Electrochemical Reactions

Electrolytic cells and galvanic cells

• Define the galvanic cell as a cell in which chemical energy is converted into electrical energy. A galvanic (voltaic) cell has self-sustaining electrode reactions.
• Define the electrolytic cell as a cell in which electrical energy is converted into chemical energy.
• Define oxidation and reduction in terms of electron (e^-) transfer:
  Oxidation is a loss of electrons.
  Reduction is a gain of electrons.
• Define oxidation and reduction in terms of oxidation numbers:
  Oxidation: An increase in oxidation number
  Reduction: A decrease in oxidation number
• Define an oxidising agent and a reducing agent in terms of oxidation and reduction:
  Oxidising agent: A substance that is reduced/gains electrons.
  Reducing agent: A substance that is oxidised/loses electrons.
• Define an anode and a cathode in terms of oxidation and reduction:
  Anode: The electrode where oxidation takes place
  Cathode: The electrode where reduction takes place
• Define an electrolyte as a solution/liquid/dissolved substance that conducts electricity through the movement of ions
• Electrolysis: The chemical process in which electrical energy is converted to chemical energy OR the use of electrical energy to produce a chemical change.

Relation of current and potential difference to rate and equilibrium

• Give and explain the relationship between current in an electrolytic cell and the rate of the reaction.
• State that the potential difference of a galvanic cell (V_cell ) is related to the extent to which the spontaneous cell reaction has reached equilibrium.
• State and use the qualitative relationship between V_cell and the concentration of product ions and reactant ions for the spontaneous reaction, namely V_cell decreases as the concentration of product ions increases and the concentration of reactant ions decreases until equilibrium is reached at which the V_cell = 0 (the cell is 'flat'). (Qualitative treatment only. Nernst equation is NOT required.)

Understanding of the processes and redox reactions taking place in galvanic cells

• Describe the movement of ions in the solutions.
• State the direction of electron flow in the external circuit.
• Write down the half-reactions that occur at the electrodes.
• State the function of the salt bridge.
• Use cell notation or diagrams to represent a galvanic cell.
  When writing cell notation, the following convention should be used:
  • The H₂|H⁺ half-cell is treated just like any other half-cell.
  • Cell terminals (electrodes) are written on the outside of the cell notation.
  • Active electrodes:
    reducing agent|oxidised species||oxidising agent|reduced species
  • Inert electrodes (usually Pt or C):
    Pt | reducing agent|oxidised species||oxidising agent|reduced species|Pt
    Example:
    
• Predict the half-cell in which oxidation will take place when two half-cells are connected.
• Predict the half-cell in which reduction will take place when connected to another half- cell.
• Write down the overall cell reaction by combining two half-reactions.
• Use the Table of Standard Reduction Potentials to calculate the emf of a standard galvanic cell.
• Use a positive value of the standard emf as an indication that the reaction is spontaneous under standard conditions.

Standard Electrode Potentials

• Write down the standard conditions under which standard electrode potentials are determined.
• Describe the standard hydrogen electrode and explain its role as the reference electrode.
• Explain how standard electrode potentials can be determined using the reference electrode and state the convention regarding positive and negative values.

Understanding the processes and redox reactions taking place in electrolytic cells

• Describe the movement of ions in the solution.
• State the direction of electron flow in the external circuit.
• Write equations for the half-reactions taking place at the anode and cathode.
• Write down the overall cell reaction by combining two half-reactions.
• Describe, using half-reactions and the equation for the overall cell reaction as well as the layout of the particular cell using a schematic diagram, the following electrolytic processes:
  • The decomposition of copper(II) chloride
  • Electroplating, e.g. the electroplating of an iron spoon with silver/nickel
  • Refining of copper
  • The electrolysis of a concentrated solution of sodium chloride and its use in the chlor-alkali industry
  • The recovery of aluminium metal from bauxite (South Africa uses bauxite from Australia.)
• Describe risks to the environment of the following electrolytic processes used industrially:
  • The production of chlorine (the chemical reactions of the chloro-alkali industry)
  • The recovery of aluminium metal from bauxite