Problem 26
Question
A solution of sodium sulphate in water is electrolysed using inert electrodes. The products at the cathode and anode are respectively [1987 - 1 Mark] (a) \(\mathrm{H}_{2}, \mathrm{O}_{2}\) (b) \(\mathrm{O}_{2}, \mathrm{H}_{2}\) (c) \(\mathrm{O}_{2}, \mathrm{Na}\) (d) \(\mathrm{O}_{2}, \mathrm{SO}_{2}\)
Step-by-Step Solution
Verified Answer
The products are
\(
\text{H}_2
\)
at the cathode and
\(
\text{O}_2
\)
at the anode. (a) is correct.
1Step 1: Understand the Electrolysis Process
In the electrolysis of a sodium sulfate solution, water undergoes electrolysis because sodium sulfate is a neutral salt in solution and does not get discharged at the electrodes.
2Step 2: Identify Reactions at the Cathode
At the cathode, water (\(2\,\text{H}_2\text{O}\)) is reduced to hydrogen gas and hydroxide ions:\(2\,\text{H}_2\text{O} + 2\,\text{e}^- \rightarrow \text{H}_2\,(g) + 2\,\text{OH}^-\). So, \(\text{H}_2\) gas is generated at the cathode.
3Step 3: Identify Reactions at the Anode
At the anode, water is oxidized to oxygen gas and hydrogen ions:\(2\,\text{H}_2\text{O} \rightarrow \text{O}_2\,(g) + 4\,\text{H}^+ + 4\,\text{e}^-\). Hence, \(\text{O}_2\) gas is produced at the anode.
4Step 4: Conclusion of Electrolysis Reactions
In the electrolysis of sodium sulfate solution, \(\text{H}_2\) is produced at the cathode and \(\text{O}_2\) is produced at the anode. Therefore, the correct answer to the options provided is option (a): \(\text{H}_2, \text{O}_2\).
Key Concepts
Sodium SulphateInert ElectrodesCathode and Anode Reactions
Sodium Sulphate
Sodium sulphate is a white crystalline salt that is highly soluble in water. In the context of electrolysis, sodium sulphate acts as an electrolyte, meaning it helps conduct electricity through a solution without undergoing significant chemical change itself. It dissociates completely in water to form sodium ions \( \text{Na}^+ \) and sulfate ions \( \text{SO}_4^{2-} \), but these do not discharge at the electrodes.
During electrolysis, it is important to note that sodium sulphate remains largely unchanged, because the cations and anions that originate from water, rather than the salt itself, participate in the electrode reactions. This property is due to what is called the 'discharge potential,' where water is more readily reduced and oxidized compared to the sodium and sulfate ions.
In the electrolysis of sodium sulphate, the focus is primarily on the water present, which splits into hydrogen and oxygen gases at the electrodes. Thus, sodium sulphate serves more to facilitate the movement of ions and sustain the flow of current throughout the solution.
During electrolysis, it is important to note that sodium sulphate remains largely unchanged, because the cations and anions that originate from water, rather than the salt itself, participate in the electrode reactions. This property is due to what is called the 'discharge potential,' where water is more readily reduced and oxidized compared to the sodium and sulfate ions.
In the electrolysis of sodium sulphate, the focus is primarily on the water present, which splits into hydrogen and oxygen gases at the electrodes. Thus, sodium sulphate serves more to facilitate the movement of ions and sustain the flow of current throughout the solution.
Inert Electrodes
In the context of electrolysis, inert electrodes are electrodes that do not participate in the chemical reactions during the process. Common materials for inert electrodes include platinum and graphite. These electrodes are largely unreactive, simply serving to conduct electricity into and out of the solution. Their primary purpose is to provide a surface for the electrolysis reactions to occur.
Using inert electrodes avoids complications that might arise if the electrodes themselves were reactive and took part in the electrochemical reactions. This simplicity is crucial in experiments where you need a clear understanding of the behavior of the electrolytic solution alone, without interference from the electrode material.
By using inert electrodes, the products formed during electrolysis are solely derived from the solution—sodium sulphate in this case—ensuring accurate identification of the reactions taking place with water molecules.
Using inert electrodes avoids complications that might arise if the electrodes themselves were reactive and took part in the electrochemical reactions. This simplicity is crucial in experiments where you need a clear understanding of the behavior of the electrolytic solution alone, without interference from the electrode material.
By using inert electrodes, the products formed during electrolysis are solely derived from the solution—sodium sulphate in this case—ensuring accurate identification of the reactions taking place with water molecules.
Cathode and Anode Reactions
The electrolysis of an aqueous sodium sulphate solution involves distinct reactions at the cathode and the anode. At these electrodes, water molecules undergo reduction and oxidation, respectively, resulting in the production of gases.
**Cathode reaction:** At the cathode, reduction takes place. The water molecules gain electrons to form hydrogen gas and hydroxide ions. The balanced reaction is given by:
\[ 2\,\text{H}_2\text{O} + 2\,\text{e}^- \rightarrow \text{H}_2\,(g) + 2\,\text{OH}^- \]
This means hydrogen gas bubbles off at the cathode.
**Anode reaction:** At the anode, oxidation occurs. Water molecules lose electrons to form oxygen gas and hydrogen ions. The reaction is represented by:
\[ 2\,\text{H}_2\text{O} \rightarrow \text{O}_2\,(g) + 4\,\text{H}^+ + 4\,\text{e}^- \]
Thus, oxygen gas bubbles off at the anode.
These reactions highlight how water covers the entire electrolysis process, demonstrating how hydrogen is released at the cathode, and oxygen at the anode, rather than the sodium and sulfate ions directly reacting.
**Cathode reaction:** At the cathode, reduction takes place. The water molecules gain electrons to form hydrogen gas and hydroxide ions. The balanced reaction is given by:
\[ 2\,\text{H}_2\text{O} + 2\,\text{e}^- \rightarrow \text{H}_2\,(g) + 2\,\text{OH}^- \]
This means hydrogen gas bubbles off at the cathode.
**Anode reaction:** At the anode, oxidation occurs. Water molecules lose electrons to form oxygen gas and hydrogen ions. The reaction is represented by:
\[ 2\,\text{H}_2\text{O} \rightarrow \text{O}_2\,(g) + 4\,\text{H}^+ + 4\,\text{e}^- \]
Thus, oxygen gas bubbles off at the anode.
These reactions highlight how water covers the entire electrolysis process, demonstrating how hydrogen is released at the cathode, and oxygen at the anode, rather than the sodium and sulfate ions directly reacting.
Other exercises in this chapter
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