Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Which reaction is not feasible? (a) \(2 \mathrm{KI}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{KBr}+\mathrm{I}_{2}\) (b) \(2 \mathrm{KBr}+\mathrm{I}_{2} \longrightarrow 2 \mathrm{KI}+\mathrm{Br}_{2}\) (c) \(2 \mathrm{KBr}+\mathrm{Cl}_{2} \longrightarrow 2 \mathrm{KCl}+\mathrm{Br}_{2}\) (d) \(2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{~F}_{2} \longrightarrow 4 \mathrm{HF}+\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

Reaction (b) is not feasible.

Step 1: Understand the Chemical Reactivity

The feasibility of these reactions is determined by the reactivity of the halogens involved. Halogens are ranked by their tendency to gain electrons: F>Cl>Br>I. A halogen can displace any halide ion below it in this reactivity series.

Step 2: Analyze Reaction (a)

In reaction (a), bromine is reacting with potassium iodide to produce potassium bromide and iodine. Bromine can displace iodine because bromine is more reactive than iodine (Br>I). Thus, this reaction is feasible.

Step 3: Analyze Reaction (b)

In reaction (b), iodine reacts with potassium bromide to produce potassium iodide and bromine. Iodine cannot displace bromine because iodine is less reactive than bromine (Br>I). Thus, this reaction is not feasible.

Step 4: Analyze Reaction (c)

In reaction (c), chlorine reacts with potassium bromide to produce potassium chloride and bromine. Chlorine can displace bromine because chlorine is more reactive than bromine (Cl>Br). Thus, this reaction is feasible.

Step 5: Analyze Reaction (d)

In reaction (d), fluorine reacts with water to produce hydrogen fluoride and oxygen. Fluorine is incredibly reactive and can even displace oxygen from water, making this reaction feasible.

Key Concepts

Displacement Reactions

Displacement reactions, specifically single displacement reactions, involve one element being replaced by another in a compound. This type of reaction is prevalent in halogen chemistry due to the order of reactivity among halogens, which are powerful reactants. An example of a displacement reaction is when a more reactive element displaces a less reactive element in a compound, creating a new compound in the process.
Let's take a closer look at the halogens, which are elements found in Group 17 of the periodic table. When a halogen is more reactive, it has a greater tendency to gain electrons compared to another halogen. This difference in reactivity helps us predict the outcome of displacement reactions. If you understand the reactivity order of halogens, you can efficiently determine which halogen will displace another from a compound.
For displacement reactions involving halogens to occur, there should be a gradient in reactivity. This means the halogen attempting to displace another must be higher in the reactivity series. If the conditions are right, this will result in a successful displacement, as seen in the reaction where bromine displaces iodine from potassium iodide.

Chemical Reactivity Series

The chemical reactivity series is a practical ranking of elements based on their reactivity, showing the most reactive at the top and the least reactive at the bottom. In the context of halogens, this series is crucial in predicting and understanding the outcomes of displacement reactions.
Halogens show a reactivity series where a higher position indicates a stronger tendency to accept electrons and thus be more reactive. This property influences their behavior in chemical reactions, such as displacement reactions, where more reactive halogens will replace less reactive ones in compounds.
In the context of the given reactions, fluorine, being the most reactive halogen, can displace oxygen from water. Similarly, chlorine, which is ranked higher than bromine, can displace bromine from potassium bromide. Understanding the reactivity series is essential for predicting these kinds of chemical changes.

Halogen Reactivity Order

Halogen reactivity order is an array displaying halogens from the most reactive to the least. The order generally is given as fluorine (F) > chlorine (Cl) > bromine (Br) > iodine (I). This sequence is vital when evaluating the feasibility of halogen displacement reactions.
The reason fluorine is highly reactive lies in its small atomic size and high electronegativity, allowing it to attract electrons readily. On the other hand, iodine, being larger and less electronegative, is much less reactive. This difference is visible in displacement reactions, where a halogen can displace another halide ion only if it is more reactive.
For example, in the reactions provided: chlorine—a highly reactive halogen—successfully displaces bromine from a compound because of its higher position in the reactivity order. Conversely, iodine cannot displace bromide, demonstrating its lower reactivity level. By always referring to this reactivity order, we can predict which displacements are feasible in chemical reactions involving halogens.