3.4.2 Electrolysis of Aqueous Solutions

• Aqueous solutions will always have water (H₂O)
• In the electrolysis of aqueous solutions, the water molecules dissociate producing H⁺ and OH^– ions:

H₂O ⇌ H⁺ + OH^–

• These ions are also involved in the process and their chemistry must be considered
• We now have an electrolyte that contains ions from the compound plus ions from the water
• Which ions get discharged and at which electrode 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

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, gains electrons and forms a halogen (chlorine, bromine or iodine)
• If no halide ions are present, then OH^- is discharged at the anode, gains electrons and forms oxygen
• In both cases the other negative ion remains in solution

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 the 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 more reactive ions will remain in solution, causing the least reactive ion to be discharged
• Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced

The reactivity series of metals including hydrogen and carbon

• The electrode products are shown below for a series of common electrolytes

Common Electrolytes Table

Inert Electrodes

• Inert electrodes, such as graphite are usually used in electrolysis as they don't take part in the process, they just provide a surface for the reactions to happen on

• The experimental process for the electrolysis of an aqueous solution using inert electrodes is:
  • Set up the apparatus as shown in the diagram
  • Add the aqueous solution to the beaker
  • Add two graphite rods as the electrodes and connect this to a power pack or battery
  • Turn on the power pack or battery and allow electrolysis to take place
  • Record the results in a suitable table (see below) and repeat for another solution, checking the electrodes in between runs to see if any metal has been deposited
  • The gases produced can be collected in the test tubes to be tested later

Diagram showing electrolysis using inert electrodes 

Non-Inert Electrodes

• Sometimes, non-inert or active electrodes are used which will take part in the electrolysis reactions
• These are used for electroplating and purifing copper 
• Purifying copper is necessary as the copper obtained from its ore is not pure enough for use in, for example, electrical wires
• Electroplating is a process used to coat metals in a thin layer of a different metal
• Examples of this include:
  • coating copper or nickel jewellery with silver
  • coating taps with chromium
  • coating steel cutlery with silver 

Purifying copper

• The practical set up is similar to that of inert electrodes except both electrodes are made from copper 
• The anode is impure copper ore
• The cathode is pure copper 
• The electrolyte is copper (II) sulfate solution 
• Once the power supply is turned up
  • At the anode Copper atoms lose electrons to become ions in the electrolyte 

Cu  ⟶  Cu²⁺  +  2e^-

• • As a result the anode decreases in mass and impurities collect underneath
  • At the cathode the copper ions from the electrolyte gain electrons to form copper atoms 

Cu²⁺ + 2e^-  ⟶  Cu

• • Copper collects on the cathode causing the mass to increase 

Electroplating

• The cathode is the object you want to coat
• The anode is the metal you want to coat it with
• The electrolyte is a solution containing ions of the coating metal 
• During the process:
  • At the anode metal atoms lose electrons to form ions in the electrolyte
  • At the cathode metal ions from the electrolyte gain electrons and are discharged on the surface of the metal that needs coating