I/GCSE Chemsitry - Electrolysis

• Let revisit some basic ideas about electrolysis in GCSE Chemistry unit 5
• Electrolysis is the process of passing direct current through a solution 
• compounds are then broken down and discharged some of the elements at the electrode

Electrolysis

Definition:

• ionic substances are decomposed into simpler substances when an electric current is passed through them.

Keywords:

1. anode: attracts negative ions(anions)
2. cathode: attracts positive ions(cations)
3. electrolyte: Ionic compounds that are in a molten state, provides mobile ions 

Mechanism:

• dc current is on to promote ions/electrons flow
• Positive charged ions(cations) move towards cathode
• Negative charged ions(anions) move towards anode

Half equations:

• Oxidation occurs at anode (An-ox)
• Reduction occurs at cathode (Red-cat)

• e.g. Balanced-equation: 2Al₂O₃(aq) --> 4Al(s) + 3O₂(g)
• Reduction half-equation: Al³⁺ + 3e^- --> Al   (Cathode)
• Oxidation half-equation: 2O^2- --> O₂ + 4e^-   (Anode)

Remarks:

**Cations are usually non-metal ions (e.g. O2-,Cl-,OH-…)

**Anions are usually metal ions (Na+,Al3+,H+…)

Reactivity series:

• In GCSE Chemistry, reactivity series is mentioned in unit 2.1
• determines which species in solution is oxidized/reduced first
• Halides ions (Cl^-,Br^-,I^-,F^-) > OH^-
• More reactive metals (K⁺,Na⁺,Ca²⁺,Mg²⁺…Pb²⁺) > H⁺ > Less reactive metals (Cu²⁺,Ag⁺,Au⁺, Pt⁺)
• if OH^- is discharged --> O₂ (g) forms
• if H⁺ is discharged --> H₂ (g) forms

Method of extraction - Electrolysis

• suitable for reactive metals which CANNOT be extracted by carbon reduction  (Usually at top of the reactivity series)
• large amount of energy is use, thus these metals can be expensive

Example: Extraction of aluminum

• Ores: bauxite
• purified bauxite is mixed with cryolite --> melt in steel electrolysis cell
• electrodes: carbon-lined cathode + suspended carbon anodes

Mechanism

• positive aluminum ions migrate towards cathode and discharged to form molten metal
• negative oxide ions migrate towards anode , O₂ (g) produced
• suspended carbon anodes react with oxygen, CO₂ (g) produced

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Image Reference:

• Electrolysis - Definition, Process, Applications - https://byjus.com/jee/electrolysis/
• Reactivity series - https://kaiserscience.wordpress.com/chemistry/the-periodic-table/metal-reactivity-series/ 

Drafted by Yoyo (Chemistry)