Electrolysis of Aqueous Salt Solutions Chemistry Tutorial

Key Concepts

• Dissolving a salt in water to produce an aqueous solution enables it to conduct electricity.
  

  non-conductor	→	conductor
  MX(s)	H2O(l) →	M+(aq) + X-(aq)

• The species present in the electrolyte are:

  (i) water (as the solvent)

  (ii) cations (from the salt)

  (iii) anions (from the salt)

• The use of inert electrodes, electrodes made of a material that will not take part in the reactions, means the only species present that can take part in the electrolytic cell reactions are the anions and cations of which the salt is composed and water.
• At the anode, either water is oxidized or the anions of the salt are oxidized.

  As a first approximation it is likely that if:

  (i) water is a stronger reductant than the anions, water will be oxidized at the anode.

  (ii) the anion is a stronger reductant than water, anions will be oxidized at the anode.

• At the cathode, either water is reduced or the cations of the salt are reduced.

  As a first approximation it is likely that if

  (i) water is a stronger oxidant than the cations, water will be reduced at the cathode.

  (ii) the cation is a stronger oxidant than water, cations will be reduced at the cathode.

• Electrolysis is a non-spontaneous reaction:

  E^o for the electrolytic cell is negative

• Applied voltage (emf) must be greater than the emf for the cell, ie greater than -E^o_{(electrolytic cell)}.
• Mass of substance produced electrolytically is proportional to the quantity of electricity flowing.

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Electrolytic Cell for Electrolysis of Aqueous Salt Solution

The possible electrolytic cell reactions are:

Determining which oxidation and reduction reactions occur based on E⁰ values:

At the anode:

• If E⁰_x > -1.23 V then X^- will be oxidized to X

  Example: Br^- → ½Br_2(l) + e^-     E⁰ = -1.08 V
  -1.08 V > -1.23 V so the Br^- anion will be oxidized at the anode NOT water.

• If E⁰_x < -1.23V then water will be oxidized to oxygen gas

  Example: F^- → ½F_2(g) + e^-     E⁰ = -2.89 V
  -2.89 V < -1.23 V so water will be oxidized at the anode NOT the F^- anion.

At the cathode:

• If E⁰_m > -0.83 V then M⁺ will be reduced to M

  Zn2++2e-→Zn	Fe2++2e-→Fe	Ni2++2e-→Ni	Sn2++2e-→Sn	Pb2++2e-→Pb	Cu2++2e-→Cu
  E0=-0.76V	E0=-0.44V	E0=-0.24V	E0=-0.14V	E0=-0.13V	E0=+0.34V
  For all these metal cations, E0 > -0.83V so the cation will be reduced at the cathode NOT water.

• If E⁰_m < -0.83V, water will be reduced to hydrogen gas

  K++e-→K	Ba2++2e-→Ba	Ca2++2e-→Ca	Na++e-→Na	Mg2++2e-→Mg	Al3++3e-→Al
  E0=-2.94V	E0=-2.91V	E0=-2.87V	E0=-2.71V	E0=-2.36V	E0=-1.68V
  For all these active metal cations, E0 < -0.83 V so water will be reduced at the cathode NOT the cation.

Example : Electrolysis of KI_(aq)

Consider the electrolytic cell shown below for the electrolysis of a standard aqueous solution of potassium iodide, KI_(aq):

A minimum of 1.37 V would need to be supplied by an external power supply in order for this electrolysis reaction to proceed.

This discussion assumes that all species are present in their standard states so that the electrode potentials are standard electrode potentials.