Problem 31 - Chemistry in 30 days

Question

From the following list of ions, select the one which is discharged in preference to the others during electrolysis? (a) \(\mathrm{SO}_{4}^{2-}\) b) \(\mathrm{NO}_{3}^{-}\) c) \(\mathrm{OH}\) d) \(\mathrm{Cl}\)

Step-by-Step Solution

Answer

The \(\text{OH}^-\) ion is discharged in preference to the others.

Step 1: Understand Electrolysis

Electrolysis is a process where electrical energy is used to drive a chemical reaction. During electrolysis, ions from an electrolyte solution are discharged at the electrodes.

Step 2: Recall Discharge Potential

The ease with which ions are discharged during electrolysis is based on their discharge potential. Ions with lower discharge potential are discharged in preference to those with higher values.

Step 3: Compare Discharge Potential of Anions

The discharge potential of common anions can be ranked as follows: \(\text{OH}^-\) has a lower discharge potential compared to \(\text{Cl}^-\), \(\text{SO}_4^{2-}\), and \(\text{NO}_3^-\). Thus, \(\text{OH}^-\) is discharged more easily than the others.

Step 4: Select Preferred Ion

According to the discharge potential, \(\text{OH}^-\) will be discharged in preference to \(\text{Cl}^-\), \(\text{SO}_4^{2-}\), and \(\text{NO}_3^-\) during electrolysis.

Key Concepts

Discharge potential

In the process of electrolysis, not all ions are equally likely to be discharged at the electrodes. Whether an ion is discharged or not depends on its discharge potential. Discharge potential refers to the minimum energy required for an ion to be discharged at the electrode. Think of it as a competition between ions; the one with the lowest discharge potential will be discharged first. For instance, in our exercise, the hydroxide ion (\(\text{OH}^-\)) has a lower discharge potential compared to chloride \(\text{Cl}^-\), sulfate \(\text{SO}_4^{2-}\), and nitrate \(\text{NO}_3^-\) ions.
This means that when a solution containing all these ions goes through electrolysis, the \(\text{OH}^-\) ion is the one that will be discharged at the electrode because it requires the least amount of energy to do so.
Understanding discharge potential helps in predicting and controlling the outcome of electrolysis processes.

Electrochemical series

The electrochemical series is like a ranking chart for ions based on their discharge potentials or electrode potentials. It's a list that organizes ions in order of their tendencies to be discharged during electrolysis.
This series helps determine which ions will be released during the process.

• More negative values in the series suggest a higher likelihood of being released as compared to those with more positive values.
• For anions (negatively charged ions), the lower the position in the series, the easier it is for them to be discharged.

In our example, \(\text{OH}^-\) ranks lower than \(\text{Cl}^-\), \(\text{SO}_4^{2-}\), and \(\text{NO}_3^-\), making it more likely to discharge during electrolysis.
The electrochemical series is a crucial tool in electrochemistry as it provides insight into the behavior of ions during chemical reactions and helps in the design of various electrochemical cells.

Ion discharge in electrolysis

Ion discharge in electrolysis involves the movement of ions towards oppositely charged electrodes where they lose or gain electrons and eventually form neutral elements or compounds.
Here is how ions typically get discharged:

• At the anode (positive electrode), anions like \(\text{OH}^-\) migrate to release electrons and form neutral molecules.
• At the cathode (negative electrode), cations receive electrons to become neutral atoms.

In the given exercise, the \(\text{OH}^-\) ion moves towards the anode where it releases electrons, turning into water and oxygen gas in the process. This consumption of ions alters the chemical nature of the solution and generates its observable changes.
Understanding ion discharge is fundamental to applications like metal plating, chemical synthesis, and refining metals.