• Electrolysis?is the breaking down of a compound into its elements using an?electric current
• For example, the electrolysis of zinc chloride (ZnCl₂) into its elements zinc and chlorine

ZnCl₂(s) → Zn(s) + Cl₂(g)

• This method is often used to:
  • Extract?metals?from their?metal ores?when the metals cannot be extracted by heating their ores with carbon
  • Purify metals
  • Produce non-metals such as fluorine
• Electrolysis is carried out in an electrolysis cell which consists of:
  • An?electrolyte?- this is the compound that is broken down during electrolysis and it is either a?molten ionic compound?or a?concentrated aqueous solution of ions
  • Two?electrodes?- these are metal or graphite rods conduct electricity to the electrolyte and away from the electrolyte
    • The positive electrode is called the?anode
    • The negative electrode is called the?cathode
  • The power supply, which is?direct current

Electrolysis takes place in an electrochemical cell;

this type of electrochemical cell is called an electrolytic cell

Electrolysis of molten electrolytes

• Cations?(positively charged ions) move to the?negatively charged cathode?where they?gain electrons
  • Reduction?takes place at the cathode
  • If a?metal?is formed, a layer of metal is deposited on a cathode or it forms a?molten layer?in the cell
  • If?hydrogen?gas is formed, bubbles are seen
  • For example, silver and hydrogen both form positively charged ions which would be?reduced?at the cathode as follows:

Ag⁺?+ e^-?→ Ag

2H^+?+ 2e^-?→ H₂

• Anions?(negatively charged ions) move to the?positively charged?anode?where they?lose electrons
  • Oxidation?takes place at the anode
  • For example, bromine forms negatively charged ions which would be?oxidised?at the anode as follows:

2Br^-?→ Br₂?+ 2e^-

Products formed by electrolysis when a pure molten ionic compound containing two simple ions is electrolysed table

Electrolysis of aqueous solutions

• Aqueous?solutions?have more than one cation and anion in solution due to the presence of?water
• Water contributes H⁺?and OH^-?ions to the solution, which makes things more complicated
  • Water is a weak electrolyte and splits into H⁺?and OH^-?ions as follows:

H₂O ? H⁺?+ OH^-

• The actual ions that are?discharged?during electrolysis will depend on:
  • The relative electrode potential of the ions
  • The concentration of the ions

Relative electrode potential of ions

• The?relative electrode potential?(E^?) of ions describes how easily an ion is discharged during electrolysis
• The?positively charged?cation?with the most?positive?E^??will be discharged at the?cathode?as this is the cation that is most easily?reduced
  • For example, a concentrated?aqueous solution?of NaF will contain hydrogen (H⁺) and sodium (Na⁺) ions
  • The half-equations for the reduction of these ions and their?E^??values are as follows:

2H⁺(aq) + 2e^-??? H₂(g)? ? ?E^??=??0.00 V

Na⁺(aq) + e^-??? Na(s)? ? ?E^??=?-2.71 V

• Since H^+?ions have a higher?E^??value, hydrogen gas (H₂) is formed at the?cathode?instead of sodium (Na)
• The?negatively charged anion?with the most?negative?E^??will be discharged at the?anode, as this is the anion that is most easily?oxidised
  • For example, a concentrated aqueous solution of NaF will contain hydroxide (OH^-) and fluoride (F^-) ions
  • The half-equations for the oxidation of these ions and their?E^??values are as follows:

4OH^-(aq)?^?→ O₂(g) + 2H₂O(l) + 4e^{-? ? ? ??}E^??=?-0.40 V

2F^-(aq)?^?→ F₂(g) + 2e^{-? ? ? ??}E^??=?-2.87 V

• Since F^-?ions have a lower?E^??value than OH^-?ions, fluorine (F₂) gas is formed at the anode

Concentration of ions

• Ions that are present in?higher?concentrations?are more likely to be discharged
• For example, when a?concentrated?solution of NaF is electrolysed, there are far more?fluoride?ions which are discharged at the anode, instead of the?hydroxide?ions as the fluoride ions are in higher concentration
  • So, mainly fluorine will form at the electrode
• However, if a?very?dilute?solution of NaF is electrolysed, there will be much more?oxygen?and much less fluorine gas formed at the anode
  • In reality, a mixture of both oxygen and fluorine gas is formed