Question:
In electrolysis of NaCl when Pt electrode is taken then $H_2$ is liberated at cathode while with Hg cathode it forms sodium amalgam :-
In electrolysis of NaCl when Pt electrode is taken then $H_2$ is liberated at cathode while with Hg cathode it forms sodium amalgam :-
Updated On: May 5, 2024
- Hg is more inert than Pt
- More voltage is required to reduce $H^+$at Hg than at Pt
- Na is dissolved in Hg while it does not dissolve in Pt
- Conc, of $H^+$ ions is larger when Pt electrode is taken
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The Correct Option is B
Solution and Explanation
Sodium chloride in water dissociates as
$NaCl \rightleftharpoons Na^++Cl^-$
$H_2O \rightleftharpoons H^++OH^- $
When electric current is passed through this solution using platinum electrodes, $Na^+ \, and \, H^+ $ move towards cathode whereas $Cl^- \, and \, OH^- $ ions move towards anode.
At cathode
$H^++e^- \rightarrow H $
$H+H \rightarrow H_2 $
At anode
$Cl^- \rightarrow Cl+e^- $
$Cl+Cl \rightarrow Cl_2 $
If mercury is used as cathode, $H^+ $ ions are not discharged at mercury cathode because mercury has a high hydrogen over voltage. $Na^+ $ ions are discharged at cathode in preference of $H^+ $ ions yielding sodium, which dissolves in mercury to form sodium amalgam.
$NaCl \rightleftharpoons Na^++Cl^-$
$H_2O \rightleftharpoons H^++OH^- $
When electric current is passed through this solution using platinum electrodes, $Na^+ \, and \, H^+ $ move towards cathode whereas $Cl^- \, and \, OH^- $ ions move towards anode.
At cathode
$H^++e^- \rightarrow H $
$H+H \rightarrow H_2 $
At anode
$Cl^- \rightarrow Cl+e^- $
$Cl+Cl \rightarrow Cl_2 $
If mercury is used as cathode, $H^+ $ ions are not discharged at mercury cathode because mercury has a high hydrogen over voltage. $Na^+ $ ions are discharged at cathode in preference of $H^+ $ ions yielding sodium, which dissolves in mercury to form sodium amalgam.
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Concepts Used:
Electrochemical Cells
An electrochemical cell is a device that is used to create electrical energy through the chemical reactions which are involved in it. The electrical energy supplied to electrochemical cells is used to smooth the chemical reactions. In the electrochemical cell, the involved devices have the ability to convert the chemical energy to electrical energy or vice-versa.
Classification of Electrochemical Cell:
Cathode
- Denoted by a positive sign since electrons are consumed here
- A reduction reaction occurs in the cathode of an electrochemical cell
- Electrons move into the cathode
Anode
- Denoted by a negative sign since electrons are liberated here
- An oxidation reaction occurs here
- Electrons move out of the anode
Types of Electrochemical Cells:
Galvanic cells (also known as Voltaic cells)
- Chemical energy is transformed into electrical energy.
- The redox reactions are spontaneous in nature.
- The anode is negatively charged and the cathode is positively charged.
- The electrons originate from the species that undergo oxidation.
Electrolytic cells
- Electrical energy is transformed into chemical energy.
- The redox reactions are non-spontaneous.
- These cells are positively charged anode and negatively charged cathode.
- Electrons originate from an external source.