Why does conc. H2SO4 give H2 gas and H2S2O8 on electrolysis while dil. H2SO4 give O2 and H2 gas? Is it because of the conc. of SO4 2- ions?

To understand the different outcomes of electrolysis when using concentrated versus dilute sulfuric acid (H2SO4), we need to delve into the behavior of the acid and the ions present in each solution. The concentration of sulfuric acid significantly influences the electrochemical reactions that occur during electrolysis.

Electrolysis of Dilute H2SO4

When you perform electrolysis on dilute sulfuric acid, the solution primarily contains water and a lower concentration of sulfate ions (SO4²⁻). The electrolysis process can be summarized as follows:

• At the Anode: Water molecules are oxidized to produce oxygen gas (O2) and protons (H⁺). This reaction can be represented as:
• 2 H2O → O2 + 4 H⁺ + 4 e⁻
• At the Cathode: The protons (H⁺) are reduced to form hydrogen gas (H2):
• 2 H⁺ + 2 e⁻ → H2

Thus, the overall reaction in dilute H2SO4 yields hydrogen gas and oxygen gas.

Electrolysis of Concentrated H2SO4

In contrast, concentrated sulfuric acid has a much higher concentration of sulfate ions and fewer water molecules. This changes the dynamics of the electrolysis process:

• At the Anode: Instead of water being oxidized, the sulfate ions can participate in the reaction. The oxidation of sulfate ions can lead to the formation of peroxydisulfate (H2S2O8), which is a more complex reaction:
• 2 SO4²⁻ → H2S2O8 + 2 e⁻
• At the Cathode: Similar to the dilute case, protons are still reduced to produce hydrogen gas:
• 2 H⁺ + 2 e⁻ → H2

Therefore, the electrolysis of concentrated H2SO4 results in the production of hydrogen gas and peroxydisulfate (H2S2O8).

Role of Ion Concentration

The concentration of sulfate ions indeed plays a crucial role in determining the products of electrolysis. In concentrated solutions, the abundance of sulfate ions allows for different oxidation reactions compared to dilute solutions, where water is the primary source of protons and oxygen. This shift in the source of ions leads to the formation of different products, such as H2S2O8 in concentrated sulfuric acid.

Summary of Key Differences

To summarize:

• In dilute H2SO4: The primary reactions involve water, leading to the production of O2 and H2.
• In concentrated H2SO4: The presence of a high concentration of sulfate ions allows for the formation of H2S2O8, alongside H2 gas.

Understanding these differences highlights the importance of concentration in electrochemical processes and how it can significantly alter the products formed during reactions. This principle is fundamental in both theoretical chemistry and practical applications in electrolysis.