Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{I}_{2}\) is passed through \(\mathrm{KCl}, \mathrm{KF}, \mathrm{KBr}\) (a) \(\mathrm{Cl}_{2}\) is evolved (b) \(\mathrm{Cl}_{2}\) and \(\mathrm{Br}_{2}\) are evolved (c) \(\mathrm{Cl}_{2}, \mathrm{~F}_{2}\) and \(\mathrm{Br}_{2}\) are evolved (d) none of these

Step-by-Step Solution

Answer

(d) none of these

Step 1: Analyze the Reaction with Iodine

Iodine, \(\mathrm{I}_2\), is a halogen. When iodine is passed through potassium halides such as \(\mathrm{KCl}\), \(\mathrm{KF}\), and \(\mathrm{KBr}\), we need to consider the reactivity of halogens. In the halogen group, reactivity decreases down the group, meaning fluorine is the most reactive, followed by chlorine, bromine, and iodine being the least reactive. This means that iodine cannot oxidize Cl\(^-\), F\(^-\), or Br\(^-\) to \(\mathrm{Cl}_2\), \(\mathrm{F}_2\), or \(\mathrm{Br}_2\).

Step 2: Evaluate Possible Outcomes

Based on the reactivity series of the halogens, iodine does not have the sufficient oxidizing power to replace fluorine, chlorine or bromine in their respective potassium salts. Therefore, no reactions occur that would lead to the evolution of \(\mathrm{Cl}_2\), \(\mathrm{F}_2\), or \(\mathrm{Br}_2\).

Step 3: Conclusion on Halogen Behavior

As a result of the iodine's inability to oxidize any of the halides in \(\mathrm{KCl}\), \(\mathrm{KF}\), and \(\mathrm{KBr}\), it concludes that none of these reactions occur. Hence, none of \(\mathrm{Cl}_2\), \(\mathrm{F}_2\), or \(\mathrm{Br}_2\) are evolved.

Key Concepts

Iodine Reactivity

Iodine is an interesting member of the halogen family. Amongst the halogens, it is known for its unique properties and distinctive purple vapor. However, when it comes to reactivity, iodine sits at the bottom of the halogen reactivity series. This means it's less reactive compared to its siblings like chlorine and bromine. Reactivity in halogens decreases as you move down the group in the periodic table.

• Fluorine is the most reactive, and as such, it can replace other halogens in reactions.
• Chlorine is next in line, followed by bromine, with iodine being the least reactive.

This reduced reactivity is due to iodine's larger atomic size and lower ability to attract electrons compared to the smaller, more electronegative fluorine and chlorine. This characteristic is crucial when considering its behavior in chemical reactions, such as its interaction with potassium halides.

Potassium Halides

Potassium halides are a group of compounds where potassium is combined with halogen elements like fluorine, chlorine, or bromine. Common examples include potassium chloride (\(\mathrm{KCl}\)), potassium fluoride (\(\mathrm{KF}\)), and potassium bromide (\(\mathrm{KBr}\)). These compounds are typically ionic, consisting of a potassium ion \(\mathrm{K}^+\) and a halide ion \(\mathrm{X}^-\), where \(\mathrm{X}\) represents a halogen.

• Each potassium halide has its own specific properties, but they usually form white crystalline solids and dissolve readily in water.
• Despite their solubility and solid form, the key chemical behavior comes from the halide ions.

When iodine passes over these potassium halides, its interaction depends heavily on the reactivity difference between iodine and the halogen atoms present in the halides. Since iodine is lower in reactivity than the halogens involved in these salts, it doesn’t have the ability to oxidize them to their elemental form.

Oxidizing Power of Halogens

The oxidizing power of halogens decreases down the group in the periodic table. This means that fluorine, being the strongest oxidizing agent, can easily gain electrons and thus oxidize other substances. Conversely, iodine, with the weakest oxidizing power among the common halogens, struggles to achieve such feats.

• The ability of a halogen to act as an oxidizing agent is linked to its ability to accept electrons, which is why fluorine can readily displace other halogens from their compounds.
• Chlorine and bromine have moderate oxidizing powers, able to displace halogens below them.
• Iodine, on the other hand, lacks sufficient oxidizing strength to replace fluorine, chlorine, or bromine from their respective potassium halides.

Therefore, when iodine passes through potassium chloride, fluoride, or bromide, it cannot oxidize the halide ions to their elemental form, aligning with the prediction that no chlorine, fluorine, or bromine gas is evolved in such a scenario.