Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{H}_{2} \mathrm{O}_{2}\) exists as .... in alkaline medium. (a) \(\mathrm{HO}_{2}+\) (b) \(\mathrm{H}_{2} \mathrm{O}+\) (c) \(\mathrm{HO}_{2}-\) (d) \(\mathrm{O}_{2}^{2}-\)

Step-by-Step Solution

Answer

(c) HO2−

Step 1: Ionization in Alkaline Medium

In an alkaline medium, water acts as a source of hydroxide ions (OH−). These ions have the potential to interact with hydrogen peroxide (H2O2), altering its structure.

Step 2: Consider the Reaction of H2O2

In an alkaline solution, H2O2 can react to form ions. The basic (alkaline) conditions encourage deprotonation. The likely product of H2O2 in basic conditions is the conjugate base formed by losing a proton.

Step 3: Formation of the Peroxide Ion

When H2O2 deprotonates in the presence of a hydroxide ion under alkaline conditions, it forms the peroxide ion (HO2−). The reaction can be represented as follows:\[ ext{H}_2 ext{O}_2 ightarrow ext{HO}_2^- + ext{H}^+\]

Step 4: Determine the Correct Option

Based on the reaction that occurs in an alkaline medium, the resulting ion from H2O2 is the hydroperoxide ion, which is symbolized as HO2−. Therefore, the correct answer among the options given is (c) \( ext{HO}_2^- \).

Key Concepts

Alkaline Medium

An alkaline medium is an essential concept in chemistry, particularly when discussing reactions involving hydrogen peroxide. This medium is defined by its basic nature, which means it has a pH greater than 7.
In an alkaline medium, the concentration of hydroxide ions (\( ext{OH}^- \)) is relatively high.
These ions can significantly influence chemical reactions by facilitating processes like deprotonation, where a hydrogen atom is removed from a molecule.

• An alkaline environment is created by adding a base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH).
• This environment makes the solution conducive for certain reactions, especially those involving acids or molecules like peroxide that can donate protons.

When hydrogen peroxide (\( ext{H}_2 ext{O}_2 \)) is present in an alkaline medium, it is more likely to react with the hydroxide ions present in the solution. This interaction often results in the formation of other ions, as alkaline conditions facilitate the breaking and forming of bonds.

Peroxide Ion

The peroxide ion is a fascinating component derived from hydrogen peroxide, characterized in an alkaline medium. When hydrogen peroxide (\( ext{H}_2 ext{O}_2 \)) is in such a medium, it has the potential to transform into its ionized forms:
either as the complete version (\( ext{O}_2^{2-} \)) or the hydroperoxide version (\( ext{HO}_2^- \)).

• The hydroperoxide ion (\( ext{HO}_2^- \)) forms when hydrogen peroxide loses a proton (\( ext{H}^+ \)).
• Meanwhile, the peroxide ion (\( ext{O}_2^{2-} \)) is another form, although in many alkaline conditions, the hydroperoxide ion is more common.

The creation of these ions is primarily due to the action of hydroxide ions that are abundant in alkaline environments. These ions break the bond within hydrogen peroxide, leading to the formation of the peroxide ion or hydroperoxide ion. Such transformations are critical in understanding the behavior of hydrogen peroxide in various chemical settings.

Conjugate Base

Every acid, when it loses a proton, transforms into its conjugate base. This concept is fundamental in understanding acid-base reactions, particularly in alkaline mediums.
For hydrogen peroxide (\( ext{H}_2 ext{O}_2 \)), when it loses a proton in an alkaline solution, it forms its conjugate base, which is the hydroperoxide ion (\( ext{HO}_2^- \)).

• The hydroperoxide ion is considered the conjugate base of hydrogen peroxide after losing a proton to the hydroxide ion present in the alkaline medium.
• This process showcases the dynamic equilibrium that can exist in solutions, balancing ion formation and decomposition.

Understanding conjugate bases helps predict the behavior of substances in various chemical environments, such as the alkaline medium here, providing insights into reactivity and stability. These principles are widely applicable across chemistry, aiding in the study of reactions and the design of chemical processes.