Question

When sodium chloride is electrolysed in Nelson's cell, hydrogen is liberated at cathode and not sodium, because: (a) \(\mathrm{H}^{+}\) does not react with water (b) \(\mathrm{H}^{+}\) has lower electrode potential (c) \(\mathrm{H}^{+}\) has greater mobility than \(\mathrm{Na}^{+}\) (d) \(\mathrm{H}^{+}\) is smaller than \(\mathrm{Na}^{+}\)

Short answer

(b) $H^+$ has lower electrode potential.

Step-by-step solution

Identify the Key Concepts

Electrolysis involves different ions migrating to the electrodes, where reduction occurs at the cathode. Examine which ions are viable candidates for reduction by comparing their electrode potentials.

Understand Electrode Potentials

The electrode potential of a species determines its likelihood to gain electrons (reduce) first. Generally, ions with lower (more negative) reduction potential get reduced at the cathode.

Compare 6$H^+$ and $Na^+$ Electrode Potentials

The standard reduction potential of $H^+$ to $H_2$ is 0 V, while $Na^+$ to Na is -2.71 V. This suggests $H^+$ is easier to reduce than $Na^+$ because it has a higher (less negative) reduction potential.

Evaluate the Correct Reasoning

Since $H^+$ is more readily reduced due to its higher electrode potential compared to $Na^+$, hydrogen gas is liberated. Therefore, the correct answer relates to electrode potentials.

Key concepts

Electrode Potential

The concept of electrode potential is key to understanding which ions will be reduced during electrolysis. It refers to the voltage or electromotive force associated with a particular ion's ability to gain electrons. This potential is often measured under standard conditions and is vital for predicting and comparing the reduction ability of different species.

When we say an ion has a higher (or less negative) reduction potential, it means that this ion is more likely to gain electrons and be reduced at the cathode compared to ions with lower (more negative) reduction potentials.

For example, when considering the reduction of hydrogen ions \(H^+\) and sodium ions \(Na^+\), \(H^+\) has a standard electrode potential of 0 V, while \(Na^+\) has a much lower potential of -2.71 V. This indicates that \(H^+\) is more easily reduced at the cathode, resulting in the liberation of hydrogen gas during the electrolysis of sodium chloride.

Reduction Reactions

Reduction reactions are half of the process in electrolysis, where ions gain electrons and become neutral or positively charged species. At the cathode, where reduction takes place, ions in the solution compete to gain electrons.

Understanding which ions get reduced involves examining their standard electrode potentials. During sodium chloride electrolysis, hydrogen ions are preferentially reduced over sodium ions because of their more favorable electrode potential of 0 V compared to sodium ions at -2.71 V.

This means \(H^+\) ions, which have a higher affinity to gain electrons, will undergo the reduction reaction: \[ 2H^+ + 2e^- \rightarrow H_2(g) \]

As this reaction illustrates, \(H^+\) ions gain electrons to form hydrogen gas, demonstrating the essence of reduction reactions in determining the outcome of the electrolysis process.

Hydrogen Liberation

Hydrogen liberation is a notable outcome of the electrolysis of solutions like sodium chloride, and it's instrumental in various industrial applications. It occurs when \(H^+\) ions gain electrons at the cathode, turning into diatomic hydrogen gas.

The advantages of hydrogen liberation include its utility as a clean fuel source and its role in various chemical processes. During sodium chloride electrolysis, the liberation process is dictated by the high reduction potential of \(H^+\) ions, making them preferentially reduced over ions like \(Na^+\).

This is expressed by the reaction \[ 2H^+ + 2e^- \rightarrow H_2(g) \] where hydrogen gas bubbles form and are collected as they rise from the cathode. The process is efficient and highlights the practical application of understanding electrode potentials in predicting and controlling chemical reactions.

Sodium Chloride Electrolysis

Sodium chloride electrolysis is a fundamental process in producing important chemicals like chlorine, sodium hydroxide, and hydrogen. Conducted in a specialized setup like Nelson's cell, it involves the passage of an electric current through an aqueous \(NaCl\) solution, causing it to dissociate into its constituent ions.

At the cathode, where reduction occurs, \(H^+\) ions are reduced to hydrogen gas due to their more favorable electrode potential compared to \(Na^+\). Meanwhile, at the anode, chloride ions \(Cl^-\) are oxidized to form chlorine gas.

• Cathode Reaction: \(2H^+ + 2e^- \rightarrow H_2(g)\)
• Anode Reaction: \(2Cl^- - 2e^- \rightarrow Cl_2(g)\)

These reactions illustrate the splitting of water and sodium chloride molecules, integral to producing the key substances. Understanding this process not only sheds light on industrial chemical production but also demonstrates the practical relevance of electrode potentials in influencing electrolysis outcomes.