Learning Objective of the post
After going through the post, you would enhance the core concepts about
- Preferential Electrolysis
Electrolysis, a chemical decomposition produced by passing an electric current through a solution containing ions (referred to as electrolytic solution) or a molten (liquid) state.
In electrolysis, metal electrodes are connected to terminals of the battery, dipped in an electrolytic solution.
The positive terminal of the battery (cathode) when connected to the electrode attracts anions (negatively charged ions), so the electrode is termed an anode. Similarly, the negative terminal of the battery (anode) when connected to the electrode attracts cations (positively charged ions), as termed as Cathode.
Types of cells:
There are two types of cells
- Electrochemical cells
- Galvanic cells
The cells tend to convert electrical energy into chemical energy.
The cells tend to convert chemical energy into electrical energy.
In both types of cells oxidation-reduction (redox) reactions take place. In electrochemical cells a non-spontaneous reaction (ΔG > 0) is made spontaneous (ΔG < 0) by passing electric current.
No reaction takes place on melting sodium chloride (NaCl), the free energy change for the reaction is greater than zero (ΔG > 0). But by applying electrical energy to molten NaCl, Na+, and Cl– ions start moving towards oppositely charged electrodes. The free energy change for the reaction is less than zero (ΔG < 0) and the reaction becomes spontaneous.
When the Zinc rod is placed in the Copper sulfate solution, the blue color of the copper sulfate solution disappears. The reaction is spontaneous (ΔG < 0) as zinc is more reactive than copper. The reaction may be written as
Preferential electrolysis is about the preferred product obtained as a result of the electrolysis process. The products of electrolysis are based on the discharge potential or electrode potentials of various ions present in the solution.
According to the latest sign conventions, the electrode where reduction takes place in comparison to the hydrogen electrode has reduction potential with a positive sign. Whereas the electrode where oxidation takes place in comparison to the hydrogen electrode has a reduction potential with a negative sign.
Standard reduction potentials (Electrochemical series)
Preferential electrolysis products
When Sodium chloride is electrolyzed in an aqueous state and molten states the products obtained in both cases are different.
Let’s try to understand the concepts.
In the case of aqueous sodium chloride, the dissociation reactions include dissociation of salt and self-ionization of water. The reactions are
The likely reactions that can occur at the cathode can be a reduction of sodium ions and a reduction of the water molecule. The reactions are
The reduction is preferred for ions having a greater reduction potential. Water has a greater reduction potential (-0.083 V). The reduction of water occurs and hydrogen gas is one of the products of electrolysis of aqueous sodium chloride.
The likely reactions that can occur at the anode can be oxidation of chloride ions and oxidation of the water molecule. The reactions are
The above reactions are expressed with oxidation potentials. By reversing the signs, they can be converted into reduction potentials. Thus Lower the value of standard reduction potential higher is the chances to get oxidized. Thus oxidation of water should preferably occur. But the evolution of oxygen gas is kinetically slow. This is referred to as overvoltage.
Overvoltage is the excess potential required for the discharge of an ion at an electrode over and above the equilibrium potential of the electrode.
Both Chloride ions (-1.36 V) and a water molecule (-1.23 V) to get oxidized require potentials with very less difference but due to overvoltage, the oxidation of chloride ions is preferred.
Thus, the preferred product of oxidation reaction is chlorine gas.
So, when aqueous sodium chloride is electrolyzed the products obtained are hydrogen gas and chlorine gas.
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