Problem 57 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(3 \mathrm{Br}_{2}+6 \mathrm{CO}_{3}^{2-}+3 \mathrm{H}_{2} \mathrm{O} \longrightarrow 5 \mathrm{Br}+\mathrm{BrO}_{3}^{-}+6 \mathrm{HCO}_{3}^{-}\) (a) bromine is oxidized and carbonate is reduced (b) bromine is reduced and water is oxidized (c) bromine is neither reduced nor oxidized (d) bromine is both reduced and oxidized

Step-by-Step Solution

Answer

(d) bromine is both reduced and oxidized

Step 1: Identify Redox Elements

Firstly, identify which elements in the reaction undergo oxidation or reduction. In this reaction, bromine ( Br_{2} ) will be assessed for any changes in oxidation state.

Step 2: Determine Oxidation States

Determine the oxidation states of bromine in Br_{2} and the products. In Br_{2} , bromine is in the 0 state. In the products, bromine exists as Br^{-} (oxidation state of -1) and BrO_{3}^{-} (oxidation state of +5).

Step 3: Analyze Changes in Oxidation States

Bromine in Br_{2} changes into two forms: Bromine changing from 0 to -1 (reduction) and bromine changing from 0 to +5 (oxidation). This indicates that bromine is undergoing both reduction and oxidation.

Step 4: Conclusion

Since bromine is both reduced (gaining electrons) and oxidized (losing electrons), it is involved in a disproportionation reaction where a single element undergoes both reduction and oxidation in the same chemical reaction.

Key Concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are fundamental processes in chemistry where the transfer of electrons occurs between chemical species. These reactions are characterized by two essential half-processes: oxidation, where a species loses electrons, and reduction, where a species gains electrons.
Understanding how to identify and interpret redox reactions is crucial in many areas of chemistry, including inorganic, organic, and biochemistry.

• In any redox reaction, the substance that donates electrons is called the reducing agent, and the one accepting electrons is the oxidizing agent.
• The overall reaction can sometimes be broken down into two half-reactions, one representing oxidation and the other reduction.
• Balancing redox reactions is a systematic process that often involves separating the equation into half-reactions to ensure both mass and charge are conserved.

Recognizing the key features of redox reactions can help in understanding the broader chemical processes they drive, such as metabolism and combustion.

Oxidation State

The oxidation state, often called oxidation number, is a useful tool for tracking electron transfer in redox reactions. It provides insight into how electrons are distributed among atoms in a molecule or compound.
Determining oxidation states can help identify which elements in a reaction are oxidized and which are reduced.

• The oxidation state is represented as an integer, which can be positive, negative, or zero.
• In the compound Br₂, for instance, each bromine atom has an oxidation state of 0. In BrO₃⁻, the oxidation state of bromine is +5.
• The calculation of oxidation states follows a set of rules, such as the oxidation state of oxygen is usually -2 and hydrogen is +1.

Identifying changes in oxidation states from reactants to products is key to determining which elements undergo oxidation or reduction. This understanding is integral to identifying disproportionation reactions, like the one involving bromine in our exercise.

Electron Transfer

Electron transfer is the cornerstone of redox reactions, as it involves the movement of electrons from one reactant (the reducing agent) to another (the oxidizing agent).
In the context of disproportionation reactions, a single element both donates and accepts electrons, showcasing its dual role.

• In our specific reaction, bromine acts uniquely by undergoing both oxidation and reduction.
• Electrons are transferred from bromine in the form of Br₂, splitting into Br⁻ (gaining an electron) and BrO₃⁻ (losing electrons).
• This dual behavior is what defines a disproportionation reaction, where the same element is oxidized and reduced.

Comprehensively understanding electron transfer not only helps elucidate the behavior of elements in redox reactions but also in broader areas like electron transport chains found in biological systems.