Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

The halogen which oxidizes water to \(\mathrm{O}_{2}\) exothermally is (a) \(\mathrm{I}_{2}\) (c) \(\mathrm{Cl}_{2}\) (b) \(\mathrm{Br}_{2}\) (d) \(\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

(d) \(\text{F}_2\)

Step 1: Understand the chemical reaction

To determine which halogen can oxidize water to form oxygen exothermally, we focus on the reaction of halogens with water: \( \text{X}_2 + \text{H}_2\text{O} \rightarrow \text{HOX} + \text{H}^+ + \text{O}_2 \). Not all halogens react with water to give \( \text{O}_2 \) and release energy.

Step 2: Consider the reactivity of halogens

Halogens have differing reactivity levels based on their position in the periodic table. Fluorine \((\text{F}_2)\), being higher up in Group 17, is the most reactive, while iodine \((\text{I}_2)\) is the least reactive. Reactivity is key in their ability to oxidize water.

Step 3: Assess thermodynamic favorability

Exothermic reactions release heat, indicating they are thermodynamically favorable. Among halogens, \(\text{F}_2\) can oxidize water to \(\text{O}_2\) and is highly exothermic due to fluorine's strong oxidizing potential.

Step 4: Identify the correct halogen

Based on the understanding of reactivity and the exothermic nature of fluorine's reaction with water, we conclude that \(\text{F}_2\) is the halogen that oxidizes water to \(\text{O}_2\) exothermally.

Key Concepts

Oxidation of Water

The oxidation of water refers to the process where water molecules react with another substance that causes the removal of electrons from the water itself. This reaction often leads to the production of oxygen gas \((\text{O}_2\)), which is a hallmark of this process. When halogens are involved, not all of them react with water in the same manner.

• Halogens are elements like Fluorine, Chlorine, Bromine, and Iodine, found in Group 17 of the periodic table.
• Their reactivity with water varies significantly from one halogen to another due to their inherent chemical properties.

In essence, for a halogen to oxidize water, it must be able to effectively remove electrons from water. This process leads to the generation of oxygen gas. Among the halogens, only specific ones have the capability to perform this oxidation to a significant extent that it becomes noticeable through energy changes.

Exothermic Reactions

Exothermic reactions are chemical processes that release energy, typically in the form of heat. These reactions are energetically favorable and often occur spontaneously under the right conditions.

• A good example of an exothermic reaction is when a halogen reacts with water, provided the reaction involves the right halogen.
• The release of heating energy indicates that the formation of products is more stable than the reactants.

In the context of halogens, when a substance reacts exothermically, it suggests that the change in energy between reactants and products results in energy being released to the surroundings. This is a critical indicator that the products formed possess lower energy and hence higher stability compared to the initial reactants.

Fluorine Reactivity

Fluorine \((\text{F}_2\)) is renowned for its incredibly high reactivity. It is the most reactive halogen due to several factors, including its electronegativity and position in the periodic table. Being at the top of Group 17, fluorine has a high tendency to attract electrons.

• Its reactivity means that fluorine can participate in reactions that other halogens cannot.
• Fluorine's ability to oxidize water exothermically is a significant example of its strong oxidizing power.

When fluorine reacts with water, it not only changes the water molecules into different products but does so by releasing considerable amounts of energy, demonstrating its characteristic reactivity and efficacy in forming stable bonds. This makes fluorine a unique and extremely potent element in chemical reactions compared to its group counterparts.