Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

Aqueous sodium hydroxide reacts with white phosphorous to form phosphine and (a) \(\mathrm{P}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{P}_{2} \mathrm{O}_{5}\) (c) \(\mathrm{NaH}_{2} \mathrm{PO}_{2}\) (d) \(\mathrm{Na}_{3} \mathrm{PO}_{3}\)

Step-by-Step Solution

Answer

The reaction produces phosphine and (c) NaH₂PO₂.

Step 1: Understanding the Reaction

The reaction describes the interaction between aqueous sodium hydroxide (NaOH) and white phosphorous (P). White phosphorous is known to react with bases like NaOH to produce phosphine (PH₃) as one product.

Step 2: Identifying the Other Product

Along with phosphine, the other product formed is typically a sodium-based compound with a phosphate group resulting from the oxidation states involved. We need to determine which of the given options matches this outcome.

Step 3: Analyzing the Options

Upon reacting with NaOH, white phosphorous forms phosphine and sodium hypophosphite (2). The formula of sodium hypophosphite is NaH₂PO₂. Hence, the correct answer corresponds to option (c) 2.

Key Concepts

White Phosphorus

White phosphorus is a common form of the element phosphorus, known for its distinctive properties. It appears as a waxy, white or yellowish solid and is highly reactive, especially with oxygen in the air. Due to its reactivity, it is stored underwater or in inert atmospheres to prevent it from catching fire spontaneously. This allotrope of phosphorus consists of tetrahedral P₄ molecules, making it less stable than other forms like red phosphorus.

Some essential characteristics of white phosphorus include:

• Highly flammable and oxidizes readily in air.
• Insoluble in water but soluble in certain organic solvents like carbon disulfide.
• Used in the production of phosphoric acid and phosphine gas.

White phosphorus is reactive with bases such as sodium hydroxide, which is why it readily participates in the formation of other chemical compounds through chemical reactions.

Phosphine Formation

Phosphine formation is a chemical process where phosphine gas (\( \text{PH}_3 \)) is produced. This typically occurs through the reaction of white phosphorus with a strong base like sodium hydroxide. Phosphine is a colorless, flammable gas, often referred to by its chemical name, hydrogen phosphide.

Here’s what happens in the reaction:

• Sodium hydroxide provides the necessary alkaline condition for the reaction.
• White phosphorus undergoes hydrolysis, releasing phosphine gas.
• This results in a combination of phosphine and a sodium-containing byproduct.

Phosphine is toxic and should be handled with care. It is known for its unpleasant, garlic-like odor and is used in pest control and in the semiconductor industry. The major challenge in working with phosphine is ensuring safe handling due to its explosive nature when mixed with air.

Sodium Hypophosphite

Sodium hypophosphite (\( \text{NaH}_2\text{PO}_2 \)) forms during the reaction of white phosphorus with sodium hydroxide. It is a compound containing sodium and a hypophosphite anion (\( \text{H}_2\text{PO}_2^- \)). This compound plays a critical role as a reducing agent in various chemical applications.

Some important properties and uses include:

• White crystalline solid that is deliquescent, meaning it can absorb moisture from the air.
• Widely used as a reducing agent in electroless nickel plating, which is a technique to deposit nickel-phosphorus alloy coatings.
• Serves in the formulation of certain pharmaceuticals and in nutritional products.

In the context of our reaction, sodium hypophosphite is one of the products formed alongside phosphine, highlighting the versatility and significance of phosphorus chemistry. Its application in industries ranging from electronics to food supplements showcases its broad utility and importance.