Electrolysis

• Aqueous solutions always contain water (H2O), which dissociates into H+ and OH- ions.
• An electrolyte contains ions from the compound plus ions from the water, and the discharge of these ions depends on the relative reactivity of the elements involved.
• Concentrated and dilute solutions of the same compound give different products.
• For anions, the more concentrated ion will tend to get discharged over a more dilute ion.
• Hydrogen copper ions are discharged at the cathode, gain electrons and are reduced to form copper metal.
• The half equation for the reaction at the cathode is: Cu2+ + 2e- —> Cu.
• The half equation for the reaction at the anode is: 4OH- —> O2 + 2H2O + 4e-.
• The cathode increases in mass while the anode decreases; this occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at the cathode, forming copper atoms.
• OH- lose electrons and are oxidised to form oxygen gas.
• The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode; therefore the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode.
• SO4(2-) and OH- are both attracted to the anode.
• OH- ions lose electrons more readily than SO4(2-).
• The concentration of the Cu2+ ions in the solution remains constant.
• The concentration of the solution also affects which ion is discharged if a concentrated halide solution is being electrolysed, the halogen is discharged first.
• If a dilute halide solution is being electrolyzed, oxygen is formed.
• For a concentrated solution of barium chloride, the Cl ions are discharged more readily than the OH ions, so chlorine gas is produced at the anode.
• Positive electrode (anode) negatively charged OH- ions and non-metal ions are attracted to the positive electrode if halide ions (Cl-, Br-, I-) and OH- are present, then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine) if no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen gas; in both cases, the other negative ion remains in the solution.
• If the solution is dilute, however, only the OH ion is discharged, and so oxygen would be formed.
• In the case of aqueous copper sulfate, Cu2+ and H+ will both be attracted to the cathode but the less reactive ion will be discharged; in this case, copper is less reactive than hydrogen.
• Negative electrode (cathode) positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons; either hydrogen gas or metal will be produced if the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode; this is because the ions of the more reactive metal will remain in the solution, causing the ions of the least reactive metal to be discharged.
• electrolysis definition


  the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
• When ions reach an electrode, they gain or lose electrons, as a result, they form atoms or molecules of elements

  • positive ions (cations) gain electrons from the negatively charged cathode
  • negative ions lose electrons at the positively charged anode
• Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis:

  • Pb2+ ions gain electrons at the cathode and become Pb atoms
  • Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules

  Lead forms at the negative electrode and bromine forms at the positive electrode.
• half equation at the cathode of molten lead bromide

  Pb2+ + 2e- → Pb
• half equation at the anode of molten lead bromide

  2Br- → Br2 + 2e-
• Electrolysis of dilute sulfuric acid

  • dilute sulfuric acid contains water. The ions present in this mixture are H+ and OH- (from the water) and H+ and SO42- from the sulfuric acid.
  • he H+ ions are attracted to the cathode and the two negative ions are attracted to the anode but it is the OH- ion which loses electrons.
  • H+ ions are attracted to the cathode, gain electrons and form hydrogen gas = 2H+ + 2e- → H2
  • OH- ions are attracted to the anode, lose electrons and form oxygen gas and water = 4OH- → O2 + 2H2O + 4e-
• Brine is a concentrated solution of aqueous sodium chloride and can bel electrolyzed using inert electrodes made from platinum carbon, or graphite.

  • when electrolyzed it produces bubbles of gas at both electrodes as chlorine and hydrogen are produced leaving behind sodium hydroxide solution
• product at the cathode in brine

  The H+ ions are discharged as they are less reactive than sodium ions. The H+ ions gain electrons to form hydrogen gas 

  •  2H+ + 2e– → H2
• product at the anode in brine

  The Cl– ions are discharged and lose electrons and chlorine gas forms

  • The Na+ and OH– ions remain behind and form the NaOH solution
  • 2Cl– → Cl2 + 2e–
• If the gas produced at the anode relights a glowing splint dipped into a sample of the gas, then the gas is oxygen

  
• If the gas produced at the anode bleaches damp litmus paper then the gas is chlorine

  
• If the gas produced at the cathode burns with a 'pop' when a sample is lit with a lighted splint then the gas is hydrogen