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) \(\mathrm{Br}_{2}\) added to a KI solution (d) \(\mathrm{Cl}_{2}\) added to an \(\mathrm{NaF}\) solution

Step-by-Step Solution

Answer

Options (a) and (c) will cause displacement: chlorine displaces bromine, and bromine displaces iodine.

Step 1: Understand Displacement Reactions

In a halogen displacement reaction, a more reactive halogen can displace a less reactive halide ion in a solution. The reactivity of halogens decreases as we go down the group in the periodic table; thus, the order of reactivity is: \( \text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2 \).

Step 2: Analyzing Reaction (a)

For option (a), chlorine \( (\text{Cl}_2) \) is added to potassium bromide \( (\text{KBr}) \) solution. Chlorine is more reactive than bromine, so \( \text{Cl}_2 \) will displace bromide ions \( (\text{Br}^-) \) to form \( \text{KCl} \) and liberate bromine. The reaction is: \[ \text{Cl}_2 + 2 \text{KBr} \rightarrow 2 \text{KCl} + \text{Br}_2 \].

Step 3: Analyzing Reaction (b)

For option (b), bromine \( (\text{Br}_2) \) is added to sodium chloride \( (\text{NaCl}) \) solution. Bromine is less reactive than chlorine, so \( \text{Br}_2 \) cannot displace chloride ions \( (\text{Cl}^-) \). No reaction occurs.

Step 4: Analyzing Reaction (c)

For option (c), bromine \( (\text{Br}_2) \) is added to potassium iodide \( (\text{KI}) \) solution. Bromine is more reactive than iodine, so \( \text{Br}_2 \) will displace iodide ions \( (\text{I}^-) \) to form \( \text{KBr} \) and liberate iodine. The reaction is: \[ \text{Br}_2 + 2 \text{KI} \rightarrow 2 \text{KBr} + \text{I}_2 \].

Step 5: Analyzing Reaction (d)

For option (d), chlorine \( (\text{Cl}_2) \) is added to sodium fluoride \( (\text{NaF}) \) solution. Since fluorine is more reactive than chlorine, \( \text{Cl}_2 \) cannot displace fluoride ions \( (\text{F}^-) \). No reaction occurs.

Key Concepts

Halogen Reactivity

In chemistry, halogen reactivity is an important topic, especially when discussing displacement reactions. Halogens are a group of elements found in Group 17 of the periodic table. These include fluorine ( F ), chlorine ( Cl ), bromine ( Br ), and iodine ( I ). Reactivity of halogens decreases as you move down the group. This means fluorine is the most reactive while iodine is the least. The reactivity is primarily due to the number of electrons and their ability to attract electrons (electronegativity). In a displacement reaction, a more reactive halogen will replace a less reactive halide ion. For instance, chlorine ( Cl_2 ) can displace bromine ( Br^− ) because chlorine is more reactive. This reactivity trend is crucial for predicting the outcomes of chemical reactions involving halogens. It helps chemists determine what compounds will form when different halogens and halides interact.

Oxidation and Reduction

Oxidation and reduction, often referred to as redox reactions, are at the core of halogen displacement. In these reactions, one species is oxidized and loses electrons, while another is reduced and gains electrons. - In a typical displacement reaction involving halogens, the more reactive halogen undergoes reduction, gaining electrons from the less reactive halide ion, which undergoes oxidation by losing electrons. - For example, when chlorine ( Cl_2 ) is added to a solution containing potassium bromide ( KBr ), chlorine is reduced, forming chloride ions ( Cl^− ), while bromide ions ( Br^− ) lose electrons and are oxidized to form bromine ( Br_2 ). Redox reactions illustrate the transfer of electrons between atoms, helping us understand how substances change their oxidation states during chemical reactions. It's a fundamental concept that explains the movement of electrons and the energy changes involved, which is especially evident in energetic reactions involving halogens.

Chemical Reactions

Chemical reactions are processes in which substances, called reactants, change into new substances, called products. These reactions frequently involve the breaking and forming of chemical bonds, resulting in a change of the chemical composition of the reacting substances. In the context of halogen displacement, chemical reactions showcase the reactivity sequence of halogens. - When Cl_2 is added to a KBr solution, a displacement reaction occurs, forming KCl and liberating Br_2 . - Conversely, if Br_2 is added to NaCl , no reaction occurs because bromine is less reactive than chlorine. Chemical reactions are governed by principles such as conservation of mass and the reactivity of the elements involved. Understanding these reactions helps grasp why certain combinations of reactants lead to reactions and others do not.

Periodic Table Trends

Periodic table trends play an essential role in predicting the behavior of elements during chemical interactions. The periodic table is structured in a way that elements with similar properties are grouped together. - For halogens, the reactivity trend is a key feature, decreasing from top to bottom due to increasing atomic size and decreasing electronegativity. - The period number tells us about the energy levels of electrons, with higher numbers indicating electrons farther from the nucleus, leading to decreased energy to attract additional electrons. Understanding these trends is vital in predicting reactions, especially for students studying chemistry. They offer insights into why elements behave a certain way when they form compounds. As seen in the displacement reactions, periodic trends across Group 17 allow us to predict which halogen will react or remain unreactive based solely on their position in the periodic table.