Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A solution of KBr is treated with each of the following. Which one would liberate bromine [1980] (a) \(\mathrm{Cl}_{2}\) (b) HI (c) \(\mathrm{I}_{2}\) (d) \(\mathrm{SO}_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Cl}_{2}\) liberates bromine from a KBr solution.

Step 1: Understand the Question

We need to determine which of the given reagents would liberate bromine from a solution of potassium bromide (KBr). This involves understanding the reaction that would occur and identifying if bromine is produced as a result.

Step 2: Review the Reactivity of Halogens

Bromine can be displaced from its compounds by more reactive halogens. Chlorine, being more reactive than bromine, can substitute bromine in a compound. This is possible due to its position above bromine in the halogen group of the periodic table.

Step 3: Consider the Reaction with Chlorine

When chlorine is added to a solution of KBr, there is a reaction: \[ \text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2 \]. Here, chlorine displaces bromine, liberating bromine gas \(\text{Br}_2\).

Step 4: Analyze Other Options

- HI (hydroiodic acid) cannot liberate bromine as iodine is less reactive than bromine. - \(\text{I}_2\) (iodine) is also less reactive and cannot displace bromine. - \(\text{SO}_2\) (sulfur dioxide) does not have a reaction pathway that liberates bromine from KBr.

Step 5: Conclusion of the Solution

The only reagent that can effectively liberate bromine from a KBr solution by displacing it is chlorine (\(\text{Cl}_2\)). Therefore, the answer is \(\text{Cl}_2\).

Key Concepts

Bromine Liberation

In chemical reactions, bromine liberation refers to the process where bromine is released from its compound form. For example, potassium bromide (KBr) can be treated with other chemicals to free bromine molecules. This is specifically important in understanding its role in reactions involving halogens. When chlorine gas (Cl_2) is introduced to a KBr solution, a displacement reaction occurs:

• Chlorine, being more reactive, takes bromine's place in the compound.
• This results in the formation of potassium chloride (KCl) and free bromine (Br_2).

The equation representing this reaction is:\[ ext{Cl}_2 + 2 ext{KBr} \rightarrow 2 ext{KCl} + ext{Br}_2\]Here, bromine is liberated as bromine gas from its salt form. This ability to displace bromine highlights the dynamic nature of halogen reactions.

Displacement Reactions

Displacement reactions are a type of chemical reaction where an element replaces another in a compound. Within the context of halogen chemistry, these are particularly interesting.
Halogens, by nature, have varying reactivities, which determine their ability to displace one another in compound reactions. For instance, given the scenario of a KBr solution:

• Chlorine replaces bromine due to its higher reactivity.
• This results in the formation of KCl and Br_2.

Displacement reactions showcase the competitive nature of elements trying to occupy compound spaces. Here’s the reaction again for clarity:\[ ext{Cl}_2 + 2 ext{KBr} \rightarrow 2 ext{KCl} + ext{Br}_2\]Such reactions help students visualize chemical processes and understand the underlying principles of chemical reactivity.

Halogen Group Reactivity

The halogen group, consisting of elements like fluorine, chlorine, bromine, and iodine, is known for its unique reactivity characteristics. Reactivity in halogens decreases as one moves down the group in the periodic table. This sequence of reactivity is essential for predicting displacement reactions.

• Chlorine ( Cl_2 ) is more reactive than bromine ( Br_2 ) and iodine ( I_2 ).
• This allows chlorine to displace bromine from solutions like KBr, but iodine cannot do the same.
• Halogen reactivity is often utilized in synthesizing new compounds or extracting elements from their compounds.

Understanding this concept allows students to predict outcomes of chemical processes involving halogens and to grasp how elemental positions dictate reactivity trends.