Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

When white phosphorus reacts with caustic soda, the products are \(\mathrm{PH}_{3}\) and \(\mathrm{NaH}_{2} \mathrm{PO}_{2} .\) This reaction is an example of (a) oxidation (b) reduction (c) disproportionation (d) neutralization

Step-by-Step Solution

Answer

The reaction is a disproportionation reaction (c).

Step 1: Understand the Definitions

First, understand the terms given in the options. Oxidation involves the loss of electrons, reduction involves the gain of electrons, disproportionation is a specific type of redox reaction where an element is simultaneously oxidized and reduced, and neutralization is a reaction where an acid and base react to form water and a salt.

Step 2: Analyze the Reaction

Write the chemical equation for the reaction: \( \text{P}_4 + \text{NaOH} + \text{H}_2\text{O} \rightarrow \text{PH}_3 + \text{NaH}_2\text{PO}_2 \). Here, phosphorus is initially in the form of white phosphorus (\(\text{P}_4\)), which changes to phosphine gas (\(\text{PH}_3\)) and sodium hypophosphite (\(\text{NaH}_2\text{PO}_2\)).

Step 3: Determine Oxidation State Changes

Determine the oxidation states of phosphorus in the reactants and products. In \(\text{P}_4\), phosphorus has an oxidation state of 0. In \(\text{PH}_3\), it is -3, and in \(\text{NaH}_2\text{PO}_2\), it is +1. This shows phosphorus is both oxidized (0 to +1) and reduced (0 to -3).

Step 4: Identify the Reaction Type

Since phosphorus is both oxidized and reduced, the reaction is a disproportionation reaction, where a single element undergoes both oxidation and reduction simultaneously.

Key Concepts

Understanding Oxidation State

Oxidation state is a crucial concept in chemistry that helps us understand how electrons are distributed in a molecule. It indicates the degree of oxidation of an atom within a compound. Essentially, the oxidation state reflects the number of electrons an atom has gained or lost compared to its elemental form.

Here’s a quick guide to determining oxidation states:

• For pure elements, the oxidation state is 0. For example, in white phosphorus (\( \text{P}_4 \)), each phosphorus atom has an oxidation state of 0.
• For monoatomic ions, the oxidation state equals the ion charge. For instance, \( \text{Na}^+ \) has an oxidation state of +1.
• In compounds, hydrogen typically has an oxidation state of +1, and oxygen usually has -2, unless specified otherwise.

The oxidation state changes when a reaction involves the transfer of electrons. In the context of the given exercise, phosphorus changes its oxidation state from 0 in \( \text{P}_4 \) to -3 in \( \text{PH}_3 \) and +1 in \( \text{NaH}_2\text{PO}_2 \), a key aspect of a disproportionation reaction.

The Nature of Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are processes where there is a transfer of electrons between two substances. They encompass two key sub-processes:

• Oxidation: This involves the loss of electrons. When a substance loses electrons, its oxidation state increases.
• Reduction: Here, a substance gains electrons, resulting in a decrease in its oxidation state.

These reactions are fundamental to various chemical processes, including combustion, corrosion, and cellular respiration.

In a redox reaction, one substance is oxidized while another is reduced. Disproportionation is a special type of redox reaction where one element in a single species is both oxidized and reduced, as seen in the provided exercise example where phosphorus does both, simultaneously investigated by the transfer of electrons within phosphorus itself. Understanding these concepts helps us predict reaction outcomes and manipulate processes in practical contexts.

Properties of Sodium Hypophosphite

Sodium hypophosphite (\( \text{NaH}_2\text{PO}_2 \)) is an important compound in chemistry and industry, acting primarily as a reducing agent. Here are some key points:

• It forms as a product of reactions involving the reduction of phosphorus, like in the given exercise where phosphorus transitions from \( \text{P}_4 \) to \( \text{NaH}_2\text{PO}_2 \) while being oxidized to +1.
• Sodium hypophosphite is widely used in electroplating processes, especially for nickel plating. It functions effectively to reduce nickel ions to nickel metal.
• The compound is soluble in water and is frequently used in aqueous solutions for various applications.

Understanding the behavior and roles of sodium hypophosphite deepens insight into not just chemical reactions but also industrial applications. Grasping its redox activity is crucial in leveraging its potential as a reducing agent in diverse chemical and industrial processes.