Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following sequence is correct with reference to the oxidation number of iodine? (a) \(\mathrm{HI}<\mathrm{I}_{2}<\mathrm{ICl}<\mathrm{HIO}_{4}\) (b) \(\mathrm{I}_{2}<\mathrm{HI}<\mathrm{ICl}<\mathrm{HIO}_{4}^{4}\) (c) \(\mathrm{HIO}_{4}<\mathrm{ICl}<\mathrm{I}_{2}<\mathrm{HI}\) (d) \(\mathrm{I}_{2}<\mathrm{ICl}<\mathrm{HI}<\mathrm{HIO}_{4}\)

Step-by-Step Solution

Answer

The correct sequence is option (a): \( HI < I_2 < ICl < HIO_4 \).

Step 1: Identify the Oxidation Number of Iodine in HI

In HI (hydrogen iodide), iodine is combined with hydrogen, which has an oxidation number of +1. Iodine typically has an oxidation number of -1 when paired with hydrogen. Thus, the oxidation state of iodine in HI is -1.

Step 2: Identify the Oxidation Number of Iodine in I2

In diatomic iodine (I_2), iodine is in its elemental form. Elements in their pure forms, not combined with other elements, have an oxidation number of 0. Hence, iodine has an oxidation state of 0 in I_2.

Step 3: Determine the Oxidation Number of Iodine in ICl

In ICl (iodine monochloride), chlorine, being more electronegative, typically has an oxidation number of -1. Therefore, iodine must have an oxidation number of +1 to balance the compound's charge, as ICl is neutral.

Step 4: Find the Oxidation Number of Iodine in HIO4

In HIO_4 (periodic acid), the hydrogen has a +1 oxidation state, and oxygen usually has a -2 oxidation state. To find iodine's oxidation number, let it be x:\[1 + x + 4(-2) = 0\]Solving this gives:\[x = 8 - 1 = +7\]So the oxidation number of iodine in HIO_4 is +7.

Step 5: Arrange Oxidation Numbers in Ascending Order

Now compare the oxidation numbers determined:- HI: -1- I_2: 0- ICl: +1- HIO_4: +7Arranging them in ascending order gives: \( HI < I_2 < ICl < HIO_4 \).

Step 6: Identify the Correct Option

The sequence matching our ordered oxidation numbers is \( HI < I_2 < ICl < HIO_4 \), which corresponds to option (a).Thus, the correct answer is option (a).

Key Concepts

Understanding Iodine Compounds

Iodine is versatile because it forms a variety of compounds with significant differences in properties. It's found in simple diatomic forms, like \(\mathrm{I}_2\), or combined with hydrogen to form hydrogen iodide \(\mathrm{HI}\). Meanwhile, in compounds such as iodine monochloride \(\mathrm{ICl}\), it combines with chlorine, showing further diversification.
Each compound exhibits unique chemical behavior. Understanding these behaviors requires examining their chemical structure and how bonds are formed between iodine and other elements. For instance, in \(\mathrm{HI}\), iodine pairs with hydrogen, while \(\mathrm{ICl}\) involves a halogen-halogen bond, both illustrating iodine's ability to bond with different atoms.

Oxidation State Determination

Oxidation states are crucial to understanding chemical reactions and bonding patterns. The oxidation state represents the degree of oxidation of an atom within a compound.
To determine an atom's oxidation number, certain rules are applied, such as:

• Elements in their pure form have an oxidation number of zero.
• Hydrogen normally has an oxidation number of +1.
• Oxygen usually has an oxidation number of -2.
• Fluorine always has an oxidation number of -1.

These rules help in recognizing that iodine in \(\mathrm{HI}\) has an oxidation number of -1, while in \(\mathrm{HIO}_4\), it's +7 due to its bonding with highly electronegative oxygen atoms. Following these steps helps to systematically ascertain the oxidation numbers of elements.

Periodic Acid Oxidation

Periodic acid \(\mathrm{HIO}_4\) is a noteworthy iodine compound, especially when considering its oxidation number. Here, iodine achieves one of its highest oxidation states at +7.
This high oxidation state is due to the four highly electronegative oxygen atoms pulling electrons away from iodine, leading to the assignment of a higher oxidation number. It's often used in various organic and inorganic reactions, particularly as a strong oxidizing agent.
This acid plays a crucial role in organic chemistry, especially in transforming certain compounds by splitting carbon-carbon bonds, making it a valuable reagent in synthetic applications.

The Role of Elemental Iodine

Elemental iodine, represented as \(\mathrm{I}_2\), is the basic form of iodine without any charge as it remains uncombined with other elements. Hence, in \(\mathrm{I}_2\), each iodine atom has an oxidation state of zero.
This zero state is central to iodine’s chemical significance, as it represents a neutral, stable form, which can be further reacted with other elements to form different compounds. Elemental iodine is commonly used in analytical chemistry for titrations and as an antiseptic in healthcare.
The neutral, simple form of iodine is pivotal for various synthesizing reactions, making it a cornerstone in both industrial and laboratory chemistry.

Hydrogen Iodide and Its Properties

Hydrogen iodide \(\mathrm{HI}\) is a compound of iodine where iodine typically has an oxidation state of -1. This results from iodine’s combination with hydrogen, a less electronegative element.
Hydrogen iodide is important in several chemical processes, notably as a reagent and precursor in pharmaceuticals, organic syntheses, and industrial processes.
It also notably forms a strong acid upon dissolution in water, known as hydroiodic acid, which is among the strongest acids available. The interplay of these properties underscores the diverse applications and significance of hydrogen iodide in chemistry.