Problem 42 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

White P reacts with caustic soda. The products are \(\mathrm{PH}_{3}\) and \(\mathrm{NaH}_{2} \mathrm{PO}_{2}\). This reaction is an example of \([1980]\) (a) Oxidation (b) Reduction (c) oxidation and reduction (d) Neutralisation

Step-by-Step Solution

Answer

(c) oxidation and reduction

Step 1: Identify the Reaction Type

White phosphorus \((P)\) reacts with caustic soda \((NaOH)\) to produce phosphine \((PH_3)\) and sodium hypophosphite \((NaH_2PO_2)\). To determine the type of reaction, we must check the change in oxidation states of phosphorus in the reaction.

Step 2: Determine Oxidation States

First, identify the oxidation states of phosphorus in the reactants and products. In white phosphorus \((P_4)\), each phosphorus atom is in the zero oxidation state. In \(PH_3\), phosphorus has an oxidation state of -3. In \(NaH_2PO_2\), phosphorus has an oxidation state of +1.

Step 3: Identify Changes in Oxidation States

Observe the changes in oxidation states for phosphorus. From \(P_4\) to \(PH_3\), phosphorus goes from 0 to -3 (reduction). From \(P_4\) to \(NaH_2PO_2\), phosphorus goes from 0 to +1 (oxidation).

Step 4: Classify the Reaction

Since phosphorus undergoes reduction (0 to -3) in forming \(PH_3\) and oxidation (0 to +1) in forming \(NaH_2PO_2\), both oxidation and reduction are occurring. Thus, this reaction is an example of both oxidation and reduction.

Key Concepts

Oxidation States

Understanding oxidation states is key to unraveling redox reactions. An oxidation state, also known as oxidation number, is used to describe the loss or gain of electrons by an atom in a chemical reaction. It signifies the degree of oxidation (loss of electrons) or reduction (gain of electrons).

In a compound, each atom is assigned an oxidation number that reflects its ability to acquire, donate, or share electrons during a chemical reaction. These values can be positive, negative, or zero.

For example, in the reaction involving white phosphorus ( P_4 ) and caustic soda ( NaOH ), variable oxidation states are observed for phosphorus. Initially found in a zero oxidation state in P_4, phosphorous manifests a range of oxidation states as it is both oxidized and reduced throughout the process.

White Phosphorus

White phosphorus is a form of the element phosphorus and is composed of P_4 molecules. This allotrope of phosphorus is less stable than the red and black forms, making it highly reactive.

Due to its inherent instability, white phosphorus readily undergoes chemical reactions, such as the one with caustic soda where it converts into phosphine and sodium hypophosphite. It is important to note that in P_4, each phosphorus atom is in the zero oxidation state.

• Highly reactive due to its strained P_4 molecular structure.
• Used in reactions as a reducing agent.
• Known to convert into several compounds through redox reactions.

White phosphorus should always be handled with care due to its dangerous reactivity.

Caustic Soda

Caustic soda, or sodium hydroxide (NaOH), is a powerful base used in various industrial processes. In chemical reactions, it often serves to balance equations and influence the overall reaction mechanism.

When reacting with white phosphorus, caustic soda provides an environment for the oxidation and reduction processes of phosphorus to occur. It acts as an essential medium in driving the conversion of P_4 into phosphine ( PH_3 ) and sodium hypophosphite ( NaH_2PO_2 ).

• Strongly alkaline in nature.
• Facilitates the dissociation and recombination of atomic species in a reaction.
• Serves to neutralize acidic by-products in reactions.

In a controlled laboratory setting, caustic soda must be managed carefully, noting its corrosive nature.

Phosphine

Phosphine, or PH_3, is a gaseous compound resulting from the reaction between white phosphorus and caustic soda. Formed during the reduction phase, phosphine has phosphorus in the -3 oxidation state.

This compound exhibits several unique characteristics:

• Colorless, with a distinctive unpleasant odor similar to "fish-like" smells.
• Highly flammable and toxic in nature.
• Occurs commonly in various industrial and biological processes as a by-product.

Phosphine's lower oxidation state compared to the phosphorus in P_4 signifies its receipt of electrons during the reaction—a classic indicator of reduction.

Sodium Hypophosphite

Sodium hypophosphite (NaH_2PO_2) emerges as a product in the reaction of white phosphorus with caustic soda. In this compound, phosphorus has an oxidation state of +1, indicating an oxidation process.

Characteristics of sodium hypophosphite include:

• White crystalline solid.
• Used in electroless nickel plating.
• Serves as a reducing agent in various chemical applications.

As phosphorus transitions from P_4 to NaH_2PO_2, it undergoes oxidation, shifting from its elemental to a higher oxidation state. This addition of oxidation states exemplifies the dual nature of phosphorus behavior in redox reactions.