Chemisty

• Electrolysis


  When an electric current is passed through a molten ionic compound, the compound decomposes or breaks down

  
• Electrode


  A rod of metal or graphite through which an electric current flows into or out of an electrolyte
• Electrolyte


  The ionic compound in molten or dissolved solution that conducts the electricity
• Anode


  The positive electrode of an electrolysis cell
• Anion


  A negatively charged ion which is attracted to the anode
• Cathode


  The negative electrode of an electrolysis cell
• Cation


  A positively charged ion which is attracted to the cathode
• Electrolysis


  The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity
• Electrolysis of molten lead(II) bromide, concentrated hydrochloric acid, concentrated aqueous sodium chloride, dilute sulfuric acid

  1. Cathode (-): Hydrogen produced (unless metal less reactive than hydrogen), Metal produced (if metal less reactive)

  2. Anode (+): Oxygen produced (unless halide ions present), Halogen produced (if halide ions present)
• Electrolysis of molten lead(II) bromide

  1. Pb2+ to cathode, Pb(s) produced

  2. Br- to anode, Br2(l) produced
• Electrolysis of concentrated hydrochloric acid

  1. H+ to cathode, H2(g) produced

  2. Cl- to anode, Cl2(g) produced
• Electrolysis of sodium chloride solution

  1. H+ to cathode, H2(g) produced

  2. Cl- to anode, Cl2(g) produced
• Electrolysis of dilute sulfuric acid

  1. H+ to cathode, H2(g) produced

  2. OH- to anode, O2(g) produced
• Electrolysis of aqueous copper(II) sulfate using carbon electrodes

  1. Cu2+ to cathode, Cu(s) produced

  2. OH- to anode, O2(g) produced
• Electrolysis of aqueous copper(II) sulfate using copper electrodes

  1. Cu2+ ions from anode move to cathode, pure Cu deposited

  2. Impurities form sludge below anode
• Electrolysis: General Principles

  1. Positive electrode (anode): Non-metal ions (other than hydrogen) are attracted to the positive electrode, Non-metal ions will lose electrons to form the non-metal

  2. Negative electrode (cathode): H+ and metal ions attracted to the negative electrode, but only one will gain electrons, If the metal is above hydrogen in reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode

  
• Metals or hydrogen are formed at the negative electrode (cathode), and that non-metals are formed at the positive electrode (anode)
• Predicting products of electrolysis of molten ionic compounds

  1. Identify ions in compound

  2. Positive ions go to cathode

  3. Negative ions go to anode
• Predicting products of electrolysis of aqueous halide solutions

  Halide ions (Cl-, Br-, I-) go to anode, producing Cl2, Br2, I2
• Electroplating process

  1. Negative electrode = object to be coated

  2. Positive electrode = metal to coat with

  3. Electrolyte = solution of coating metal ions
• Uses of electroplating

  • Coating cheaper metal with more expensive one (e.g. silver cutlery, copper pans)
  • Improving appearance and corrosion resistance
• Oxidation


  Loss of electrons
• Electrolysis of molten lead(II) bromide

  Negative bromide ions move to the positive electrode (anode) and each loses one electron to form bromine molecules, Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode

  
• Reduction


  Gain of electrons
• Aqueous solutions will always have water present
• Reduction


  When a substance gains electrons

  
• OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)
• At the anode


  Negatively charged ions lose electrons and are oxidised

  
• At the cathode


  Positively charged ions gain electrons and are reduced

  
• Half equations

  Show the oxidation and reduction of the ions involved

  
• Exothermic reaction


  Thermal energy is transferred to the surroundings, so the temperature of the surroundings increases
• Endothermic reaction


  Thermal energy is taken in from the surroundings, so the temperature of the surroundings decreases
• If more energy is released when new bonds are formed

  The reaction is exothermic

  
• If more energy is absorbed to break bonds

  The reaction is endothermic
• Activation energy (Ea)


  The minimum energy that colliding particles must have in order to react
• Electrolysis of CuSO4 using inert electrodes


  1. Produces oxygen and copper

  2. Changing the electrodes changes the products at each electrode
• Enthalpy change (ΔH)


  The transfer of thermal energy during a reaction, which is either positive (endothermic) or negative (exothermic)
• Copper Refining


  1. Anode is impure copper, cathode is pure copper

  2. Copper atoms at the anode lose electrons and go into solution as ions, impurities fall to the bottom

  3. Copper ions in the solution are attracted to the cathode where they gain electrons and form purified copper atoms

  
• Exothermic reactions

  • Combustion
  • Oxidation
  • Neutralisation
• Electroplating

  • Process where the surface of one metal is coated with a layer of a different metal
  • Anode is made from the pure metal to coat the object with
  • Cathode is the object to be electroplated
  • Electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode