Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

In electrolysis of dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\), what is liberated at anode? (a) \(\mathrm{H}_{2}\) (b) \(\mathrm{SO}_{4}^{2-}\) (c) \(\mathrm{SO}_{2}\) (d) \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

Oxygen ( O_2 ) is liberated at the anode in the electrolysis of dilute H_2 SO_4 .

Step 1: Understanding Anode Reaction

In electrolysis, the anode is the positive electrode where oxidation occurs. For the electrolysis of dilute sulfuric acid ( H_2 SO_4 ), the solution primarily contains water and H^+ and SO_4^{2-} ions.

Step 2: Possible Reactions at Anode

At the anode, only two main types of reactions can occur: oxidation of water to form oxygen gas ( O_2 ) or oxidation of sulfate ions ( SO_4^{2-} ). Oxidation of water to O_2 is more feasible under electrolysis conditions.

Step 3: Identifying the Anode Product

The standard electrode potential for water oxidation to O_2 is lower compared to sulfate ions, making it more likely. Therefore, in the electrolysis of dilute H_2 SO_4 , oxygen ( O_2 ) is liberated at the anode.

Key Concepts

Anode Reaction

In the electrolysis of dilute sulfuric acid ( H_2SO_4 ), understanding what happens at the anode is essential. The anode is the positive electrode where oxidation reactions occur. During the process, ions present in the solution are attracted to the anode to undergo oxidation.

In our exercise, the dilute sulfuric acid solution mainly consists of water, hydrogen ions ( H^+ ), and sulfate ions ( SO_4^{2-} ). However, it's important to note that not every ion undergoes oxidation at the anode. Only specific reactions are energetically favorable.

• Oxidation of water: This reaction results in the formation of oxygen gas ( O_2 ) and hydrogen ions ( H^+ ).
• Oxidation of sulfate ions: Less favored in this dilution, rarely taking place.

The dominant process in this scenario, due to the conditions specific to dilute solutions, is water oxidation producing oxygen.

Oxidation Process

Oxidation is a fundamental chemical process where an atom or molecule loses electrons. In electrolysis, oxidation happens at the anode, where ions in solution give up electrons.

Let's delve into the oxidation processes that occur in the context of dilute sulfuric acid electrolysis:

• Water oxidation: The water molecules lose electrons to form oxygen gas (O_2), H^+ ions, and electrons. This can be represented by the equation:
  
  \[ 2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^- \]
• Sulfate ion oxidation: Although theoretically possible, this reaction requires higher energy and is usually not observed in dilute solutions.

The essence of the oxidation process lies in the preference for reactions that require less energy, making water oxidation the primary reaction.

Standard Electrode Potential

Standard electrode potential ( E^0 ) is a valuable tool in predicting the likelihood of a redox reaction. It measures the tendency of a chemical species to be reduced (gain electrons) or oxidized (lose electrons).

Within the electrolysis of dilute sulfuric acid, the consideration of standard electrode potentials helps determine which reaction occurs at the anode more favorably. For example, the standard electrode potential for the oxidation of water to oxygen is more favorable (less positive) compared to the oxidation of sulfate ions.

• Water to O_2 : E^0 = +1.23 ext{ V}
• Sulfate ions: More positive and less likely

With lower energy barriers, the oxidation of water is more spontaneous under standard conditions in the electrolysis of H_2SO_4 . This is why oxygen ( O_2 ) is the primary product liberated at the anode.