Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will displace the halogen from the solution of the halide? (a) \(\mathrm{Cl}_{2}\) added to a KBr solution (b) \(\mathrm{Br}_{2}\) added to an \(\mathrm{NaCl}\) solution (c) \(\mathrm{Br}\), added to a KI solution (d) \(\mathrm{Cl}_{2}^{2}\) added to an \(\mathrm{NaF}\) solution

Step-by-Step Solution

Answer

The displacements occur in (a) Cl₂ in KBr and (c) Br₂ in KI.

Step 1: Understand Halogen Reactivity

Halogens are more reactive the higher they are in the group in the periodic table. In terms of displacement, a more reactive halogen can displace a less reactive halide from its compound. The reactivity trend is: \[ \text{Fluorine (F)} > \text{Chlorine (Cl)} > \text{Bromine (Br)} > \text{Iodine (I)} \] This means Cl can displace Br or I but not F. Similarly, Br can displace I but not Cl or F.

Step 2: Assess Each Reaction Possibility

Each option involves a potential displacement reaction of a halide by a halogen. Consider the reactivity differences:- (a) \( \text{Cl}_2 + \text{KBr} \rightarrow \text{KCl} + \text{Br}_2 \): Chlorine is more reactive than bromine, so Cl will displace Br.- (b) \( \text{Br}_2 + \text{NaCl} \rightarrow \text{No Reaction} \): Bromine is less reactive than chlorine, so no displacement occurs.- (c) \( \text{Br}_2 + \text{KI} \rightarrow \text{KBr} + \text{I}_2 \): Bromine is more reactive than iodine, so Br will displace I.- (d) \( \text{Cl}_2 + \text{NaF} \rightarrow \text{No Reaction} \): Chlorine is less reactive than fluorine, so no displacement occurs.

Step 3: Conclusion

The feasible displacement reactions are option (a) and option (c), where the added halogen is more reactive than the halide present in the compound.

Key Concepts

Halogen Reactivity

In the world of chemistry, halogens are elements belonging to Group 17 of the periodic table. These include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Understanding the reactivity of halogens is crucial because they tend to participate in numerous reactions, including displacement reactions. Reactivity among halogens is determined by their position in the periodic table. The rule is simple: the higher up in the group, the more reactive the halogen. Hence, the reactivity order is:
\[ \text{Fluorine (F)} > \text{Chlorine (Cl)} > \text{Bromine (Br)} > \text{Iodine (I)} \]
This implies that fluorine is the most reactive halogen, capable of displacing any other halide ion in a compound. Chlorine follows, able to displace bromine and iodine, but not fluorine. Similarly, bromine can displace iodine but is less reactive than chlorine and fluorine.

Displacement Reactions

Displacement reactions are fascinating chemical processes where a more reactive element displaces a less reactive element from its compound. When it comes to halogens, this means a more reactive halogen can replace a less reactive halide ion in a compound. Let's see a few examples:

• Chlorine gas added to potassium bromide will displace bromine, forming potassium chloride and bromine gas.
• Bromine, being less reactive than chlorine, will not displace chlorine from sodium chloride.

This principle of displacement helps predict the outcomes of reactions involving halogens, making it easier for chemists to understand and foresee reaction patterns.

Periodic Table Trends

The periodic table isn't just a roadmap of elements. It reflects trends and patterns that help predict how elements like halogens will behave in reactions. Halogen reactivity is just one of these trends. As mentioned, halogen reactivity decreases as you move down the group in the periodic table. This is due to:

• Increasing atomic size: Atoms grow larger as additional electron shells are added.
• Decreasing ionization energy: It becomes easier for the outer electrons to be removed.
• Decreasing electronegativity: The ability of an atom to attract electrons diminishes.

These changes impact how halogens interact with other elements and compounds, guiding chemists in predicting reaction feasibility.

Reactivity Series

The reactivity series is a tool used by chemists to determine how different elements will interact. For halogens, the series follows the order of reactivity from fluorine to iodine. This helps us understand which halogen can displace another from its halide. Remember the key aspect:

• A more reactive halogen can displace a less reactive halide from a solution.
• This concept is crucial in predicting outcomes of displacement reactions.

Consider practical examples such as mixing chlorine with bromide solutions, where chlorine, being more reactive, will successfully displace bromine, illustrating this principle beautifully.

Chemical Reactions in Aqueous Solutions

Reactions involving halogens, particularly in aqueous solutions, are an exciting area of study. These reactions follow certain predictable patterns:

• Halogens dissolve in water to react with halide solutions based on their reactivity.
• Displacement occurs when halogens in solutions react with halide ions to form new compounds.

For instance, if bromine is added to a potassium iodide aqueous solution, bromine will displace iodine because of its higher reactivity. Recognizing these patterns is invaluable for understanding the behavior of halogens in real-world reactions, further illuminating the intricacies of chemistry and aiding in the prediction of reaction outcomes.