Types Of Electrodes In Electrolysis


The discharge of ions during electrolysis can be influenced by the types of electrodes used. Some electrodes are inert (do not take part in electrolytic reaction) while other electrodes are reactive (which may influence the ionic discharge). Platinum or carbon electrodes are examples of inert electrodes.

One example of reactive electrodes changing the ionic discharge will be the electrolysis of Copper (II) sulphate using copper electrodes.

Recall: If the electrolysis of copper (II) sulphate is done using carbon electrodes, oxygen is released at the anode and copper is deposited at the cathode.

Case Study: Electrolysis Of Copper (II) Sulphate Using Copper Electrodes

Electrodes: Copper

Electrolyte: Copper (II) sulphate solution

$$CuSO_{4} (aq) \rightarrow Cu^{2+} (aq) + SO_{4}^{2-} (aq)$$

$$H_{2}O (l) \rightleftharpoons H^{+} (aq) + OH^{-} (aq)$$

Ions present in the electrolyte: $H^{+}$, $OH^{-}$, $Cu^{2+}$ and $SO_{4}^{2-}$

Reaction at CATHODE:

  • $H^{+}$ and $Cu^{2+}$ ions are attracted to the cathode.
  • $Cu^{2+}$ ions are preferentially discharged as $Cu^{2+}$ is lower in the electrochemical series than $H^{+}$
  • Each $Cu^{2+}$ ion gains two electrons from the cathode to form one copper atom.
  • Copper metal is deposited on the cathode, which resulting in the copper cathode to become larger.
  • $Cu^{2+} (aq) + 2e^{-} \rightarrow Cu (s)$

Reaction at ANODE:

  • $OH^{-}$ and $SO_{4}^{2-}$ are attracted to the anode.
  • However, neither $OH^{-}$ nor $SO_{4}^{2-}$ ions are discharged as the electrode is an active electrode.
  • The copper electrode loses electrons more readily than $OH^{-}$ and $SO_{4}^{2-}$ ions.
  • Each Cu atom loses two electrons to form $Cu^{2+}$ ion.
  • The copper anode will dissolve away to form $Cu^{2+}$ ions.
  • $Cu (s) \rightarrow Cu^{2+} (aq) + 2e^{-}$

Overall Reaction:

  • Concentration of copper (II) sulphate (electrolyte) remains unchanged. $\rightarrow$ Copper (II) sulphate solution remains blue in colour
  • Copper from the anode is transferred from the anode to the cathode.

Additional Use:

  • This method can be used to refine copper.
  • Impure copper is used as the anode while a piece of pure copper is used as the cathode.
  • Pure copper from the impure copper anode will dissolve into the electrolyte, and pure copper is then deposited onto the copper cathode.
  • Impurities are left on the anode, which will then fall off to collect below the anode.

 

Case Study: Electrolysis Of Silver Nitrate Using Silver Electrodes

Electrodes: Silver

Electrolyte: Silver nitrate solution

$$AgNO_{3} (aq) \rightarrow Ag^{+} (aq) + NO_{3}^{-} (aq)$$

$$H_{2}O (l) \rightarrow 2H^{+} (aq) + O^{2-} (aq)$$

Ions present in solution: $H^{+}$, $Ag^{+}$, $OH^{-}$ and $NO_{3}^{-}$

Reaction at CATHODE:

  • $H^{+}$ and $Ag^{+}$ ions are attracted to the cathode.
  • $Ag^{+}$ ions are preferentially discharged as $Ag^{+}$ is lower than $H^{+}$ in the electrochemical series.
  • Each $Ag^{+}$ ion gains one electron to form one silver atom.
  • Silver is deposited on the silver cathode.
  • $Ag^{+} (aq) + e^{-} \rightarrow Ag (s)$

Reaction at ANODE:

  • $OH^{-}$ and $NO_{3}^{-}$ ions are attracted to the anode.
  • Neither $OH^{-}$ nor $NO_{3}^{-}$ are discharged as the electrode is an active electrode.
  • Silver atom loses electrons more readily than $OH^{-}$ and $NO_{3}^{-}$ ions.
  • Each silver atom loses one electron to form one $Ag^{+}$ ion.
  • Silver anode dissolves away to form $Ag^{+}$ ion.
  • $Ag (s) \rightarrow Ag^{+} (aq) + e^{-}$

Overall Reaction:

  • Silver nitrate is not decomposed $\rightarrow$ concentration of silver nitrate solution remains unchanged.
  • Silver is transferred from the anode to the cathode.

 


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