2.4 Faraday's Laws of Electrolysis

Faraday's laws of electrolysis are fundamental principles in electrochemistry. They describe how the mass of substances changed during electrolysis relates to the amount of electricity used and the substance's properties.

These laws help us understand and predict the outcomes of electrolytic processes. They're crucial for calculating the amount of product formed, determining the efficiency of electrolytic cells, and considering factors that affect the overall process.

Faraday's Laws of Electrolysis

Faraday's first law of electrolysis

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• States the mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode
• Quantity of electricity refers to the amount of electrical charge, typically measured in coulombs (C)
• Doubling the charge will double the mass of the substance altered
• Mathematical representation: $m = k \times Q$
  • $m$ is the mass of the substance altered at the electrode (grams)
  • $k$ is the electrochemical equivalent of the substance (grams per coulomb)
  • $Q$ is the total electric charge passed through the substance (coulombs)

Applications of Faraday's second law

• States the mass of a substance altered at an electrode during electrolysis is directly proportional to its equivalent weight
• Equivalent weight is the molar mass divided by the number of electrons transferred per formula unit
• Substances with different equivalent weights will have different masses altered when the same quantity of electricity is passed
• Mathematical representation: $m = \frac{Q \times M}{F \times z}$
  • $m$ is the mass of the substance altered at the electrode (grams)
  • $Q$ is the total electric charge passed through the substance (coulombs)
  • $M$ is the molar mass of the substance (grams per mole)
  • $F$ is Faraday's constant, approximately 96,485 coulombs per mole of electrons
  • $z$ is the number of electrons transferred per formula unit

Charge, current, and time in electrolysis

• Relationship between charge, current, and time: $Q = I \times t$
  • $Q$ is the total electric charge (coulombs)
  • $I$ is the current (amperes)
  • $t$ is the time (seconds)
• To calculate the mass of a substance altered during electrolysis, combine the equation for charge with Faraday's second law: $m = \frac{I \times t \times M}{F \times z}$
• Ensure units are consistent and make necessary conversions
  • Convert time to seconds before using in the equation

Efficiency factors in electrolytic cells

• Current efficiency is the ratio of the actual yield to the theoretical yield based on the quantity of electricity passed
  • Factors reducing current efficiency:
    • Competing side reactions consuming current without producing the desired product
    • Product loss due to dissolution, evaporation, or other means
  • Lower current efficiency results in a smaller quantity of the desired product formed compared to the theoretical yield
• Overpotential is the additional potential required beyond the thermodynamic potential to drive an electrolytic reaction at a desired rate
  • Factors contributing to overpotential:
    • Activation overpotential, the energy needed to overcome the activation energy barrier
    • Concentration overpotential, arising from concentration differences between the electrode surface and the bulk solution
  • Higher overpotentials can lead to increased energy consumption and may promote unwanted side reactions, reducing efficiency
• Mass transport limitations can affect efficiency by restricting the supply of reactants to the electrode surface
  • Factors influencing mass transport:
    • Diffusion of reactants from the bulk solution to the electrode surface
    • Convection induced by stirring or flow of the electrolyte
    • Migration of charged species under the influence of the electric field
  • Insufficient mass transport can lead to concentration polarization, where the reactant concentration at the electrode surface becomes depleted, limiting the reaction rate and efficiency