RI 2025 Electrochemistry (II)

1 RAFFLES INSTITUTION YEAR 6 H2 CHEMISTRY 2025 Lecture Notes 21b – Electrochemistry 2 (Electrolysis) _____________________________________________________________________________ A Content Electrolysis (i) Factors affecting the amount of substance liberated during electrolysis (ii) Industrial uses of electrolysis (iii) The Faraday constant; the Avogadro constant; their relationship B Learning outcomes Candidates should be able to: (a) state the relationship, F = Le, between the Faraday constant, the Avogadro constant and the charge on the electron (b) predict the identity of the substance liberated during electrolysis from the state of electrolyte (molten or aqueous), position in the redox series (electro de potential) and concentration (c) calculate: (i) the quantity of charge passed during electrolysis (ii) the mass and/or volume of substance liberated during electrolysis (d) explain, in terms of the electrode reactions, the industrial processes of: (i) the anodising of aluminium (ii) the electrolytic purification of copper [technical details are not required] C References 1. Chemistry for Advanced Level by Peter Cann and Peter Hughes 2. Chemistry the Central Science by T L Brown and H E Lemay 3. Chemistry by Zumdahl D Online Resources 1. http://www.chemguide.co.uk/physical/redoxeqiamenu.html#top

2 1 INTRODUCTION 1.1 Basic set up • The process of passing electricity (from an external source) to force a non–spontaneous redox reaction to occur is called electrolysis. • The cell in which electrolysis occurs is called the electrolytic cell. • A typical electrolytic cell has the following components: • During electrolysis, there are two possibilities for “chemical change”: a) electrolyte – its components are oxidised or reduced. b) electrode – a ___________anode is oxidised. • Due to the redox processes occurring, some possible physical observations are: a) the electrolyte changes colour or colour intensity, b) bubbling at the cathode or anode (i.e. a gas is evolved), c) a solid metal deposits on the cathode, d) the anode decreases in size and mass (i.e. it dissolves). Note: • Substances are described as being liberated (for substances produced) or discharged (for species that underwent redox reaction) during electrolysis. • Any oxidation always takes place at the anode. • Any reduction always takes place at the cathode. A + – e– e–  External source of electrical energy i.e. battery  Electrolyte (molten or aqueous)  Electrodes (cathode/anode) e– flow to this electrode • chemical species accept e– from this electrode • reduction occurs here This electrode is the cathode. e– flow from this electrode • chemical species lose e– here • oxidation occurs here This electrode is the anode.

3 1.2 Comparison between Electrochemical (or Voltaic) Cell & Electrolytic Cell Type of Cell Electrochemical (or Voltaic) Cell Electrolytic Cell Diagram Spontaneous redox reaction in cell produces electricity. Electricity f