Electrolysis  Section 19.2

Electrolysis Occurs in an electrolytic cell Can be the molten salt, or ions in solution Cations are attracted to the cathode where reduction takes place Anions are attracted to the anode where oxidation takes place Electrodes are usually made of inert conductors like graphite or platinum Other considerations if an aqueous solution is used

Water Considerations Electrolysis of water could occur Cathode: reduction to H 2 Anode: oxidation to O 2

Reduction Occurs unless the metal ions in the electrolyte are low in the electrochemical series (like Cu or Ag) 2H 2 O (l) + 2 e - → H 2(g) + 2OH - (aq) Can be regarded as the reduction of hydrogen ions produced from the dissociation of water: 2H + (aq) + 2 e - → H 2(g)

Oxidation Occurs unless the electrolyte contains easily oxidized anions (like Br - or I - ) 2H 2 O (l) → O 2(g) + 4H e - Can be regarded as the oxidation of hydroxide ions produced from the dissociation of water: 4OH - (l) → O 2(g) + 2H 2 O (l) + 4 e -

Case to Consider Copper electrodes Aqueous copper sulfate is the electrolyte Copper is transferred during electrolysis from the anode (decreases in mass) to the cathode (increases in mass) This process is used to purify copper

Electroplating Process of creating a thin layer of metal over another metal by electrolysis Uses: Decoration Protection against corrosion

Products of the Electrolysis of an Aqueous Solution Cathode: Formation of the metal or formation of hydrogen The one with the most positive E Θ will take place Hydrogen is formed unless Cu 2+ or Ag + is present

Continued Anode: Formation of a non-metal, formation of oxygen or oxidation of the electrode Usually the one with the most negative E Θ will take place Oxygen is formed unless Br - or I - are present

More Water Considerations Pure water cannot be electrolyzed (concentration of ions is too low) If an electrolyte produces the same products as water, or is less easily oxidized/reduced, H 2 will form at the cathode and O 2 at the anode There will be twice as much H 2 as O 2 4 H 2 O (l) + 4 e - → 2 H 2(g) + 4 OH - (aq) and: 2 H 2 O (l) → O 2(g) + 4H e -

Determining Factors for the Amount of Product that Results Concentration of the electrolyte (saturated solution) reduces E Θ The number of electrons required to produce one mole of product Magnitude of the current (rate of flow of electrons) Time for which the current has passed

Case to Consider Same current, same amount of time Aqueous solution of silver ions and an aqueous solution of copper ions Twice as much metal will be produced at the cathode of the silver cell Ag + + e - → Ag compared to Cu e - → Cu Same amount of copper would require doubling the current or doubling the time