Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following molecules, in which does bromine show the highest oxidation number? (a) \(\mathrm{Hg}_{2}\left(\mathrm{BrO}_{3}\right)_{2}\) (b) \(\mathrm{Br}-\mathrm{Cl}\) (c) \(\mathrm{KBrO}_{4}\) (d) \(\mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

Bromine shows the highest oxidation number in (c) \( \mathrm{KBrO}_{4} \), which is +7.

Step 1: Understand Oxidation Numbers

The oxidation number of an element in a compound indicates the number of electrons it has gained, lost, or shared in chemical bonding. To determine the highest oxidation number for bromine, evaluate each option.

Step 2: Analyze (a) \( \mathrm{Hg}_{2}\left(\mathrm{BrO}_{3}\right)_{2} \)

In \( \mathrm{BrO}_{3}^{-} \), the oxidation number of oxygen is \(-2\). Let the oxidation number of bromine be \(x\).\ \(3(-2) + x = -1\ \Rightarrow x = +5.\) Hence, Bromine's oxidation number is +5.

Step 3: Analyze (b) \( \mathrm{Br}-\mathrm{Cl} \)

In \( \mathrm{Br}-\mathrm{Cl} \), chlorine is more electronegative and is assigned \(-1\) oxidation number. Thus, bromine has \(+1\) oxidation number.

Step 4: Analyze (c) \( \mathrm{KBrO}_{4} \)

In \( \mathrm{KBrO}_{4} \), potassium (K) is \(+1\), and oxygen is \(-2\). Let bromine have an oxidation number \(x\).\ \(1 + x + 4(-2) = 0\ \Rightarrow x = +7.\) Hence, Bromine's oxidation number is +7.

Step 5: Analyze (d) \( \mathrm{Br}_{2} \)

In \( \mathrm{Br}_{2} \), the oxidation number is \(0\) for each bromine atom as this is an elemental form of bromine.

Step 6: Compare Oxidation Numbers

Upon review, bromine has the oxidation numbers: (a) +5, (b) +1, (c) +7, and (d) 0. The highest oxidation number is found in \( \mathrm{KBrO}_{4} \).

Key Concepts

Bromine Chemistry

Bromine, symbolized as Br, is a member of the halogen family and is essential in chemistry due to its versatile properties. It is a reddish-brown liquid at room temperature, making it unique among the other elements in its group. As a halogen, bromine can exist in several oxidation states, prominently affecting how it participates in chemical reactions.
Bromine's common oxidation states include

• 0 (elemental bromine, such as in \( \text{Br}_2 \))
• -1 (common in many bromine compounds)
• +1, +3, +5, and +7 (in various oxyanions and other compounds)

Understanding these states is crucial, especially when predicting the reactivity and chemical behavior of bromine in different compounds.
For example, in the compound \( \text{KBrO}_4 \), bromine exhibits its highest possible oxidation state of +7, showing its ability to form stable, yet reactive compounds.

Chemical Bonding

Chemical bonding refers to the forces holding atoms together in molecules or compounds. Bromine participates actively in chemical bonding due to its ability to achieve stable configurations. Three main types of chemical bonds are important:

• Covalent Bonds: Sharing of electrons between atoms. \( \text{Br}_2 \) is a perfect example of a simple covalent molecule where each bromine atom shares an electron to form a stable diatomic molecule.
• Ionic Bonds: Transfer of electrons from one atom to another, resulting in oppositely charged ions. Potassium bromate \( \text{KBrO}_4 \) showcases ionic characteristics between potassium (K) and the bromate ion \( \text{BrO}_4^- \).
• Polar Covalent Bonds: Unequal sharing of electrons due to differences in electronegativity, such as between bromine and chlorine \( \text{Br-Cl} \). Chlorine, being more electronegative, pulls the shared electrons closer, granting bromine a +1 oxidation state in this compound.

Comprehending these bond types helps in predicting the structure, stability, and reactions of bromine-containing compounds.

Oxidation States

An oxidation state or number is attributed to an element in a compound to indicate the degree of oxidation (loss of electrons) or reduction (gain of electrons) it experiences during a reaction. The concept is essential in bromine chemistry for predicting compound reactivity and stability.
Each bromine compound depicts a specific oxidation state:

• \( \text{Hg}_2(\text{BrO}_3)_2 \): Bromine takes an oxidation state of +5, due to the compound's structure and the roles of oxygen.
• \( \text{Br-Cl} \): Here, bromine has a +1 oxidation state as chlorine is more electronegative.
• \( \text{KBrO}_4 \): Exhibiting bromine's highest oxidation state at +7, it is emphasized by the strong oxidizing property of the compound.
• \( \text{Br}_2 \): In its elemental state, bromine's oxidation number is 0, signifying a neutral state with no net electron gain or loss.

Recognizing these states helps in balancing chemical equations, predicting product formation, and understanding the energetics of reactions involving bromine.