrolysis is very expensive due to the high energy required to melt the compounds and produce the electric current
• Electrolysis


  The process of using electricity to drive a chemical reaction
• Electrolysis of aqueous solutions

  • Need to consider the ions present in water (H+ and OH-)
  • Need to consider the reactivity series to determine which ions will be reduced at the cathode
• Electrolysis of copper sulfate solution

  1. Copper ions (Cu2+) are reduced at the cathode to form copper atoms

  2. Oxygen gas is produced at the anode from the oxidation of hydroxide ions
• Electrodes used in electrolysis should be inert, meaning they do not react with the chemicals being produced
• Half-equation for cathode reaction


  Cu2+ + 2e- → Cu (reduction)
• The reactions at the cathode and anode during electrolysis of aqueous solutions are important for higher tier students to understand
• Aqueous


  Dissolved in water
• Water molecules splitting


  1. Produce hydrogen ion H+

  2. Produce hydroxide ion OH-
• Sodium chloride


  Has sodium ion Na+ and chloride ion Cl-
• Electrolysis of sodium chloride solution


  1. Ions attracted to cathode: sodium ion Na+ and hydrogen ion H+

  2. Ions attracted to anode: chloride ion Cl- and hydroxide ion OH-
• Cathode

  • Hydrogen gas is produced
• Anode

  • Chlorine gas is produced
• If the aqueous solution contains halide ions such as chloride, then the halogen will be produced at the anode
• Reaction at cathode


  1. Hydrogen ion H+ gains 1 electron to form hydrogen atom

  2. Hydrogen atoms pair to form hydrogen molecule H2
• Reaction at anode


  1. Chloride ion Cl- loses 1 electron to form chlorine atom

  2. Chlorine atoms pair to form chlorine molecule Cl2
• The half equations for the reactions at the cathode and anode can be written in different ways but show the same reactions
• Investigating what happens when aqueous solutions undergo electrolysis

  1. Pour approximately 50 cm³ of copper(II) chloride solution into a beaker

  2. Place a plastic petri dish with two holes over the beaker

  3. Insert a carbon graphite rod into each hole (electrodes)

  4. Attach crocodile leads to the rods and connect to a low-voltage power supply

  5. Select 4 volts on the power supply and switch it on
• Electrodes


  • Carbon graphite rods are unreactive (inert)
  • The two electrodes must not touch each other to avoid a short-circuit
• Copper is less reactive than hydrogen


  Copper is discharged at the cathode
• Electrolyzing a solution containing a halide ion


  The halogen is discharged at the anode
• Holding a piece of damp blue litmus paper near the anode shows the gas produced is chlorine (it becomes bleached)
• Electrolysis of copper(II) chloride solution

  Copper is discharged at the cathode, chlorine gas is discharged at the anode
• Investigating electrolysis of sodium chloride solution


  1. Pour approximately 50 cm³ of sodium chloride solution into a beaker

  2. Turn on the low-voltage power supply
• Sodium is more reactive than hydrogen


  Hydrogen gas is discharged at the cathode
• Collecting the gas at the cathode and testing with a lit splint shows it is hydrogen (produces a squeaky pop)
• Mixture


  Any combination of any different types of elements and compounds that aren't chemically bonded together
• Solution


  A mixture of a solute dissolved in a solvent
• Separating mixtures


  1. Filtration for large insoluble particles

  2. Crystallization to leave solute behind after evaporating solvent

  3. Distillation to separate liquids with different boiling points
• Physical changes


  • No new substances are being made
  • Breaking intermolecular forces, not chemical bonds
• States of matter


  • Solid - particles vibrate in fixed positions
  • Liquid - particles can move past each other
  • Gas - particles move randomly and have high energy
• Gases can be compressed, solids and liquids cannot
• To melt or evaporate a substance, energy must be supplied to overcome intermolecular forces
• Atom models


  • Plum pudding model - positive charge with electrons dotted around
  • Nuclear model - positive nucleus with electrons orbiting relatively far away