Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is the best description of the behaviour of bromine in the reaction given below? \(\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}_{2} \longrightarrow \mathrm{HOBr}+\mathrm{HBr}\) (a) reduced only (b) oxidized only (c) both oxidized and reduced (d) proton acceptor only

Step-by-Step Solution

Answer

Bromine is both oxidized and reduced (option c).

Step 1: Identify the Redox Process

Look at the reactants and products to determine if a redox (reduction-oxidation) process is occurring. Redox involves changes in the oxidation states of elements. In the reaction \[\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}_{2} \longrightarrow \mathrm{HOBr}+\mathrm{HBr}\]the element of interest is bromine.

Step 2: Determine Oxidation States in the Reactants

For the molecule \( \mathrm{Br}_2 \), each bromine atom is in its elemental state with an oxidation state of 0 because it is a diatomic molecule.

Step 3: Determine Oxidation States in the Products

In the products, bromine appears as part of two compounds: \( \mathrm{HOBr} \) and \( \mathrm{HBr} \). In \( \mathrm{HOBr} \), bromine is bonded to oxygen; here, its oxidation state is +1. In \( \mathrm{HBr} \), bromine is bonded to hydrogen; thus, its oxidation state is -1.

Step 4: Analyze Changes in Oxidation States

Comparing the oxidation states from reactants to products, in \( \mathrm{Br}_2 \):- **Reduction**: The oxidation state goes from 0 in \( \mathrm{Br}_2 \) to -1 in \( \mathrm{HBr} \). This indicates a gain of electrons, or reduction.- **Oxidation**: The oxidation state goes from 0 in \( \mathrm{Br}_2 \) to +1 in \( \mathrm{HOBr} \). This indicates a loss of electrons, or oxidation.

Step 5: Conclude the Behaviour of Bromine

Since one bromine atom is oxidized (loses electrons) and the other is reduced (gains electrons) during the reaction, bromine is acting as both an oxidizing and reducing agent.

Key Concepts

Oxidation States

Oxidation states are used to keep track of electron transfers during chemical reactions. They are a formal way of describing the degree of oxidation of an atom in a compound. Here are the basics:

• In a pure element like bromine gas (\( \mathrm{Br}_2 \)), the oxidation state is zero. This is because the electrons are shared equally between the two identical bromine atoms.
• When bromine forms compounds, its oxidation state changes based on the number of electrons it shares or gains from other atoms. For example, in \( \mathrm{HBr} \), bromine takes an electron from hydrogen, giving it an oxidation state of -1. This implies that bromine has gained an electron, increasing its degree of reduction.
• In \( \mathrm{HOBr} \), bromine is bonded to oxygen, which attracts electrons more strongly. This results in an oxidation state of +1 for bromine as it loses electrons.

Oxidation states thus serve as a bookkeeping device, making it easier to identify which atoms are oxidized and which are reduced in reactions.

Reduction

Reduction involves the gain of electrons by an atom, resulting in a decrease in its oxidation state. In the provided reaction, reduction occurs when bromine transitions from a higher oxidation state to a lower one:

• For bromine in \( \mathrm{Br}_2 \), both atoms start with an oxidation state of 0.
• One bromine atom, in forming \( \mathrm{HBr} \), gains an electron, going from 0 to -1. This represents reduction.

Reduction can be remembered with the acronym OIL RIG: "Oxidation Is Loss, Reduction Is Gain" of electrons. In this case, the gaining of electrons by one bromine atom results in it being reduced.

Oxidation

Oxidation is the process of losing electrons, which results in an increase in the oxidation state of an atom. In the reaction involving bromine:

• Both bromine atoms in \( \mathrm{Br}_2 \) start with an oxidation state of 0.
• One bromine atom loses electrons when forming \( \mathrm{HOBr} \), changing its oxidation state from 0 to +1.

This indicates oxidation, as losing electrons increases the oxidation state. Remember the acronym OIL RIG: "Oxidation Is Loss" of electrons. As an atom loses electrons, it undergoes oxidation.

Oxidizing and Reducing Agents

In redox reactions, substances that cause other substances to be oxidized or reduced are referred to as oxidizing and reducing agents. Here's how they work:

• An oxidizing agent is a substance that gains electrons and is reduced in the chemical reaction. In our case, bromine displays dual roles.
• As a component of \( \mathrm{Br}_2 \), one bromine atom acts as a reducing agent by losing electrons (being oxidized) when \( \mathrm{HOBr} \) is formed, increasing its oxidation state.
• At the same time, it acts as an oxidizing agent when it gains electrons while forming \( \mathrm{HBr} \), decreasing its oxidation state.

A good way to remember this is LEO the lion says GER: "Losing Electrons means Oxidation, Gaining Electrons means Reduction." This role-shifting makes bromine both an oxidizing and reducing agent in the reaction.