Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following reaction: \(6 \mathrm{NaOH}+3 \mathrm{Cl}_{2} \longrightarrow 5 \mathrm{NaCl}+\mathrm{A}+3 \mathrm{H}_{2} \mathrm{O}\) What is the oxidation number of chlorine in 'A'? (a) \(-1\) (b) \(+1\) (c) \(+3\) (d) \(+5\)

Step-by-Step Solution

Answer

The oxidation number of chlorine in 'A' is +5.

Step 1: Identify Changes in Oxidation States

First, we determine the oxidation states of chlorine in the reactants and products. In Cl₂, each chlorine has an oxidation state of 0. In NaCl, chlorine has an oxidation state of -1, since Na is +1 and NaCl is neutral.

Step 2: Assign Oxidation Numbers

Analyze the reaction: 6 NaOH + 3 Cl₂ → 5 NaCl + A + 3 H₂O. In NaCl, the oxidation state of Cl is -1. Since A involves chlorine, we need to consider its oxidation state in A.

Step 3: Calculate Total Moles of Electrons Transferred

In 3 Cl₂, chlorine consists of 6 atoms of Cl, each with an initial oxidation state of 0. In 5 NaCl, 5 Cl atoms have an oxidation state of -1 (total change in oxidation state = 5 units). The remaining 1 Cl atom must accommodate the change in oxidation number, so we set the equation for total change: \[6(0) - 5(-1) - x = 0\]where \(x\) is the change due to Cl in A.

Step 4: Solve for x

Solve the equation from step 3:\[6(0) - 5(-1) - x = 0\]\[x = 1\]This means that the single Cl atom left must have an oxidation state such that when combined with the reaction, the total change in oxidation state sums to zero.

Step 5: Determine the Oxidation State of Chlorine in A

Since we have determined that \(x = 1\), we conclude that the oxidation number of chlorine in A is +5 to maintain the total balance of oxidation and reduction in the reaction.

Key Concepts

Redox Reactions

Redox reactions are an essential part of chemistry that involve the transfer of electrons between two species. These reactions are characterized by the change in oxidation numbers of elements involved in the reaction. When an element undergoes a redox reaction, it can either gain or lose electrons, which corresponds to reduction or oxidation, respectively.

In a typical redox process, one reactant donates electrons, becoming oxidized, while another accepts electrons, becoming reduced. This electron transfer is significant because it drives the chemical reactions forward. An easy way to remember this is through the acronym "OIL RIG": Oxidation Is Loss, Reduction Is Gain. Understanding this concept helps in identifying how substances transform in a chemical reaction.

• Oxidation: When a substance loses electrons, its oxidation number increases.
• Reduction: When a substance gains electrons, its oxidation number decreases.

If you pay attention to the changes in oxidation states, you can determine which substances are oxidized and which are reduced.

Electron Transfer

Electron transfer is at the heart of all redox reactions. Specifically, it describes how electrons move between atoms or ions. This transfer is crucial in explaining the dynamics of chemical changes and transformations.

In the exercise you’re studying, the molecule Cl₂ undergoes electron transfer during the reaction. Initially, Cl₂ has an oxidation number of 0, but in different products, the oxidation states change. Some chlorine atoms become part of NaCl, where they gain electrons and have an oxidation number of -1. However, in compound A, chlorine's oxidation state is +5, indicating that it has lost electrons in the transfer process.

• Initial State: Cl₂, with oxidation number of 0.
• Product State: NaCl, chlorine with oxidation number -1; Compound A, chlorine with oxidation number +5.

By following the trail of electron transfer and changes in oxidation states, chemists can balance reactions and predict products.

Chlorine Chemistry

Chlorine is a versatile element in chemistry, often participating in redox reactions because of its ability to exist in multiple oxidation states ranging from -1 to +7. In the given exercise, chlorine starts in a neutral diatomic molecule, Cl₂, at an oxidation state of 0 and transforms into different oxidation states in the products.

Chlorine's chemistry is illustrated by its ability to form various compounds. In the reaction provided, we see chlorine in two forms:

• NaCl: Formation involves chlorine being reduced from 0 to -1 when it accepts an electron from sodium.
• Compound A: Here, the oxidation state of chlorine is +5, signifying oxidation as it loses electrons.

Understanding chlorine chemistry allows chemists to predict and manipulate reactions, which is valuable in applications like disinfection, bleach production, and organic synthesis. Chlorine's adaptability is key to its widespread industrial use and its pivotal role in the redox reactions.