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: Understanding Reactivity Series

To determine which reaction is not feasible, we need to know the reactivity of halogens (Group 17 elements: F, Cl, Br, I). In decreasing order of reactivity, they are: \( F_2 > Cl_2 > Br_2 > I_2 \). A more reactive halogen can displace a less reactive halogen from a compound.

Step 2: Evaluating Reaction (a)

In reaction (a) \(2 \text{KI} + \text{Br}_2 \rightarrow 2 \text{KBr} + \text{I}_2\), bromine (\( Br_2 \)), which is more reactive than iodine (\( I_2 \)), displaces iodine from potassium iodide to form potassium bromide. This reaction is feasible.

Step 3: Evaluating Reaction (b)

In reaction (b) \(2 \text{KBr} + \text{I}_2 \rightarrow 2 \text{KI} + \text{Br}_2\), iodine (\( I_2 \)), which is less reactive than bromine (\( Br_2 \)), cannot displace bromine from potassium bromide. Therefore, this reaction is not feasible.

Step 4: Evaluating Reaction (c)

In reaction (c) \(2 \text{KBr} + \text{Cl}_2 \rightarrow 2 \text{KCl} + \text{Br}_2\), chlorine (\( Cl_2 \)), which is more reactive than bromine (\( Br_2 \)), displaces bromine from potassium bromide. This reaction is feasible.

Step 5: Evaluating Reaction (d)

In reaction (d) \(2 \text{H}_2 \text{O} + 2 \text{F}_2 \rightarrow 4 \text{HF} + \text{O}_2\), fluorine (\( F_2 \)), which is the most reactive halogen, reacts with water to form hydrofluoric acid and oxygen. This is a feasible reaction.

Key Concepts

Reactivity Series

The reactivity series is like a ladder of elements showing how easily they react. For halogens, this series helps us understand which is more reactive and capable of displacing others in reactions. The halogens in order from most to least reactive are: fluorine ( F_2 ), chlorine ( Cl_2 ), bromine ( Br_2 ), and iodine ( I_2 ).

This order is crucial in determining whether a reaction involving a halogen displacement is feasible. A more reactive halogen can easily replace a less reactive halogen in a compound. Understanding this hierarchy allows us to predict and evaluate chemical reactions involving these elements.

Chemical Feasibility

Chemical feasibility refers to whether a chemical reaction can realistically occur under given conditions. For halogen displacement reactions, this involves checking if the more reactive halogen in the reactivity series will actually displace the less reactive one.

If the species meant to be displaced is more reactive than the displacing element, the reaction won't occur. For instance, in reaction (b) from the exercise, iodine is less reactive than bromine but is expected to displace it, which is why the reaction is not feasible.

• Feasibility helps rule out reactions that won't go forward under standard conditions.
• This understanding aids in predicting which chemical processes are likely to produce the desired outcomes.

Halogen Reactivity

Halogen reactivity is the ability of halogens to engage in chemical reactions. This property varies across the group, primarily due to the number of electrons available for bonding and their position in the periodic table.

As you move from fluorine to iodine in the reactivity series, the ability to displace other halogens decreases.

• Fluorine is so reactive that it can even react violently with water, as demonstraed in reaction (d).
• Chlorine, while less reactive than fluorine, can displace bromine and iodine.
• Bromine is less reactive than chlorine but more reactive than iodine, allowing it to displace iodine in reactions like (a).

Knowing these tendencies allows chemists to manipulate reactions to create desired compounds.

Displacement Reactions

Displacement reactions are chemical processes where one element replaces another in a compound. In the context of halogens, these reactions hinge on the relative reactivity of the halogens involved.

For a halogen displacement reaction to occur:

• The substituting halogen must be more reactive than the halogen being displaced.
• The reactivity series (a > c > b > d) provides a quick check to see if such a displacement will happen.

These reactions are not just theoretically significant; they have practical implications in industrial syntheses and laboratory analyses. For example, understanding why reaction (c) is feasible helps in predicting similar reactions involving chlorine and bromine. Displacement reactions form the basis of many techniques used in chemical production and research.